ADS (text="CLUSTERS" "GLOBULAR")

Valenti, E.: High-resolution near-infrared spectra of NGC 6624 and 6569

We present the first abundance analysis based on high-resolution infrared (IR) echelle spectra of NGC 6569 and 6624, two moderately reddened globular clusters located in the outer bulge of the Galaxy. We find [Fe/H]=-0.79 ± 0.02 and -0.69 ± 0.02 dex for NGC 6569 and 6624, respectively, and an average α-element enhancement of ≈+0.43 ± 0.02 and +0.39 ± 0.02 dex, consistent with previous measurements on other metal-rich bulge clusters. We measure accurate radial velocities of <v_r>=-47 ± 4 and +51 ± 3 km s^-1 and velocity dispersions of ≈8 and 6 km s^-1 for NGC 6569 and 6624, respectively. Finally, we find very low ¹²C/¹³C isotopic ratio (≤7 in NGC 6624 and ≈5 in NGC 6569), confirming the presence of extramixing mechanisms during the red giant branch evolution phase. Data presented herein were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California and the National Aeronautics and Space Administration. The observatory was made possible for studies by the generous support of the W. M. Keck Foundation.

van Dokkum, P. G.: Confirmation of Enhanced Dwarf-sensitive Absorption Features in the Spectra of Massive Elliptical Galaxies: Further Evidence for a Non-universal Initial Mass Function

We recently found that massive cluster elliptical galaxies have strong Na I λ8183, 8195 and FeH λ9916 Wing-Ford band absorption, indicating the presence of a very large population of stars with masses <~ 0.3 M _sun. Here we test this result by comparing the elliptical galaxy spectra to those of luminous globular clusters associated with M31. These globular clusters have similar metallicities, abundance ratios, and ages as massive elliptical galaxies but their low dynamical mass-to-light ratios rule out steep stellar initial mass functions (IMFs). From high-quality Keck spectra we find that the dwarf-sensitive absorption lines in globular clusters are significantly weaker than in elliptical galaxies and consistent with normal IMFs. The differences in the Na I and Wing-Ford indices are 0.027 ± 0.007 mag and 0.017 ± 0.006 mag, respectively. We directly compare the two classes of objects by subtracting the averaged globular cluster spectrum from the averaged elliptical galaxy spectrum. The difference spectrum is well fit by the difference between a stellar population synthesis model with a bottom-heavy IMF and one with a bottom-light IMF. We speculate that the slope of the IMF may vary with velocity dispersion, although it is not yet clear what physical mechanism would be responsible for such a relation.

Coomber, G.: The Unusual X-Ray Binaries of the Globular Cluster NGC 6652

Our 5 ks Chandra ACIS-S observation of the globular cluster NGC 6652 detected seven X-ray sources, three of which were previously unidentified. This cluster hosts a well-known bright low-mass X-ray binary, source A (or XB 1832–330). Source B shows unusual rapid flaring variability, with an average L_X (0.5-10 keV) ~2 × 10³⁴ erg s^–1, but with minutes-long flares up to L_X = 9 × 10³⁴ erg s^–1. Its spectrum can be fit by an absorbed power law of photon index Γ ~ 1.24 and hardens as the count rate decreases. This suggests that part or all of the variation might be due to obscuration by the rim of a highly inclined accretion disk. Sources C and D, with L_X ~ 10³³ erg s^–1, have soft and unusual spectra. Source C requires a very soft component, with a spectrum peaking at 0.5 keV, which might be the hot polar cap of a magnetically accreting polar cataclysmic variable. Source D shows a soft spectrum (fit by a power law of photon index ~2.3) with marginal evidence for an emission line around 1 keV its nature is unclear. The faint new sources E, F, and G have luminosities of 1-2 × 10³² erg s^–1, if associated with the cluster (which is likely). E and F have relatively hard spectra (consistent with power laws with photon index ~1.5). G lacks soft photons, suggesting absorption with N_H > 10²² cm^–2.

Colucci, J. E.: Globular Cluster Abundances from High-resolution, Integrated-light Spectroscopy. III. The Large Magellanic Cloud: Fe and Ages

In this paper, we refine our method for the abundance analysis of high-resolution spectroscopy of the integrated light of unresolved globular clusters (GCs). This method was previously demonstrated for the analysis of old (>10 Gyr) Milky Way (MW) GCs. Here, we extend the technique to young clusters using a training set of nine GCs in the Large Magellanic Cloud. Depending on the signal-to-noise ratio of the data, we use 20-100 Fe lines per cluster to successfully constrain the ages of old clusters to within a ~5 Gyr range, the ages of ~2 Gyr clusters to a 1-2 Gyr range, and the ages of the youngest clusters (0.05-1 Gyr) to a ~200 Myr range. We also demonstrate that we can measure [Fe/H] in clusters with any age less than 12 Gyr with similar or only slightly larger uncertainties (0.1-0.25 dex) than those obtained for old MW GCs (0.1 dex) the slightly larger uncertainties are due to the rapid evolution in stellar populations at these ages. In this paper, we present only Fe abundances and ages. In the next paper in this series, we present our complete analysis of ~20 elements for which we are able to measure abundances. For several of the clusters in this sample, there are no high-resolution abundances in the literature from individual member stars; our results are the first detailed chemical abundances available. The spectra used in this paper were obtained at Las Campanas with the echelle on the du Pont Telescope and with the MIKE spectrograph on the Magellan Clay Telescope. This paper includes data gathered with the 6.5 m Magellan Telescopes located at Las Campanas Observatory, Chile.

Naiman, J. P.: External Mass Accumulation onto Core Potentials: Implications for Star Clusters, Galaxies, and Galaxy Clusters

Accretion studies have been focused on the flow around bodies with point mass gravitational potentials, but few general results are available for non-point mass distributions. Here, we study the accretion flow onto non-divergent, core potentials moving through a background medium. We use Plummer and Hernquist potentials as examples to study gas accretion onto star clusters, dwarf and large galaxy halos, and galaxy clusters in a variety of astrophysical environments. The general conditions required for a core potential to collectively accrete large quantities of gas from the external medium are derived using both simulations and analytic results. The consequences of large mass accumulation in galaxy nuclei, dwarf galaxies, and star clusters are twofold. First, if the gas cools effectively star formation can be triggered, generating new stellar members in the system. Second, if the collective potential of the system is able to alter the ambient gas properties before the gas is accreted onto the individual core members, the augmented mass supply rates could significantly alter the state of the various accreting stellar populations and result in an enhanced central black hole accretion luminosity.

Abramowski, A.: H.E.S.S. Observations of the Globular Clusters NGC 6388 and M15 and Search for a Dark Matter Signal

Observations of the globular clusters (GCs) NGC 6388 and M15 were carried out by the High Energy Stereoscopic System array of Cherenkov telescopes for a live time of 27.2 and 15.2 hr, respectively. No gamma-ray signal is found at the nominal target position of NGC 6388 and M15. In the primordial formation scenario, GCs are formed in a dark matter (DM) halo and DM could still be present in the baryon-dominated environment of GCs. This opens the possibility of observing a DM self-annihilation signal. The DM content of the GCs NGC 6388 and M15 is modeled taking into account the astrophysical processes that can be expected to influence the DM distribution during the evolution of the GC: the adiabatic contraction of DM by baryons, the adiabatic growth of a black hole in the DM halo, and the kinetic heating of DM by stars. Ninety-five percent confidence level exclusion limits on the DM particle velocity-weighted annihilation cross section are derived for these DM halos. In the TeV range, the limits on the velocity-weighted annihilation cross section are derived at the 10^-25 cm³ s^-1 level and a few 10^-24 cm³ s^-1 for NGC 6388 and M15, respectively.

Roederer, I. U.: Heavy-element Dispersion in the Metal-poor Globular Cluster M92

Dispersion among the light elements is common in globular clusters (GCs), while dispersion among heavier elements is less common. We present detection of r-process dispersion relative to Fe in 19 red giants of the metal-poor GC M92. Using spectra obtained with the Hydra multi-object spectrograph on the WIYN Telescope at Kitt Peak National Observatory, we derive differential abundances for 21 species of 19 elements. The Fe-group elements, plus Y and Zr, are homogeneous at a level of 0.07-0.16 dex. The heavy-elements La, Eu, and Ho exhibit clear star-to-star dispersion spanning 0.5-0.8 dex. The abundances of these elements are correlated with one another, and we demonstrate that they were produced by r-process nucleosynthesis. This r-process dispersion is not correlated with the dispersion in C, N, or Na in M92, indicating that r-process inhomogeneities were present in the gas throughout star formation. The r-process dispersion is similar to that previously observed in the metal-poor GC M15, but its origin in M15 or M92 is unknown at present. The WIYN Observatory is a joint facility of the University of Wisconsin-Madison, Indiana University, Yale University, and the National Optical Astronomy Observatory.

Strader, J.: Star Clusters in M31. V. Internal Dynamical Trends: Some Troublesome, Some Reassuring

We present internal velocity dispersions and precise radial velocities for 200 globular clusters (GCs) in M31 that are derived using new high-resolution spectra from MMT/Hectochelle. Of these, 163 also have King model structural parameters that allow us to estimate their mass-to-light ratios. This is, by far, the largest such data set available for any galaxy, including the Milky Way. These data strongly confirm earlier suggestions that the optical and near-infrared mass-to-light ratios of M31 GCs decline with increasing metallicity. This behavior is the opposite of that predicted by stellar population models for a standard initial mass function. We show that this phenomenon does not appear to be caused by standard dynamical evolution. A shallower mass function for metal-rich GCs (with dN/dMvpropM ^-0.8-M ^-1.3 below 1 M _sun) can explain the bulk of extant observations. We also observe a consistent, monotonic correlation between mass-to-light ratio and cluster mass. This correlation, in contrast to the correlation with metallicity, is well explained by the accepted model of dynamical evolution of GCs through mass segregation and the preferential loss of low-mass stars, and these data are among the best available to constrain this process.

Richtler, T.: The dark halo of the Hydra I galaxy cluster: core, cusp, cosmological? Dynamics of NGC 3311 and its globular cluster system

Context. Some galaxy clusters exhibit shallow or even cored dark matter density profiles in their central regions rather than the predicted steep or cuspy profiles, conflicting with the standard understanding of dark matter. NGC 3311 is the central cD galaxy of the Hydra I cluster (Abell 1060).
Aims: We use globular clusters around NGC 3311, combined with kinematical data of the galaxy itself, to investigate the dark matter distribution in the central region of Hydra I.
Methods: Radial velocities of 118 bright globular clusters, based on VLT/VIMOS mask spectroscopy, are used to calculate velocity dispersions which are well defined out to 100 kpc. NGC 3311 is the most distant galaxy for which this kind of study has been performed. We also determine the velocity dispersions of the stellar component from long-slit spectroscopy of NGC 3311 acquired with VLT/FORS1 out to 20 kpc. We present a new photometric model for NGC 3311, based on deep VLT/FORS1 images in the V-band. We search for a dark halo that, in the context of a spherical Jeans model, can reproduce the kinematical data. We also compare the radial velocity distributions of globular clusters and planetary nebulae.
Results: The projected stellar velocity dispersion rises from a central low value of about 185 km s^-1 to 350 km s^-1 at a radius of 20 kpc. The globular cluster dispersion rises as well from 500 km s^-1 at 10 kpc to about 800 km s^-1 at 100 kpc, comparable to the velocity dispersion of the cluster galaxies. A dark matter halo with a core (Burkert halo) closely reproduces the velocity dispersions of stars and globular clusters simultaneously under isotropy. The central stellar velocity dispersions predicted by cosmological NFW halos do not agree well with those observed, while the globular clusters allow a wide range of halo parameters. A suspected radial anisotropy of the stellar population found in merger simulations aggravates the disagreement with observations. A slight tangential anisotropy would enable the data to be more accurately reproduced. However, we find discrepancies with previous kinematical data that we cannot resolve, which may indicate a more complicated velocity pattern.
Conclusions: Although one cannot conclusively demonstrate that the dark matter halo of NGC 3311 has a core rather than a cusp, a core seems to be most consistent with the present data. A more complete coverage of the velocity field and a more thorough analysis of the anisotropy is required to reach firm conclusions. Based on observations taken at the European Southern Observatory, Cerro Paranal, Chile, under the programs 082.B-0680, 076.B-0154, 065.N-0166, 065.N-0459.

Pasquini, L.: Measuring helium abundance difference in giants of NGC 2808

Context. Multiple populations have been detected in several globular clusters (GC) that do not display a spread in metallicity. Unusual features of their observed colour-magnitude diagrams (CMD) can be interpreted in terms of differences in the helium content of the stars belonging to the sub-populations.
Aims: Even if evidence gathered so far is compelling, differences in He abundance have never been directly observed. We attempt to measure these differences in two giant stars of NGC 2808 with very similar astrophysical parameters but different Na and O abundances, hence that presumably belong to different sub-populations, by directly comparing their He I 10 830 Å lines.
Methods: The He 10 830 Å line forms in the upper chromosphere. Our detailed models derive the chromospheric structure using the Ca II and Hα chromospheric lines, and simulate the corresponding He I 10 830 line profiles as a function of the helium abundance. We show that, at a given value of He abundance, the He I 10 830 equivalent width cannot significantly change without a corresponding much larger change in the Ca II chromospheric lines. We have used the VLT-CRIRES to obtain high-resolution spectra in the 10 830 Å region, and the VLT-UVES to obtain spectra of the Ca II and Hα lines of our target stars.
Results: The two target stars have very similar Ca II and Hα chromospheric lines, but different appearances in the He region. One line, blueshifted by 17 km s^-1 with respect to the He 10 830 rest wavelength, is detected in the spectrum of the Na-rich star, whereas the Na-poor star spectrum is consistent with a non-detection. From a detailed chromospheric modeling, we show that the difference in the spectra is consistent and most closely explained by an He abundance difference between the two stars of ΔY ≥ 0.17. Our optical observations bracket the infrared ones over a range of about 50 days and we do not observe any substantial variability in the Ca II and Hα lines.
Conclusions: We provide direct evidence of a significant He line strength difference in giant stars of NGC 2808 belonging to different sub-populations, which had been previously detected by other photometric and spectroscopic means. The use of appropriate model chromospheres allows us for the first time to provide an approximate quantitative estimate of this difference, which is clearly consistent with the expected difference in abundance required by the stellar evolution theory to account for the observed peculiarities of this cluster's cmD. Based on observations collected at ESO VLT, Chile, under programme 384.D-0283.

Misgeld, I.: A large population of ultra-compact dwarf galaxies in the Hydra I cluster

Aims: We performed a large spectroscopic survey of compact, unresolved objects in the core of the Hydra I galaxy cluster (Abell 1060), with the aim of identifying ultra-compact dwarf galaxies (UCDs) and investigating the properties of the globular cluster (GC) system around the central cD galaxy NGC 3311.
Methods: We obtained VIMOS medium-resolution spectra of about 1200 candidate objects with apparent magnitudes 18.5 < V < 24.0 mag, covering both the bright end of the GC luminosity function and the luminosity range of all known UCDs.
Results: By means of spectroscopic redshift measurements, we identified 118 cluster members, from which 52 are brighter than M_V = -11.0 mag, and can therefore be termed UCDs. The brightest UCD in our sample has an absolute magnitude of M_V = -13.4 mag (corresponding to a mass of ≳ 5 × 10⁷ M_ȯ) and a half-light radius of 25 pc. This places it among the brightest and most massive UCDs ever discovered. Most of the GCs/UCDs are both spatially and dynamically associated to the central cD galaxy. The overall velocity dispersion of the GCs/UCDs is comparable to what is found for the cluster galaxies. However, when splitting the sample into a bright and a faint part, we observe a lower velocity dispersion for the bright UCDs/GCs than for the fainter objects. At a dividing magnitude of M_V = -10.75 mag, the dispersions differ by more than 200 km s^-1 and up to 300 km s^-1 for objects within 5' around NGC 3311.
Conclusions: We interpret these results in the context of different UCD formation channels, and conclude that interaction-driven formation seems to play an important role in the centre of Hydra I. Based on observations obtained at the European Southern Observatory, Chile (Observing Programmes 082.B-0680 and 076.B-0154).

Ventura, P.: A deep insight into the Mg-Al-Si nucleosynthesis in massive asymptotic giant branch and super-asymptotic giant branch stars

The stars in globular clusters are known to differ in their surface chemistry: spectroscopic investigations in recent decades outlined the presence of star-to-star differences in the abundances of the light elements, up to aluminium (and possibly silicon), suggesting that some stars were contaminated by an advanced proton-capture nucleosynthesis. The asymptotic giant branch (AGB) stars are one of the most promising candidates in producing the pollution of the intracluster medium, via the ejection of gas processed by hot bottom burning, from which new stellar generations are formed. This work is focused on the degree of nucleosynthesis involving magnesium, aluminium and silicon that these sources may experience. The key ingredient in determining the degree of magnesium depletion, and the amount of aluminium that can be produced, is the rate of proton capture on ²⁵Mg, forming ²⁶Al; an increase in this cross-section by a factor of 2 with respect to the highest value allowed by the NACRE compilation allows the reproduction of the extent of the Mg depletion observed, and is in qualitative agreement with the positive Al-Si correlation observed in a few clusters. The main uncertainties associated with the macro- and microphysics input are discussed and commented upon, and a comparison with recent spectroscopic results for globular clusters showing some degree of Mg-Al anticorrelation and Al-Si correlation is presented.

Faucher-Giguère, C.-A.: Pulsar-black hole binaries in the Galactic Centre

Binaries consisting of a pulsar and a black hole (BH) are a holy grail of astrophysics, both for their significance for stellar evolution and for their potential application as probes of strong gravity. In spite of extensive surveys of our Galaxy and its system of globular clusters, no pulsar-black hole (PSR-BH) binary has been found to date. Clues as to where such systems might exist are therefore important. We show that if the central parsec around Sgr A^★ harbours a cluster of ˜25 000 stellar BHs (as predicted by mass-segregation arguments) and if it is also rich in recycled pulsar binaries (by analogy with globular clusters), then three-body exchange interactions should produce PSR-BHs in the Galactic Centre. Simple estimates of the formation rate and survival time of these binaries suggest that a few PSR-BHs should be present in the central parsec today. The proposed formation mechanism makes unique predictions for the PSR-BH properties: (1) the binary would reside within ˜1 pc of Sgr A^★; (2) the pulsar would be recycled, with a period of ˜1 to a few tens of milliseconds, and a low magnetic field B≲ 10¹⁰ G; (3) the binary would have high eccentricity, e˜ 0.8, but with a large scatter and (4) the binary would be relatively wide, with semimajor axis a_b˜ 0.1 -≳3 au. The potential discovery of a PSR-BH binary therefore provides a strong motivation for deep, high-frequency radio searches for recycled pulsars towards the Galactic Centre.

Leigh, N.: Dissecting the colour-magnitude diagram: a homogeneous catalogue of stellar populations in globular clusters

We present a homogeneous catalogue for blue straggler, red giant branch, horizontal branch and main-sequence turn-off stars in a sample of 35 clusters taken from the Advanced Camera for Surveys (ACS) Survey for Globular Clusters. As a result of the superior photometry and relatively large field of view offered by the ACS data, this new catalogue is a significant improvement upon the one presented in our 2007 catalogue. Using our catalogue, we study and compare the radial distributions of the different stellar populations. We have confirmed our previous result that there is a clear, but sublinear, correlation between the number of blue stragglers found in the cluster core and the total stellar mass contained within it. By considering a larger spatial extent than just the core, our results suggest that mass segregation is not the dominant effect contributing to the observed sublinearity. We also investigate the radial distributions of the different stellar populations in our sample of clusters. Our results are consistent with a linear relationship between the number of stars in these populations and the total mass enclosed within the same radius. Therefore, we conclude that the cluster dynamics does not significantly affect the relative distributions of these populations in our sample.

Foster, C.: Global properties of 'ordinary' early-type galaxies: photometry and spectroscopy of stars and globular clusters in NGC 4494

We present a comprehensive analysis of the spatial, kinematic and chemical properties of stars and globular clusters (GCs) in the 'ordinary' elliptical galaxy NGC 4494 using data from the Keck and Subaru telescopes. We derive galaxy surface brightness and colour profiles out to large galactocentric radii. We compare the latter to metallicities derived using the near-infrared Calcium Triplet. We obtain stellar kinematics out to ˜3.5 effective radii. The latter appear flattened or elongated beyond ˜1.8 effective radii in contrast to the relatively round photometric isophotes. In fact, NGC 4494 may be a flattened galaxy, possibly even an S0, seen at an inclination of ˜45°. We publish a catalogue of 431 GC candidates brighter than i₀= 24 based on the photometry, of which 109 are confirmed spectroscopically and 54 have measured spectroscopic metallicities. We also report the discovery of three spectroscopically confirmed ultra-compact dwarfs around NGC 4494 with measured metallicities of -0.4 ≲[Fe/H]≲-0.3. Based on their properties, we conclude that they are simply bright GCs. The metal-poor GCs are found to be rotating with similar amplitude as the galaxy stars, while the metal-rich GCs show marginal rotation. We supplement our analysis with available literature data and results. Using model predictions of galaxy formation, and a suite of merger simulations, we find that many of the observational properties of NGC 4494 may be explained by formation in a relatively recent gas-rich major merger. Complete studies of individual galaxies incorporating a range of observational avenues and methods such as the one presented here will be an invaluable tool for constraining the fine details of galaxy formation models, especially at large galactocentric radii.

Liu, L.: Two extreme contact binary systems in the nearest globular cluster M4

We present the results of photometric solutions for two contact binaries in the nearby globular cluster M4. We estimate their physical parameters using the light-curve program of the Wilson-Devinney code. These systems have quite different mass ratios, which indicates that they did not form at the same time. V47 is an extreme mass-ratio system (q= 0.123 ± 0.002) being dissimilar to the same kind of field contact binaries. V53 is a high mass-ratio (q= 1.230 ± 0.004), deep-contact (f= 79.7 ± 1.9 per cent) system. It is also a blue straggler.

Frank, M. J.: Spatially resolved kinematics of an ultracompact dwarf galaxy

We present the internal kinematics of UCD3, the brightest known ultracompact dwarf galaxy (UCD) in the Fornax cluster, making this the first UCD with spatially resolved spectroscopy. Our study is based on seeing-limited observations obtained with the ARGUS Integral Field Unit of the VLT/FLAMES spectrograph under excellent seeing conditions (0.5-0.67 arcsec FWHM). The velocity field of UCD3 shows the signature of weak rotation, comparable to that found in massive globular clusters. Its velocity dispersion profile is fully consistent with an isotropic velocity distribution and the assumption that mass follows the light distribution obtained from Hubble Space Telescope imaging. In particular, there is no evidence for the presence of an extended dark matter halo contributing a significant (≳33 per cent within R < 200 pc) mass fraction, nor for a central black hole more massive than ˜5 per cent of the UCD's mass. While this result does not exclude a galaxian origin for UCD3, we conclude that its internal kinematics are fully consistent with it being a massive star cluster. Based on observations obtained at the European Southern Observatory, Chile [Observing Programme 078.B-0496(B)].

Degenaar, N.: The accretion-heated crust of the transiently accreting 11-Hz X-ray pulsar in the globular cluster Terzan 5

We report on a Chandra Director's Discretionary Time observation of the globular cluster Terzan 5, carried out ˜7 weeks after the cessation of the 2010 outburst of the newly discovered transiently accreting 11-Hz X-ray pulsar. We detect a thermal spectrum that can be fitted with a neutron star atmosphere model with a temperature for an observer at infinity of kT^∞˜ 100 eV and a quiescent thermal bolometric luminosity of L_q˜ 2 × 10³³ erg s^-1 for an assumed distance of 5.5 kpc. The thermal emission is elevated above the quiescent base level measured in 2003 and 2009, i.e. prior to the recent accretion outburst. A likely explanation is that the neutron star crust was significantly heated during the recent accretion episode and needs to cool until it restores thermal equilibrium with the core. Although this has been observed for neutron star low-mass X-ray binaries that undergo accretion episodes of years to decades, it is the first time that evidence for crustal heating is detected for a transient system with a regular outburst duration of weeks. This opens up a new window to study heating and cooling of transiently accreting neutron stars.

Fiocchi, M.: The 17-min orbital period in the ultracompact X-ray binary 4U 0513-40

The ultracompact low-mass X-ray binary 4U 0513-40 in the globular cluster NGC1851 exhibits large amplitude X-ray flux variations with spectral changes from low/hard to high/soft states which have not been reported previously in other ultracompact X-ray binaries. Using BeppoSAX, Chandra and XMM-Newton archival data together with recent INTEGRAL observations, we reveal a clear sinusoidal periodic signal with a period of ˜17 min when the source is in a typical high/soft state with a dominant soft thermal component. The periodicity disappears when the source is in a low/hard state and the thermal soft component is not required any more to model the data. These properties indicate the orbital nature of the detected signal and imply an high inclination angle of the binary system (>80°).

Motta, S.: X-ray bursts and burst oscillations from the slowly spinning X-ray pulsar IGR J17480-2446 (Terzan 5)

The newly discovered 11-Hz accreting pulsar, IGR J17480-2446, located in the globular cluster Terzan 5, has shown several bursts with a recurrence time as short as a few minutes. The source shows the shortest recurrence time ever observed from a neutron star. Here we present a study of the morphological, spectral and temporal properties of 107 bursts observed by the Rossi X-ray Timing Explorer. The recurrence time and the fluence of the bursts clearly anticorrelate with the increase in the persistent X-ray flux. The ratio between the energy generated by the accretion of mass and that liberated during bursts indicates that helium is ignited in a hydrogen-rich layer. Therefore, we conclude that all the bursts shown by IGR J17480-2446 are Type I X-ray bursts. Pulsations could be detected in all the brightest bursts and no drifts of the frequency are observed within 0.25 Hz of the spin frequency of the neutron star. These are also phase-locked with respect to the pulsations observed during the persistent emission and no rise in the rms associated to the pulse frequency is observed during the burst. This behaviour would favour a scenario where the Type I burst, possibly ignited at the polar caps, immediately propagates to the entire neutron star surface.

Huxor, A. P.: Exploring the properties of the M31 halo globular cluster system

Following on from our discovery of a significant population of M31 outer halo globular clusters (GCs), and updates to the Revised Bologna Catalogue of M31 GCs, we investigate the GC system of M31 out to an unprecedented radius (≈120 kpc). We derive various ensemble properties, including the magnitude, colour and metallicity distributions, as well as the GC number density profile. One of our most significant findings is evidence for a flattening in the radial GC number density profile in the outer halo. Intriguingly, this occurs at a galactocentric radius of ˜2° (˜30 kpc) which is the radius at which the underlying stellar halo surface density has also been shown to flatten. The GCs which lie beyond this radius are remarkably uniform in terms of their blue (V-I)₀ colours, consistent with them belonging to an ancient population with little to no metallicity gradient. Structural parameters are also derived for a sample of 13 newly discovered extended clusters (ECs), and we find the lowest luminosity ECs have magnitudes and sizes similar to Palomar-type GCs in the Milky Way halo. We argue that our findings provide strong support for a scenario in which a significant fraction of the outer halo GC population of M31 has been accreted.

Norris, M. A.: The ubiquity and dual nature of ultra-compact dwarfs

We present the discovery of several ultra-compact dwarfs (UCDs) located in field/group environments. Examination of these objects, plus literature UCDs, confirms the existence of two distinct formation channels for these compact stellar systems. We find that the UCDs we have discovered around the group elliptical NGC 3923 (and most UCDs in general) have properties consistent with their being the most luminous members of the host galaxy's globular cluster (GC) system. As with GCs they are therefore likely to be the product of the most violent epochs of galaxy formation. We describe UCDs of this type as giant GCs (GGCs). In contrast, the UCD we have found associated with the isolated S0 NGC 4546 is clearly the result of the stripping of a nucleated companion galaxy. The young age (˜3.4 Gyr) of the UCD, the lack of a correspondingly young GC population, the apparently short dynamical friction decay time-scale (˜0.5 Gyr) of the UCD and the presence of a counter-rotating gas disc in the host galaxy (corotating with respect to the UCD) together suggest that this UCD is the liberated nucleus remaining after the recent stripping of a companion by NGC 4546. We infer that the presence of UCDs of either category (GGCs formed in major star-forming events, or stripped nuclei formed in minor mergers) can provide a useful probe of the assembly history of the host galaxy. We suggest a general scheme that unifies the formation of GCs, UCDs and galaxy nuclei. In this picture, 'normal' GCs are a composite population, composed of GCs formed in situ, GCs acquired from accreted galaxies and a population of lower mass stripped dwarf nuclei masquerading as GCs. Above a 'scaling onset mass' of 2 × 10⁶ M_ȯ (M_V˜-10), UCDs emerge together with a mass-size relation and a likely mass-metallicity relation (the 'blue tilt'). In the mass range up to 7 × 10⁷ M_ȯ (M_V˜-13) UCDs comprise a composite population of GGCs and stripped nuclei. Interestingly, dwarf nuclei have similar colours to blue GCs and UCDs across the scaling onset mass, smoothly extending the blue tilt, while nuclei of more massive galaxies and a prominent minority of UCDs extend the red locus of GCs. Above 7 × 10⁷ M_ȯ, UCDs must be almost exclusively stripped nuclei, as no sufficiently rich GC systems exist to populate such an extreme of the GCLF.

Hourihane, A. P.: Ground- and space-based study of two globular cluster cataclysmic variables: M22 CV1 and M5 V101

As a class of compact binaries with large binding energy, cataclysmic variables (CVs) formed through close encounters play an important role in the dynamical evolution of globular clusters. As part of a systematic search for CVs undergoing dwarf nova (DN) eruptions in globular clusters, our 2004 monitoring programme of M22 detected an outburst of the DN candidate CV1 during May. We implement the ISIS image subtraction routine to obtain a light curve for an outburst of CV1. We present the outburst light curve as well as Hubble Space Telescope/Wide-Field Planetary Camera 2 (WFPC2) photometry in the V, U and near-ultraviolet (nUV) bands and a Chandra/Advanced CCD Imaging Spectrometer (ACIS) spectrum of the object. Our results confirm the DN nature of the outburst and the CV status of the object. We also present the results of a ground-based study of another globular cluster CV, M5 V101 - including quiescent medium-resolution William Herschel Telescope (WHT)/Intermediate dispersion Spectrograph and Imaging System (ISIS) spectroscopy in the B and R bands, displaying prominent Balmer and He I emission, and R-band photometry.

Forbes, D. A.: Evidence for two phases of galaxy formation from radial trends in the globular cluster system of NGC 1407

Here we present the colours of individual globular clusters (GCs) around the massive elliptical galaxy NGC 1407 out to a projected galactocentric radius of 140 kpc or 17 galaxy effective radii (R_e). Such data are a proxy for the halo metallicity. We find steep, and similar, metallicity gradients of ˜-0.4 dex dex^-1 for both the blue (metal poor) and red (metal rich) GC subpopulations within 5-8.5 R_e (40-70 kpc). At larger radii the mean GC colours (metallicity) are constant. A similar behaviour is seen in a wide-field study of M87's GC system, and in our own Galaxy. We interpret these radial metallicity trends to indicate an inner region formed by early in situ dissipative processes and an outer halo formed by the ongoing accretion of low-mass galaxies and their GCs. These results provide observational support for the model of galaxy formation whereby massive galaxies form inside-out in two phases. We have also searched the literature for other massive early-type galaxies with reported GC metallicity gradients in their inner regions. No obvious correlation with galaxy mass or environment is found but the sample is currently small.

Assmann, P.: Star cluster collisions - a formation scenario for the extended globular cluster Scl-dE1 GC1

Recent observations of the dwarf elliptical galaxy Scl-dE1 (Sc22) in the Sculptor group of galaxies revealed an extended globular cluster (Scl-dE1 GC1), which exhibits an extremely large core radius of about 21.2 pc. The authors of the discovery paper speculate on whether this object could reside in its own dark matter halo and/or if it might have formed through the merging of two or more star clusters. In this paper, we present N-body simulations to explore thoroughly this particular formation scenario. We follow the merger of two star clusters within dark matter haloes of a range of masses (as well as in the absence of a dark matter halo). In order to obtain a remnant which resembles the observed extended star cluster, we find that the star formation efficiency has to be quite high (around 33 per cent) and the dark matter halo, if present at all, has to be of very low mass, i.e. raising the mass-to-light ratio of the object within the body of the stellar distribution by at most a factor of a few. We also find that expansion of a single star cluster following mass loss provides another viable formation path. Finally, we show that future measurements of the velocity dispersion of this system may be able to distinguish between the various scenarios we have explored.

Gieles, M.: The life cycle of star clusters in a tidal field

The evolution of globular clusters due to two-body relaxation results in an outward flow of energy and at some stage all clusters need a central energy source to sustain their evolution. Hénon provided the insight that we do not need to know the details of the energy production in order to understand the relaxation-driven evolution of the cluster, at least outside the core. He provided two self-similar solutions for the evolution of clusters based on the view that the cluster as a whole determines the amount of energy that is produced in the core: steady expansion for isolated clusters, and homologous contraction for clusters evaporating in a tidal field. The amount of expansion or evaporation per relaxation time-scale is set by the instantaneous radius or number of stars, respectively. We combine these two approximate models and propose a pair of Unified Equations of Evolution (UEE) for the life cycle of initially compact clusters in a tidal field. The half-mass radius increases during the first part (roughly half) of the evolution, and decreases in the second half, while the escape rate approaches a constant value set by the tidal field. We refer to these phases as 'expansion dominated' and 'evaporation dominated'. These simple analytical solutions of the UEE immediately allow us to construct evolutionary tracks and isochrones in terms of cluster half-mass density, cluster mass and galactocentric radius. From a comparison to the Milky Way globular clusters we find that roughly one-third of them are in the second, evaporation-dominated phase and for these clusters the density inside the half-mass radius varies with the galactocentric distance R_G as ρ_h∝R^-2_G. The remaining two-thirds are still in the first, expansion-dominated phase and their isochrones follow the environment-independent scaling ρ_h∝M², where M is the cluster mass; that is, a constant relaxation time-scale. We find substantial agreement between Milky Way globular cluster parameters and the isochrones, which suggests that there is, as Hénon suggested, a balance between the flow of energy and the central energy production for almost all globular clusters.

Majaess, D. J.: Anchoring the Universal Distance Scale Via a Wesenheit Template

A VI Wesenheit diagram featuring SX Phoenicis, δ Scuti, RR Lyrae, and type II and classical Cepheid variables is calibrated by means of geometric-based distances inferred from HST, Hipparcos, and VLBA observations (n = 30). The distance to a target population follows from the offset between the observed Wesenheit magnitudes and the calibrated template. The method is evaluated by ascertaining the distance moduli for the LMC (μ0 = 18.43 ± 0.03 σx) and the globular clusters ω Cen, M54, M13, M3, and M15. The results agree with estimates cited in the literature, although a nearer distance to M13 is favored (pending confirmation of the data's photometric zero-point) and observations of variables near the core of M15 suffer from photometric contamination. The calibrated LMC data are subsequently added to the Wesenheit template since that galaxy exhibits precise OGLE photometry for innumerable variables of differing classes, that includes recent observations for δ Scuti variables indicating the stars follow a steeper VI Wesenheit function than classical Cepheids pulsating in the fundamental mode. VI photometry for the calibrators is tabulated to facilitate further research, and includes new observations acquired via the AAVSO's robotic telescope network (e.g., VY Pyx: <V> = 7.25 and <V> - = 0.67). The approach outlined here supercedes the lead author's prior first-order effort to unify variables of the instability strip in order to establish reliable distances.

Sivakoff, G. R.: Chandra HRC confirms that M15 X-2 is the currently flaring source in M15

Radio observations of the ongoing X-ray flare in the globular cluster M15 (ATel #3356, #3363), suggested that the ultracompact X-ray binary M15 X-2 was the flaring source (ATel #3378). This is surprising as ultracompact X-ray binaries such as M15 X-2 (22 minute period, Dieball et al. 2005, ApJ, 634, L105) are generally not expected to show factors of >10 flux variations over timescales of days to weeks.

Lynch, R. S.: Six New Recycled Globular Cluster Pulsars Discovered with the Green Bank Telescope

We have completed sensitive searches for new pulsars in seven globular clusters using the Robert C. Byrd Green Bank Telescope, and have discovered six new recycled pulsars (four in NGC 6517 and two in M22), five of which are fully recycled millisecond pulsars with P < 10 ms. We report full timing solutions for all six new pulsars and provide estimates of their flux densities and spectral indices. None of the pulsars is detected in archival Chandra data down to L _X ~ 10³² erg s^-1 for NGC 6517 and L _X ~ 10³¹ erg s^-1 for M22. One of the millisecond pulsars in M22 appears to have a very low mass companion, and is likely a new "black widow." A second binary pulsar in NGC 6517 is in a long-period, mildly eccentric orbit. We are able to set some lower limits on the age of the system, and find that it may be less than a few hundred million years old, which would indicate recent pulsar recycling in NGC 6517. An isolated pulsar in NGC 6517 that lies about 20 core radii from the cluster center appears to have been ejected from the core by interacting with a massive binary. By analyzing the luminosity function of the pulsars in NGC 6517, we predict the cluster to harbor roughly a dozen pulsars. We use the observed period derivatives of three pulsars to set lower limits on the mass-to-light ratios in the cores of their host clusters, and find no evidence for large amounts of low-luminosity matter. We also discuss reasons for non-detections in some of the clusters we searched.

Barnard, R.: Four New Black Hole Candidates Identified in M31 Globular Clusters with Chandra and XMM-Newton

We have identified four new black hole candidates (BHCs) in M31 globular clusters (GCs) using 123 Chandra and 4 XMM-Newton observations of the M31 central region. The X-ray source associated with Bo 163 (XB163) is a recurrent transient with the highest observed luminosity, ~1.4× 10³⁸ erg s^-1, considerably brighter than any outbursts from neutron star transients Aql X-1 or 4U 1608-452; the outburst apparently started ~45 days earlier than the observed peak, and hence the luminosity may have been considerably higher. We identified XB082, XB153, and XB185 as BHCs by observing "low state" emission spectra at luminosities that exceed the threshold for neutron star binaries. The probability that these are neutron star systems with anisotropic emission beamed toward us is <~ 4 × 10^-4, and their variability suggests emission from a single source. We therefore conclude that these systems likely contain black holes rather than neutron stars. We have now identified four persistently bright BHCs in the region; the probability that these are all background active galactic nuclei is lsim1×10^-20. According to theory, the donors could be tidally captured main-sequence stars or white dwarfs in ultracompact binaries. We find that GCs that are particularly massive (XB082) or metal-rich (XB144) can host bright X-ray sources in addition to GCs that are both (XB163). Our method may reveal BHCs in other bright X-ray sources.

Coelho, P.: Chemical Abundance Anticorrelations in Globular Cluster Stars: The Effect on Cluster Integrated Spectra

It is widely accepted that individual Galactic globular clusters harbor two coeval generations of stars, the first one born with the "standard" α-enhanced metal mixture observed in field halo objects and the second one characterized by an anticorrelated CNONa abundance pattern overimposed on the first generation, α-enhanced metal mixture. We have investigated with appropriate stellar population synthesis models how this second generation of stars affects the integrated spectrum of a typical metal-rich Galactic globular cluster, like 47 Tuc, focusing our analysis on the widely used Lick-type indices. We find that the only indices appreciably affected by the abundance anticorrelations are Ca4227, G4300, CN₁, CN₂, and NaD. The age-sensitive Balmer line, Fe line, and the [MgFe] indices widely used to determine age, Fe, and total metallicity of extragalactic systems are largely insensitive to the second generation population. Enhanced He in second generation stars affects also the Balmer line indices of the integrated spectra, through the change of the turnoff temperature and—with the assumption that the mass-loss history of both stellar generations is the same—the horizontal branch morphology of the underlying isochrones.

Schlaufman, K. C.: Insight into the Formation of the Milky Way through Cold Halo Substructure. II. The Elemental Abundances of ECHOS

We determine the average metallicities of the elements of cold halo substructure (ECHOS) that we previously identified in the inner halo of the Milky Way within 17.5 kpc of the Sun. As a population, we find that stars kinematically associated with ECHOS are chemically distinct from the background kinematically smooth inner halo stellar population along the same Sloan Extension for Galactic Understanding and Exploration (SEGUE) line of sight. ECHOS are systematically more iron-rich, but less α-enhanced than the kinematically smooth component of the inner halo. ECHOS are also chemically distinct from other Milky Way components: more iron-poor than typical thick-disk stars and both more iron-poor and α-enhanced than typical thin-disk stars. In addition, the radial velocity dispersion distribution of ECHOS extends beyond σ ~ 20 km s^-1. Globular clusters are unlikely ECHOS progenitors, as ECHOS have large velocity dispersions and are found in a region of the Galaxy in which iron-rich globular clusters are very rare. Likewise, the chemical composition of stars in ECHOS does not match predictions for stars formed in the Milky Way and subsequently scattered into the inner halo. Dwarf spheroidal (dSph) galaxies are possible ECHOS progenitors, and if ECHOS are formed through the tidal disruption of one or more dSph galaxies, the typical ECHOS [Fe/H] ~ - 1.0 and radial velocity dispersion σ ~ 20 km s^-1 implies a dSph with M _tot >~ 10⁹ M _sun. Our observations confirm the predictions of theoretical models of Milky Way halo formation that suggest that prominent substructures are likely to be metal-rich, and our result implies that the most likely metallicity for a recently accreted star currently in the inner halo is [Fe/H] ~ - 1.0.

Roh, D.-G.: Two Distinct Red Giant Branches in the Globular Cluster NGC 288

We report the presence of two distinct red giant branches (RGBs) in the globular cluster NGC 288 from the narrowband calcium and Strömgren b and y photometry obtained at the CTIO 4 m Blanco telescope. The RGB of NGC 288 is clearly split into two in the hk [=(Ca - b) - (b - y)] index, while the split is not shown in the b - y color. Unlike other globular clusters with multiple populations reported thus far, the horizontal branch of NGC 288 is only mildly extended. Our stellar population models show that this and the presence of two distinct RGBs in NGC 288 can be reproduced if slightly metal-rich (Δ[m/H] ≈ 0.16) second generation stars are also enhanced in helium by small amount (ΔY ≈ 0.03) and younger by ~1.5 Gyr. The RGB split in the hk index is most likely indicating that the second generation stars were affected by supernovae enrichment, together with the pollution of lighter elements by intermediate-mass asymptotic giant branch stars or fast-rotating massive stars. In order to confirm this, however, spectroscopy of stars in the two distinct RGB groups is urgently required.

Clausen, D.: How to Make a Singleton sdB Star via Accelerated Stellar Evolution

Many hot subdwarf B stars (sdBs) are in close binaries, and the favored formation channels for subdwarfs rely on mass transfer in a binary system to strip a core He-burning star of its envelope. However, these channels cannot account for sdBs that have been observed in long-period binaries nor the narrow mass distribution of isolated (or "singleton") sdBs. We propose a new formation channel involving the merger of a helium white dwarf and a low-mass, hydrogen-burning star, which addresses these issues. Hierarchical triples whose inner binaries merge and form sdBs by this process could explain the observed long-period subdwarf+main-sequence binaries. This process would also naturally explain the observed slow rotational speeds of singleton sdBs. We also briefly discuss the implications of this formation channel for extreme horizontal branch morphology in globular clusters and the UV upturn in elliptical galaxies.

Linares, M.: On the Cooling Tails of Thermonuclear X-ray Bursts: The IGR J17480-2446 Link

The neutron star transient and 11 Hz X-ray pulsar IGR J17480-2446, recently discovered in the globular cluster Terzan 5, showed unprecedented bursting activity during its 2010 October-November outburst. We analyzed all X-ray bursts detected with the Rossi X-ray Timing Explorer and find strong evidence that they all have a thermonuclear origin, despite the fact that many do not show the canonical spectral softening along the decay imprinted on type I X-ray bursts by the cooling of the neutron star photosphere. We show that the persistent-to-burst power ratio is fully consistent with the accretion-to-thermonuclear efficiency ratio along the whole outburst, as is typical for type I X-ray bursts. The burst energy, peak luminosity, and daily-averaged spectral profiles all evolve smoothly throughout the outburst, in parallel with the persistent (non-burst) luminosity. We also find that the peak-burst to persistent luminosity ratio determines whether or not cooling is present in the bursts from IGR J17480-2446, and argue that the apparent lack of cooling is due to the "non-cooling" bursts having both a lower peak temperature and a higher non-burst (persistent) emission. We conclude that the detection of cooling along the decay is a sufficient, but not a necessary condition to identify an X-ray burst as thermonuclear. Finally, we compare these findings with X-ray bursts from other rapidly accreting neutron stars.

Servillat, M.: A Dwarf Nova in the Globular Cluster M13

Dwarf novae (DNe) in globular clusters (GCs) seem to be rare with only 13 detections in the 157 known Galactic GCs. We report the identification of a new DN in M13, the 14th DN identified in a GC to date. Using the 2 m Faulkes Telescope North, we conducted a search for stars in M13 that show variability over a year (2005-2006) on timescales of days and months. This led to the detection of one DN showing several outbursts. A Chandra X-ray source is coincident with this DN and shows both a spectrum and variability consistent with that expected from a DN, thus supporting the identification. We searched for a counterpart in Hubble Space Telescope Advanced Camera for Surveys/Wide Field Camera archived images and found at least 11 candidates, of which we could characterize only the 7 brightest, including one with a 3σ Hα excess and a faint blue star. The detection of one DN when more could have been expected likely indicates that our knowledge of the global Galactic population of cataclysmic variables is too limited. The proportion of DNe may be lower than found in catalogs, or they may have a much smaller mean duty cycle (~1%) as proposed by some population synthesis models and recent observations in the field.

Gray, W. J.: Formation of Compact Stellar Clusters by High-redshift Galaxy Outflows. II. Effect of Turbulence and Metal-line Cooling

In the primordial universe, low-mass structures with virial temperatures less than 10⁴ K were unable to cool by atomic line transitions, leading to a strong suppression of star formation. On the other hand, these "minihalos" were highly prone to triggered star formation by interactions from nearby galaxy outflows. In Gray & Scannapieco, we explored the impact of nonequilibrium chemistry on these interactions. Here we turn our attention to the role of metals, carrying out a series of high-resolution three-dimensional adaptive mesh refinement simulations that include both metal cooling and a subgrid turbulent mixing model. Despite the presence of an additional coolant, we again find that outflow-minihalo interactions produce a distribution of dense, massive stellar clusters. We also find that these clusters are evenly enriched with metals to a final abundance of Z ≈ 10^-2 Z _sun. As in our previous simulations, all of these properties suggest that these interactions may have given rise to present-day halo globular clusters.

Martell, S. L.: Light-element abundance variations in globular clusters

Star-to-star variations in abundances of the light elements carbon, nitrogen, oxygen, and sodium have been observed in stars of all evolutionary phases in all Galactic globular clusters that have been thoroughly studied. The data available for studying this phenomenon, and the hypotheses as to its origin, have both co-evolved with observing technology; once high-resolution spectra were available even for main-sequence stars in globular clusters, scenarios involving multiple closely spaced stellar generations enriched by feedback from moderate- and high-mass stars began to gain traction in the literature. This paper briefly reviews the observational history of globular cluster abundance inhomogeneities, discusses the presently favored models of their origin, and considers several aspects of this problem that require further study. Highlight talk Astronomische Gesellschaft 2010

Sobeck, J. S.: The Abundances of Neutron-capture Species in the Very Metal-poor Globular Cluster M15: A Uniform Analysis of Red Giant Branch and Red Horizontal Branch Stars

The globular cluster M15 is unique in its display of star-to-star variations in the neutron-capture elements. Comprehensive abundance surveys have been previously conducted for handfuls of M15 red giant branch (RGB) and red horizontal branch (RHB) stars. No attempt has been made to perform a single, self-consistent analysis of these stars, which exhibit a wide range in atmospheric parameters. In the current effort, a new comparative abundance derivation is presented for three RGB and six RHB members of the cluster. The analysis employs an updated version of the line transfer code MOOG, which now appropriately treats coherent, isotropic scattering. The apparent discrepancy in the previously reported values for the metallicity of M15 RGB and RHB stars is addressed and a resolute disparity of Δ(RHB - RGB) ≈ 0.1 dex in the iron abundance was found. The anti-correlative behavior of the light neutron-capture elements (Sr, Y, Zr) is clearly demonstrated with both Ba and Eu, standard markers of the s- and r-process, respectively. No conclusive detection of Pb was made in the RGB targets. Consequently for the M15 cluster, this suggests that the main component of the s-process has made a negligible contribution to those elements normally dominated by this process in solar system material. Additionally for the M15 sample, a large Eu abundance spread is confirmed, which is comparable to that of the halo field at the same metallicity. These abundance results are considered in the discussion of the chemical inhomogeneity and nucleosynthetic history of M15.

Gonzalez, O. A.: Alpha element abundances and gradients in the Milky Way bulge from FLAMES-GIRAFFE spectra of 650 K giants

Aims: We present the analysis of the [α/Fe] abundance ratios for a large number of stars at several locations in the Milky Way bulge with the aim of constraining its formation scenario.
Methods: We obtained FLAMES-GIRAFFE spectra (R = 22 500) at the ESO Very Large Telescope for 650 bulge red giant branch (RGB) stars and performed spectral synthesis to measure Mg, Ca, Ti, and Si abundances. This sample is composed of 474 giant stars observed in 3 fields along the minor axis of the Galactic bulge and at latitudes b = -4°, b = -6°, b = -12°. Another 176 stars belong to a field containing the globular cluster NGC 6553, located at b = -3° and 5° away from the other three fields along the major axis. Stellar parameters and metallicities for these stars were presented in Zoccali et al. (2008, A&A, 486, 177). We have also re-derived stellar parameters and abundances for the sample of thick and thin disk red giants analyzed in Alves-Brito et al. (2010, A&A, 513, A35). Therefore using a homogeneous abundance database for the bulge, thick and thin disk, we have performed a differential analysis minimizing systematic errors, to compare the formation scenarios of these Galactic components.
Results: Our results confirm, with large number statistics, the chemical similarity between the Galactic bulge and thick disk, which are both enhanced in alpha elements when compared to the thin disk. In the same context, we analyze [α/Fe] vs. [Fe/H] trends across different bulge regions. The most metal rich stars, showing low [α/Fe] ratios at b = -4° disappear at higher Galactic latitudes in agreement with the observed metallicity gradient in the bulge. Metal-poor stars ([Fe/H] < -0.2) show a remarkable homogeneity at different bulge locations.
Conclusions: We have obtained further constrains for the formation scenario of the Galactic bulge. A metal-poor component chemically indistinguishable from the thick disk hints for a fast and early formation for both the bulge and the thick disk. Such a component shows no variation, neither in abundances nor kinematics, among different bulge regions. A metal-rich component showing low [α/Fe] similar to those of the thin disk disappears at larger latitudes. This allows us to trace a component formed through fast early mergers (classical bulge) and a disk/bar component formed on a more extended timescale. Based on observations collected at the European Southern Observatory, Paranal, Chile (ESO programmes 071.B-0617 and 073.B-0074.Full Table 4 is only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/530/A54

Lane, R. R.: AAOmega spectroscopy of 29 351 stars in fields centered on ten Galactic globular clusters

Galactic globular clusters have been pivotal in our understanding of many astrophysical phenomena. Here we publish the extracted stellar parameters from a recent large spectroscopic survey of ten globular clusters. A brief review of the project is also presented. Stellar parameters have been extracted from individual stellar spectra using both a modified version of the RAdial Velocity Experiment (RAVE) pipeline and a pipeline based on the parameter estimation method of RAVE. We publish here all parameters extracted from both pipelines. We calibrate the metallicity and convert this to [Fe/H] for each star and, furthermore, we compare the velocities and velocity dispersions of the Galactic stars in each field to the Besançon Galaxy model. We find that the model does not correspond well with the data, indicating that the model is probably of little use for comparisons with pencil beam survey data such as this. The data described in Tables 1-3 are only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/530/A31

Pipino, A.: An empirical calibration of Lick indices using Milky Way globular clusters

Aims: We provide an empirical calibration relation to convert Lick indices into abundances for the integrated light of old, simple stellar populations for a large range in the observed [Fe/H] and [α/Fe]. This calibration supersedes the previously adopted ones because it is based on the real abundance pattern of the stars instead of the commonly adopted metallicity scale derived from the colours.
Methods: We carried out a long-slit spectroscopic study of 23 Galactic globular clusters (GCs) for which detailed chemical abundances in stars have previously been measured. The line-strength indices, as defined by the Lick system and Serven and collaborators, were measured in low-resolution integrated spectra of the GC light. The results were compared to average abundances and abundance ratios in stars taken from the compilation by Pritzl and collaborators, as well as synthetic models.
Results: The Fe-related indices increase linearly as a function of [Fe/H] for [Fe/H] > - 2. The Mg-related indices respond in a similar way to [Mg/H] variations, although Mgb turns out to be a less reliable metallicity indicator for [Z/H] < - 1.5. Despite our knowledge of the Mg overabundance with respect to Fe in GC stars, we are unable to infer a mean [Mg/Fe] for the integrated spectra that correlates with the resolved stars properties, because the sensitivity of the indices to [Mg/Fe] is smaller at lower metallicities. We present empirical calibrations for Ca, TiO, Ba, and Eu indices, as well as the measurements of H_α and NaD. Based on observations collected at the European Southern Observatory, Chile, programs 0.77B0195(A) and (B).Appendices are available in electronic form at http://www.aanda.org

Froebrich, D.: Old star clusters in the FSR catalogue (Froebrich+, 2010)

The sample of clusters analysed in this work is based on the FSR catalogue. They determined a star density map based on 2MASS data (Skrutskie et al. 2006, Cat. VII/233) along the entire Galactic plane with |b|<20°. Star cluster candidates were selected as local star density enhancements and a total of 1788 objects were found. These candidates were cross-referenced with the SIMBAD data base. (3 data files).

McDonald, I.: Fundamental stellar parameters in 47 Tucanae (Mcdonald+, 2011)

The photometric data used in the SEDs come from a variety of different sources covering different fields of view. These data are summarized in Figure 1 and Table 1. (2 data files).

Howe, R.: Imaging M15 with a Small Aperture Telescope by Treating the Core as a Single Star

The objective for this study will be to explore various time series methods using CCD photometry for use with amateur telescope observations of the dense globular cluster M15. Amateur telescopes are defined here as having an aperture of less than 40 cm, and are collecting photometric filtered observations over time. Specifically, we attempt to determine the light curve of the core of M15 as a "single star". This requires selection of comparison and check stars to perform differential photometry; i.e. subtraction of flux density measures between a nonvariable (reference star) and the variable "single star" of the M15 core as it changes in magnitudes over time. We explore the possibility of measuring the M15 periodicity as an aggregate of many standard stars as identified in the Stetson catalog for NGC7078 (2010). In this paper we'd like to propose methods and techniques for aggregating different cluster region's flux densities (luminosity) and periods.

Kryachko, T.: New Variable Stars in the Field of the Globular Cluster NGC5466

Not Available

di Criscienzo, M.: NGC 2419: a large and extreme second generation in a currently undisturbed cluster

We analyse complementary Hubble Space Telescope and Subaru data for the globular cluster NGC 2419. We make a detailed analysis of the horizontal branch (HB), which is composed of two main groups of stars: the luminous blue HB stars, which extend by evolution into the RR Lyrae and red HB region, and a fainter, extremely blue population. We examine the possible models for the latter group and conclude that a plausible explanation is that they correspond to a significant (˜30 per cent) extreme second generation with a strong helium enhancement (Y˜ 0.4). We also show that the colour dispersion of the red giant branch is consistent with this hypothesis, while the main-sequence data are compatible with it, although the large observational error blurs the possible underlying splitting. While it is common to find an even larger (50-80) percentage of second generation in a globular cluster, the presence of a substantial and extreme fraction of these stars in NGC 2419 might be surprising, as the cluster is at present well inside the radius beyond which the Galactic tidal field would be dominant. If a similar situation had been present in the first stages of the cluster life, then the cluster would have retained its initial mass and the percentage of second-generation stars would have been quite small (up to ˜10 per cent). Such a large fraction of extreme second-generation stars implies that the system must have been initially much more massive and in different dynamical conditions from what it is today. We discuss this issue in the light of existing models of the formation of multiple populations in globular clusters.

D'Ercole, A.: Formation of multiple populations in globular clusters: constraints on the dilution by pristine gas

The star-to-star differences in the abundance of light elements observed in the globular clusters (GCs) can be explained assuming that a second generation (SG) of stars form in the gas ejected by the asymptotic giant branch (AGB) stars belonging to a first stellar generation. However, while Na and O appear to be anticorrelated in the cluster stars, from the stellar models they turn out to be correlated in the AGB ejecta. In order to reconcile the stellar theory with the observational findings, all the GC models invoke an early dilution of AGB ejecta with pristine gas occurring during the SG formation. Despite a vast consensus about the occurrence of such a dilution, the physical process behind it is still unknown. In the present paper we set some general constraints on the pristine gas dynamics and on the possible amount of pristine gas involved in the SG formation, making use of a one-zone chemical model. We find that such a dilution is a necessary ingredient in the SG star formation to explain the observed abundance patterns. We confirm the conclusion of our previous works showing that clusters must have been initially much more massive. We also show that models assuming that clusters had an initial mass similar to their current one, and adopting a large fraction of pristine gas to form SG stars, fail to reproduce the observed Na-O anticorrelation and are not viable. We finally show that the dilution event should be restricted in time, rather than extended for the full duration of SG formation.

Das, P.: Using NMAGIC to probe the dark matter halo and orbital structure of the X-ray bright, massive elliptical galaxy, NGC 4649

We create dynamical models of the massive elliptical galaxy, NGC 4649, using the N-body made-to-measure code, NMAGIC, and kinematic constraints from long-slit and planetary nebula (PN) data. We explore a range of potentials based on previous determinations from X-ray observations and a dynamical model fitting globular cluster (GC) velocities and a stellar density profile. The X-ray mass distributions are similar in the central region but have varying outer slopes, while the GC mass profile is higher in the central region and on the upper end of the range further out. Our models cannot differentiate between the potentials in the central region, and therefore if non-thermal pressures or multi-phase components are present in the hot gas, they must be smaller than previously inferred. In the halo, we find that the PN velocities are sensitive tracers of the mass, preferring a less massive halo than that derived from the GC mass profile, but similar to one of the mass distributions derived from X-rays. Our results show that the GCs may form a dynamically distinct system, and that the properties of the hot gas derived from X-rays in the outer halo have considerable uncertainties that need to be better understood. Estimating the mass in stars using photometric information and a stellar population mass-to-light ratio, we infer a dark matter mass fraction in NGC 4649 of ˜0.39 at 1R_e (10.5 kpc) and ˜0.78 at 4R_e. We find that the stellar orbits are isotropic to mildly radial in the central ˜6 kpc depending on the potential assumed. Further out, the orbital structure becomes slightly more radial along R and more isotropic along z, regardless of the potential assumed. In the equatorial plane, azimuthal velocity dispersions dominate over meridional velocity dispersions, implying that meridional velocity anisotropy is the mechanism for flattening the stellar system.

Walker, A. R.: Constraints on the formation of the globular cluster IC 4499 from multiwavelength photometry

We present new multiband photometry for the Galactic globular cluster IC 4499 extending well past the main-sequence turn-off in the U, B, V, R, I and DDO 51 bands. This photometry is used to determine that IC 4499 has an age of 12 ± 1 Gyr and a cluster reddening of E(B-V) = 0.22 ± 0.02. Hence, IC 4499 is coeval with the majority of Galactic globular clusters, in contrast to suggestions of a younger age. The density profile of the cluster is observed to not flatten out to at least r˜ 800 arcsec, implying that either the tidal radius of this cluster is larger than previously estimated, or that IC 4499 is surrounded by a halo. Unlike the situation in some other, more massive, globular clusters, no anomalous colour spreads in the ultraviolet are detected among the red giant branch stars. The small uncertainties in our photometry should allow the detection of such signatures apparently associated with variations of light elements within the cluster, suggesting that IC 4499 consists of a single stellar population. Based in part on observations made with the European Southern Observatory telescopes obtained from the ESO/ST-ECF Science Archive Facility.

Misgeld, I.: Families of dynamically hot stellar systems over 10 orders of magnitude in mass

Dynamically hot stellar systems, whether star clusters or early-type galaxies, follow well-defined scaling relations over many orders of magnitudes in mass. These Fundamental Plane relations have been subject of several studies, which have been mostly confined to certain types of galaxies and/or star clusters so far. Here, we present a complete picture of hot stellar systems ranging from faint galaxies and star clusters of only a few hundred solar masses up to giant ellipticals (gEs) with 10¹² M_ȯ, in particular including - for the first time - large samples of compact ellipticals (cEs), ultracompact dwarf galaxies (UCDs), dwarf ellipticals (dEs) of nearby galaxy clusters and Local Group ultrafaint dwarf spheroidals (dSphs). For all those stellar systems we show the effective radius luminosity, effective radius stellar mass and effective mass surface density stellar mass plane. Two clear families of hot stellar systems can be differentiated: the 'galaxian' family, ranging from gEs over ellipticals (Es) and dEs to dSphs, and the 'star cluster' family, comprising globular clusters (GCs), UCDs and nuclear star clusters (NCs). Interestingly, massive Es have a similar size-mass relation as cEs, UCDs and NCs, with a clear common boundary towards minimum sizes, which can be approximated by R_eff≥ 2.24 × 10^-6M^4/5_★ pc. No object of either family is located in the 'zone of avoidance' beyond this limit. Even the majority of early-type galaxies at high redshift obeys this relation. The sizes of dEs and dSphs (R_eff˜ 1.0 kpc) as well as GCs (R_eff˜ 3 pc) barely vary with mass over several orders of magnitude. We use the constant galaxy sizes to derive the distances of several local galaxy clusters. The size gap between star clusters and dwarf galaxies gets filled in by low mass, resolving star clusters and the faintest dSphs at the low mass end, and by GCs/UCDs, NCs and cEs in the mass range 10⁶ < M_★ < 10⁹ M_ȯ. In the surface density-mass plane the sequences of star clusters and galaxies show the same slope, but are displaced with respect to each other by 10³ in mass and 10² in surface density. Objects that fall in between both sequences include cEs, UCDs, NCs and ultrafaint dSphs. Both, galaxies and star clusters, do not exceed a surface density of Σ_eff= 3.17 × 10¹⁰M^-3/5_★ M_ȯ pc^-2, causing an orthogonal kink in the galaxy sequence for Es more massive than 10¹¹ M_ȯ. The densest stellar systems (within their effective radius) are NCs.

Schaerer, D.: A new perspective on globular clusters, their initial mass function and their contribution to the stellar halo and the cosmic reionization

We examine various implications from a dynamical and chemical model of globular clusters (GCs), which successfully reproduces the observed abundance patterns and the multiple populations of stars in these systems assuming chemical enrichment from fast-rotating massive stars. Using the model of Decressin et al., we determine the ratio between the observed, present-day mass of GCs and their initial stellar mass as a function of the stellar initial mass function (IMF). We also compute the mass of low-mass stars ejected and the amount of hydrogen ionizing photons emitted by the proto-GCs. Typically, we find that the initial masses of GCs must be ˜8-10 times (or up to 25 times, if second-generation stars also escape from GCs) larger than the present-day stellar mass. The present-day Galactic GC population must then have contributed to approximately 5-8 per cent (10-20 per cent) of the low-mass stars in the Galactic halo. We also show that the detection of second-generation stars in the Galactic halo, recently announced by different groups, provides a new constraint on the GC IMF (GCIMF). These observations appear to rule out a power-law GCIMF, whereas they are compatible with a lognormal one. Finally, the high initial masses also imply that GCs must have emitted a large amount of ionizing photons in the early Universe. Our results reopen the question on the IMF of GCs and reinforce earlier conclusions that old GCs could have represented a significant contribution to reionize the intergalactic medium at high redshift.

Church, R. P.: Implications for the origin of short gamma-ray bursts from their observed positions around their host galaxies

We present the observed offsets of short-duration gamma-ray bursts (SGRBs) from their putative host galaxies and compare them with the expected distributions of merging compact object binaries, given the observed properties of the hosts. We find that for all but one burst in our sample the offsets are consistent with this model. For the case of bursts with massive elliptical host galaxies, the circular velocities of the hosts' haloes exceed the natal velocities of almost all our compact object binaries. Hence, the extents of the predicted offset distributions for elliptical galaxies are determined largely by their spatial extents. In contrast, for spiral hosts, the galactic rotation velocities are smaller than typical binary natal velocities and the predicted burst offset distributions are more extended than the galaxies. One SGRB, 060502B, apparently has a large radial offset that is inconsistent with an origin in a merging galactic compact binary. Although it is plausible that the host of GRB 060502B is misidentified, our results show that the large offset is compatible with a scenario where at least a few per cent of SGRBs are created by the merger of compact binaries that form dynamically in globular clusters.

Paudel, S.: Nuclei of early-type dwarf galaxies: insights from stellar populations

We present a comprehensive analysis of the spatially resolved stellar population properties of 26 early-type dwarf galaxies (dEs) in the Virgo cluster. Using Lick/IDS absorption line indices we derive simple stellar population (SSP) equivalent age, metallicity and [α/Fe] abundance ratio. In particular, we focus on the comparison of the stellar populations between the central nucleus and the surrounding galactic main body. The stellar populations of the nuclei are, for most dEs, significantly younger than those of the respective galactic main bodies, with an average difference of 3.5 Gyr. We find only five dEs with significantly older nuclei than their galactic main bodies. Furthermore, we observe most dE nuclei to be more metal rich compared to their host galaxies. These age and metallicity behaviours are shown by almost all dEs brighter than M_r=-17 mag. The metallicity of both nuclei and galactic main bodies correlates with the total luminosity of the dEs. However, the metallicity of the nuclei covers a larger range (+0.18 to -1.22 dex) than that of the galactic main bodies, which all have subsolar metallicity. The ages of dE nuclei show a statistically significant correlation with the local projected galaxy density within the cluster, such that younger ages are predominantly observed outside of the high-density central cluster region. The α-element abundance ratios are consistent with solar for both nuclei and galactic main bodies. We also examine the presence of radial gradients in the SSP parameters for a subset of 13 dEs (up to 1.2 kpc or 15 arcsec radius). We notice two different types of gradients, namely smooth profiles that include the nucleus, and profiles where a break occurs between the nucleus and the rest of the galaxy. Nevertheless, an overall trend of increasing age and decreasing metallicity with radius exists, consistent with earlier studies. The α-abundance ratio as function of radius is consistent with no gradient. Possible formation scenarios for the nuclei of dEs are discussed. The young and metal-enhanced population of nuclei suggests that these might have formed at later epochs, or the termination of star formation activity in the nuclei might have occurred relatively late, perhaps due to continuous infall of gas into the central potential well. Our stellar population analysis suggests that the merging of globular clusters is not an appropriate scenario for the formation of most dE nuclei, at least not for the brighter dEs. We speculate that there might be different formation processes which are responsible for the formation of dEs and their nuclei depending on their luminosity. Based on observations collected at the European Organization for Astronomical Research in the Southern hemisphere, Chile (programme 078.B-0178).

Bramich, D. M.: CCD time-series photometry of the globular cluster NGC 6981: variable star census and physical parameter estimates

We present the results from 10 nights of observations of the globular cluster NGC 6981 (M72) in the V, R and I Johnson wavebands. We employed the technique of difference image analysis to perform precision differential photometry on the time-series images, which enabled us to carry out a census of the understudied variable star population of the cluster. We show that 20 suspected variables in the literature are actually non-variable, and we confirm the variable nature of another 29 variables while refining their ephemerides. We also detect 11 new RR Lyrae variables and three new SX Phe variables, bringing the total confirmed variable star count in NGC 6981 to 43. We performed Fourier decomposition of the light curves for a subset of RR Lyrae stars and used the Fourier parameters to estimate the fundamental physical parameters of the stars using relations available in the literature. Mean values of these physical parameters have allowed us to estimate the physical parameters of the parent cluster. We derive a metallicity of [Fe/H]_ZW≈-1.48 ± 0.03 on the Zinn & West scale (or [Fe/H]_UVES≈-1.38 ± 0.03 on the new Carretta et al. scale) for NGC 6981, and distances of ˜16.73 ± 0.36 and ˜16.68 ± 0.36 kpc from analysis of the RR0 and RR1 stars separately. We also confirm the Oosterhoff type I classification for the cluster, and show that our colour-magnitude data are consistent with the age of ˜12.75 ± 0.75 Gyr derived by Dotter et al.

Küpper, A. H. W.: The curious case of Palomar 13: the influence of the orbital phase on the appearance of galactic satellites

We investigate the dynamical status of the low-mass globular cluster Palomar 13 by means of N-body computations to test whether its unusually high mass-to-light ratio of about 40 and its peculiarly shallow surface density profile can be caused by tidal shocking. Alternatively, we test - by varying the assumed proper motion - if the orbital phase of Palomar 13 within its orbit about the Milky Way can influence its appearance and thus may be the origin of these peculiarities, as has been suggested by Küpper et al. We find that, of these two scenarios, only the latter can explain the observed mass-to-light ratio and surface density profile. We note, however, that the particular orbit that best reproduces those observed parameters has a proper motion inconsistent with the available literature value. We discuss this discrepancy and suggest that it may be caused by an underestimation of the observational uncertainties in the proper motion determination. We demonstrate that Palomar 13 is most likely near apogalacticon, which makes the cluster appear supervirial and blown-up due to orbital compression of its tidal debris. Since the satellites of the Milky Way are on average closer to apogalacticon than perigalacticon, their internal dynamics may be influenced by the same effect, and we advocate that this needs to be taken into account when interpreting their kinematical data. Moreover, we briefly discuss the influence of a possible binary population on such measurements.

Mucciarelli, A.: NGC 1866: a milestone for understanding the chemical evolution of stellar populations in the Large Magellanic Cloud

We present new FLAMES@VLT spectroscopic observations of 30 stars in the field of the Large Magellanic Cloud (LMC) stellar cluster NGC 1866. NGC 1866 is one of the few young and massive globular clusters that is close enough so that its stars can be individually studied in detail. Radial velocities have been used to separate stars belonging to the cluster and to the LMC field, and the same spectra have been used to derive chemical abundances for a variety of elements, from [Fe/H] to the light (i.e. Na, O, Mg, etc.) to the heavy ones. The average iron abundance of NGC 1866 turns out to be [Fe/H]=-0.43 ± 0.01 dex (with a dispersion σ= 0.04 dex), from the analysis of 14 cluster member stars. Within our uncertainties, the cluster stars are homogeneous, as far as chemical composition is concerned, independent of the evolutionary status. The observed cluster stars do not show any sign of the light elements' ‘anticorrelation’ present in all the Galactic globular clusters so far studied and are also found in the old LMC stellar clusters. A similar lack of anticorrelations has been detected in the massive intermediate-age LMC clusters, indicating a different formation/evolution scenario for the LMC massive clusters younger than ˜3 Gyr with respect to the old ones. Also opposite to the Galactic globulars, the chemical composition of the older red giant branch field stars and of the young post-main-sequence cluster stars show robust homogeneity suggesting a quite similar process of chemical evolution. The field and cluster abundances are in agreement with recent chemical analysis of LMC stars, which show a distinctive chemical pattern for this galaxy with respect to the Milky Way. We discuss these findings in light of the theoretical scenario of chemical evolution of the LMC.

Matsunaga, N.: Period-luminosity relations of type II Cepheids in the Magellanic Clouds

Period-luminosity relations (PLRs) of type II Cepheids (T2Cs) in the Small Magellanic Cloud (SMC) are derived based on OGLE-III, IRSF/SIRIUS and other data, and these are compared with results for the Large Magellanic Cloud (LMC) and Galactic globular clusters. Evidence is found for a change of the PLR slopes from system to system. Treating the longer period T2Cs (W Vir stars) separately gives an SMC-LMC modulus difference of 0.39 ± 0.05 mag without any metallicity corrections being applied. This agrees well with the difference in moduli based on different distance indicators, in particular the PLRs of classical Cepheids. The shorter period T2Cs (BL Her stars) give a smaller SMC-LMC difference suggesting that their absolute magnitudes might be affected either by metallicity or by age effects. It is shown that the frequency distribution of T2C periods also changes from system to system.

Sharaf, M. A.: Analytical Solution for Stellar Density in Globular Clusters

In this paper, four parameters analytical solution will be established for the stellar density function in globular clusters. The solution could be used for any arbitrary order of outward decrease of the cluster's density.

Miller-Jones, J. C. A.: EVLA observations suggest that M15 X-2 is the currently flaring source in M15

Following the detection of an X-ray flare in the globular cluster M15 (ATel #3356) and its subsequent Swift XRT localization to the cluster core (ATel #3363), we undertook EVLA observations to determine the source of the flaring event. The 1-hour observation on 2011 May 22 (12:11-13:11 UT) comprises of two 1024-MHz bands centred at 5.0 and 7.0 GHz. The array was in its BnA configuration, providing an angular resolution of 1.25 x 0.63 srq arcsec at 5 GHz and 0.92 x 0.50 sqr arcsec at 7 GHz.

Heinke, C. O.: Swift/XRT measurement of position and X-ray spectrum of M15 X-ray binary

Following the MAXI/GSC detection of a recent flux increase and two X-ray bursts from the position of the globular cluster M15 (Atel #3356), a Swift XRT observation was conducted May 18 at 03:44 UT for 1.0 ks. The XRT data are heavily piled up, and affected by a bad column. We find the position by fitting a circle to the pileup ring, estimating a center at RA, Dec (J2000)=21:29:57.9, +12:10:06 with a 90% conf.

Bazzano, A.: Announcement of INTEGRAL Galactic Plane monitoring program and detection of 2 new hard X-ray sources.

INTEGRAL commenced a new series of Galactic Plane Scans on May 14th 2011. Under an approved INTEGRAL AO-8 proposal (PI: Bazzano) the part of the Galactic Plane visible by INTEGRAL will be scanned regularly, during every INTEGRAL revolution (about every 3 days), for parts of AO-8, for a total exposure time of 2Msec.
We here report on quick-look analysis of the observations performed in revolution 1048, covering the period from UT 2011-05-14 at 05:21:38 to 2011-05-15 at 07:51:04 and later from 2011-05-16 at 08:28:28 ending at 21:13:12 on two different regions of the sky for a total duration 64 of and 41 ks, respectively.

Morii, M.: MAXI/GSC detection of X-ray activities in the globular cluster M15

We report recent activities of the globular cluster M15. MAXI/GSC detected an X-ray burst at the scan transit centered at UT 2011-05-16T23:23:41. The burst was detected at least for 30.0 seconds within the 50.5 seconds triangular transit response of MAXI/GSC. The nominal location of the source, assuming that the source flux was constant over the transit (which probably was not), is determined as

(R.A., Dec) = (+322.27 deg, +12.14 deg) = (21 29 6, +12 08 23)(J2000)

with a rectangular 90% statistical error box with the following corners:

(R.A., Dec) = (+322.31 deg, +11.95 deg) = (21 29 14, +11 57 11)(J2000)
(R.A., Dec) = (+322.06 deg, +12.10 deg) = (21 28 15, +12 06 9)(J2000)
(R.A., Dec) = (+322.24 deg, +12.32 deg) = (21 28 57, +12 19 23)(J2000)
(R.A., Dec) = (+322.48 deg, +12.17 deg) = (21 29 56, +12 10 24)(J2000)

There is additional systematic uncertainty of 0.2 degree (90% containment radius).

De Propris, R.: Separating the Conjoined Red Clump in the Galactic Bulge: Kinematics and Abundances

We have used the AAOMEGA spectrograph to obtain R ~ 1500 spectra of 714 stars that are members of two red clumps in the Plaut Window Galactic bulge field (l, b) = (0°, - 8°). We discern no difference between the clump populations based on radial velocities or abundances measured from the Mgb index. The velocity dispersion has a strong trend with Mgb-index metallicity, in the sense of a declining velocity dispersion at higher metallicity. We also find a strong trend in mean radial velocity with abundance. Our red clump sample shows distinctly different kinematics for stars with [Fe/H] <-1, which may plausibly be attributable to a minority classical bulge or inner halo population. The transition between the two groups is smooth. The chemo-dynamical properties of our sample are reminiscent of those of the Milky Way globular cluster system. If correct, this argues for no bulge/halo dichotomy and a relatively rapid star formation history. Large surveys of the composition and kinematics of the bulge clump and red giant branch are needed to further define these trends.

King, A.: The Brightest Cluster X-ray Sources

There have been several recent claims of black hole binaries in globular clusters. I show that these candidate systems could instead be ultracompact X-ray binaries (UCXBs) in which a neutron star accretes from a white dwarf. They would represent a slightly earlier evolutionary stage of known globular cluster UCXBs such as 4U 1820-30, with white dwarf masses ~0.2 M _sun and orbital periods below 5 minutes. Accretion is slightly super-Eddington and makes these systems ultraluminous sources with rather mild beaming factors b ~ 0.3. Their theoretical luminosity function flattens slightly just above L _Edd and then steepens at ~3L _Edd. It predicts of order two detections in elliptical galaxies such as NGC 4472, as observed. The very bright X-ray source HLX-1 lies off the plane of its host S0a galaxy. If this is an indication of globular cluster membership, it could conceivably be a more extreme example of a UCXB with white dwarf mass M ₂ ~= 0.34 M _sun. The beaming here is tighter (b ~ (2.5-9) × 10^-3), but the system's distance of 95 Mpc easily eliminates any need to invoke improbable alignment of the beam for detection. If its position instead indicates membership of a satellite dwarf galaxy, HLX-1 could have a much higher accretor mass ~1000 M _sun

Roederer, I. U.: Primordial r-process Dispersion in Metal-poor Globular Clusters

Heavy elements, those produced by neutron-capture reactions, have traditionally shown no star-to-star dispersion in all but a handful of metal-poor globular clusters (GCs). Recent detections of low [Pb/Eu] ratios or upper limits in several metal-poor GCs indicate that the heavy elements in these GCs were produced exclusively by an r-process. Re-examining GC heavy element abundances from the literature, we find unmistakable correlations between the [La/Fe] and [Eu/Fe] ratios in four metal-poor GCs (M5, M15, M92, and NGC 3201), only two of which were known previously. This indicates that the total r-process abundances vary from star to star (by factors of 2-6) relative to Fe within each GC. We also identify potential dispersion in two other GCs (M3 and M13). Several GCs (M12, M80, and NGC 6752) show no evidence of r-process dispersion. The r-process dispersion is not correlated with the well-known light element dispersion, indicating that it was present in the gas throughout the duration of star formation. The observations available at present suggest that star-to-star r-process dispersion within metal-poor GCs may be a common but not ubiquitous phenomenon that is neither predicted by nor accounted for in current models of GC formation and evolution.

Guillot, S.: Neutron Star Radius Measurement with the Quiescent Low-mass X-ray Binary U24 in NGC 6397

This paper reports the spectral and timing analyses of the quiescent low-mass X-ray binary (qLMXB) U24 observed during five archived Chandra/ACIS exposures of the nearby globular cluster NGC 6397, for a total of 350 ks. We find that the X-ray flux and the parameters of the hydrogen atmosphere spectral model are consistent with those previously published for this source. On short timescales, we find no evidence of aperiodic intensity variability, with 90% confidence upper limits during five observations ranging between <8.6% rms and <19% rms, in the 0.0001-0.1 Hz frequency range (0.5-8.0 keV); and no evidence of periodic variability, with maximum observed powers in this frequency range having a chance probability of occurrence from a Poisson-deviated light curve in excess of 10%. We also report the improved neutron star (NS) physical radius measurement, with statistical accuracy of the order of ~10%: R _NS = 8.9^+0.9 _-0.6 km for M _NS = 1.4 M _sun. Alternatively, we provide the confidence regions in mass-radius space as well as the best-fit projected radius R _∞ = 11.9^+1.0 _-0.8 km, as seen by an observer at infinity. The best-fit effective temperature, kT _eff = 80⁺⁴ _-5 eV, is used to estimate the NS core temperature which falls in the range T _core = (3.0-9.8) × 10⁷ K, depending on the atmosphere model considered. This makes U24 the third most precisely measured NS radius among qLMXBs, after those in ω Cen and M13.

Murphy, B. W.: Fokker-Planck Models for M15 Without a Central Black Hole: The Role of the Mass Function

We have developed a set of dynamically evolving Fokker-Planck models for the collapsed-core globular star cluster M15, which directly address the issue of whether a central black hole is required to fit Hubble Space Telescope (HST) observations of the stellar spatial distribution and kinematics. As in our previous work reported by Dull et al., we find that a central black hole is not needed. Using local mass-function data from HST studies, we have also inferred the global initial stellar mass function. As a consequence of extreme mass segregation, the local mass functions differ from the global mass function at every location. In addition to reproducing the observed mass functions, the models also provide good fits to the star-count and velocity-dispersion profiles, and to the millisecond pulsar accelerations. We address concerns about the large neutron star populations adopted in our previous Fokker-Planck models for M15. We find that good model fits can be obtained with as few as 1600 neutron stars; this corresponds to a retention fraction of 5% of the initial population for our best-fit initial mass function. The models contain a substantial population of massive white dwarfs, that range in mass up to 1.2M_sun . The combined contribution by the massive white dwarfs and neutron stars provides the gravitational potential needed to reproduce HST measurements of the central velocity-dispersion profile.

Stacey, W. S.: Transient Extremely Soft X-ray Emission from the Unusually Bright Cataclysmic Variable in the Globular Cluster M3: A New CV X-ray Luminosity Record?

We observed the accreting white dwarf (WD) 1E1339.8+2837 (1E1339) in the globular cluster M3 in 2003 November, 2004 May, and 2005 January, using the Chandra ACIS-S detector. The source was observed in 1992 to possess traits of a supersoft X-ray source (SSS), with a 0.1-2.4 keV luminosity as large as 2 × 10³⁵ erg s^-1, after which time the source's luminosity fell by roughly two orders of magnitude, adopting a hard X-ray spectrum more typical of cataclysmic variables (CVs). Our observations confirm 1E1339's hard CV-like spectrum, with photon index Γ = 1.3 ± 0.2. We found 1E1339 to be highly variable, with a 0.5-10 keV luminosity ranging from (1.4 ± 0.3) × 10³⁴ erg s^-1 to 8.5^+4.9 _{- 4.6} × 10³² erg s^-1, with 1E1339's maximum luminosity being perhaps the highest yet recorded for hard X-ray emission from a WD. In 2005 January, 1E1339 displayed substantial low-energy emission below ~0.3 keV. Although current Chandra responses cannot properly model this emission, its bolometric luminosity appears comparable to or greater than that of the hard spectral component. This raises the possibility that the supersoft X-ray emission seen from 1E1339 in 1992 may have shifted to the far-UV.

Alonso-García, J.: Mapping Differential Reddening in the Inner Galactic Globular Cluster System

A serious limitation in the study of many globular clusters—especially those located near the Galactic center—has been the existence of large and differential extinction by foreground dust. In a series of papers, we intend to map the differential extinction and remove its effects, using a new dereddening technique, in a sample of clusters in the direction of the inner Galaxy, observed using the Magellan 6.5 m telescope and the Hubble Space Telescope. These observations and their analysis will let us produce high-quality color-magnitude diagrams of these poorly studied clusters that will allow us to determine these clusters' relative ages, distances, and chemistry and to address important questions about the formation and the evolution of the inner Galaxy. We also intend to use the maps of the differential extinction to sample and characterize the interstellar medium along the numerous low-latitude lines of sight where the clusters in our sample lie. In this first paper, we describe in detail our dereddening method along with the powerful statistics tools that allow us to apply it, and we show the kind of results that we can expect, applying the method to M62, one of the clusters in our sample. The width of the main sequence and lower red giant branch narrows by a factor of two after applying our dereddening technique, which will significantly help to constrain the age, distance, and metallicity of the cluster. Based partly on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. This paper also includes data gathered with the 6.5 m Magellan Telescopes located at Las Campanas Observatory, Chile.

Lin, C.-C.: Search For And Characterization Of Galactic Star Clusters With 2MASS And Pan-STARRS 1

We plan to conduct a comprehensive search for uncatalogued galactic open clusters using Pan-STARRS (Panoramic Survey Telescope And Rapid Response System) data in five optical bands (g, r, i, z, y) to a depth of 24 magnitude, about 100 times more sensitive than currently available surveys. A ``star-count'' pipeline has been tested on the 2MASS point-source catalog. In a demonstration region with a sky coverage between galactic longitude 2 < l < 358 degrees and galactic latitude |b| < 50 degrees, we identified 502 cluster candidates. Of these, two are galaxies, 32 (6.3%) are cluster of galaxies, 91 (18.1%) are globular clusters, and 360 (71.7%) are open clusters. Seventeen (3.3%) of our candidates are newly identified. Overall, the detection rate of stellar clusters using our algorithm is about 90%. In addition, we have characterized some of the fundamental parameters of clusters, such as age and distance, using the Pan-STARRS 1 photometry, and the results are in good agreement with previous results.

Cohen, R.: The Connection Between SX Phe and Blue Stragglers: Globular Cluster Variables as Period-Luminosity Relation Calibrators

The number of known SX Phoenicis (SX Phe) pulsators in Galactic globular clusters (GGCs) and the Local Group has more than doubled in recent years. Because all SX Phe in GGCs are likely blue stragglers, it has been suggested that their pulsational properties can constrain their evolutionary histories. We investigate the connection between SX Phe and blue stragglers via period-luminosity and color-magnitude diagrams. In particular, we verify distances to a large set of GGCs by fitting a carefully chosen set of nearby, unevolved subdwarfs to the cluster main sequences. We use these distances to analyze the pulsational properties of all known SX Phe, including SX Phe period-luminosity relations obtained using the SX Phe in these GGCs as calibrators, together with those in Local Group dwarf galaxies and their clusters. Finally, we discuss implications for blue straggler formation and evolution, and address both observational and theoretical avenues for further investigation.

Majaess, D.: Securing the Distance Scale via a Universal VI Wesenheit Template and Deep Infrared ZAMS

HST, VLBA, and HIP geometric distances to SX Phe, Delta Scuti, RR Lyrae, Type II and classical Cepheid variables are used to construct a universal VI Wesenheit template. The template uses the statistical weight of the entire variable star demographic to establish precise (<5%) distances to nearby galaxies, star clusters, etc. The reddening-free nature of the Wesenheit approach obviates the propagation of uncertainties tied to tentative total/differential extinction corrections, ensuring that further calibration may ensue directly from published or forthcoming geometric-based distances (masers, HST, GAIA, SIM). An empirical JHKs ZAMS established from deep 2MASS photometry and precise HIP parallaxes for nearby stars provides a concurrent means of securing absolute Wesenheit magnitudes for variables in stellar clusters (calibrators). The infrared ZAMS is comparatively insensitive to stellar age and [Fe/H], and yields distances to 7 of 9 benchmark open clusters that agree with the van Leeuwen (2009) revised HIP estimates (the Pleiades and Blanco 1 are discrepant cases, but should not detract from the broader consensus). In sum, the distance scale is secured to a geometrically anchored framework that consists of results from several key surveys (OGLE, NOMAD, ASAS, etc.) united by a universal VI Wesenheit template and deep infrared ZAMS. Future research entails populating the universal Wesenheit template with lower-temperature calibrators (variable red giants, Miras, longer period Cepheids, etc.) using observations acquired from the ARO, SRO (AAVSO), OMM, and DAO, and further characterizing insidious photometric contamination associated with variables occupying crowded regions near the cores of globular clusters and galaxies (including the Milky Way).

Linares, M.: On The Cooling Tails Of Thermonuclear X-ray Bursts: News From Terzan 5

The neutron star transient and 11 Hz X-ray pulsar IGR J17480-2446, recently discovered in the globular cluster Terzan 5, showed unprecedented bursting activity during its 2010 October-November outburst. We analyzed all X-ray bursts detected with the Rossi X-ray Timing Explorer and find strong evidence that they all have a thermonuclear origin, despite the fact that many do not show the canonical spectral softening along the decay imprinted on type I X-ray bursts by the cooling of the neutron star photosphere. We show that the persistent-to-burst power ratio is fully consistent with the accretion-to-thermonuclear efficiency ratio along the whole outburst, as is typical for type I X-ray bursts. The burst energy, peak luminosity and daily-averaged spectral profiles all evolve smoothly throughout the outburst, in parallel with the persistent (non-burst) luminosity. We also find that the peak burst to persistent luminosity ratio determines whether or not cooling is present in the bursts from IGR J17480-2446, and argue that the apparent lack of cooling is due to the ``non-cooling'' bursts having both a lower peak temperature and a higher non-burst (persistent) emission. We conclude that the detection of cooling along the decay is a sufficient, but not a necessary condition to identify an X-ray burst as thermonuclear. Finally, we compare these findings with X-ray bursts from other rapidly accreting neutron stars.

Maccarone, T. J.: Compact Object Formation in Globular Clusters, the Milky Way and External Galaxies

I will review the key contributions of Chandra, and associated multi-wavelength facilities, to our understanding of formation and evolution of compact objects, with a focus on compact binaries. I will discuss the key results from both studies of Galactic and extragalactic globular cluster sources and field X-ray sources, showing how Chandra observations have both solved long-standing puzzles, and created some new, interesting problems to be solved.

Barmby, P.: Globular Cluster Colors Versus Population Synthesis Models

Although the stellar populations of globular clusters are not as simple as we used to believe, they are still the simplest populations available in the nearby universe. As such, they are extremely useful for testing stellar population synthesis models. Using recent mass estimates for Local Group globular clusters, we have compiled a sample of clusters with masses large enough that stochastic effects on integrated photometry should be minimal. We have measured integrated colors in the Spitzer/IRAC bands for as many of these as possible, paying careful attention to systematics in order to get the most accurate colors. We present a comparison of the results with the predictions of the latest generation of population synthesis models, including GALEV and FSPS. Support for this work was provided by a Discovery Grant and an Undergraduate Summer Research Award from NSERC and by an Ontario Early Researcher Award.

Yoon, S.-J.: Color Bimodality of Extragalactic Globular Clusters: A Test for the Nonlinear Color-Metallicity Relation Scenario via the u-band Colors

The colors of globular clusters (GCs) in most large elliptical galaxies are bimodal. Based on the observed linear relations between GC colors and their metallicities, the bimodality is taken as evidence of two GC subsystems with different metallicities in each galaxy and has led to a number of theories in the context of galaxy formation. More recent observations and modeling of GCs, however, suggest that colors likely trace metallicities in a subtly nonlinear manner. The nonlinearity could even transform a broad, unimodal metallicity spread into a strongly bimodal color distribution. Despite the far-reaching implications, whether color-metallicity relations (CMRs) are nonlinear and whether the nonlinearity indeed causes the color bimodality are still open questions. Given that the spectroscopic refinement of CMRs is still very challenging, we here propose a new photometric tool to probe the possible nonlinear nature of CMRs. In essence, a color distribution of GCs is a "projected” distribution of their metallicities based on a given CMR. Since the form of CMRs hinges on which color is used, the shape of color distributions varies depending significantly on colors in use. Among other optical colors, the u-band colors (e.g., u-g and u-z) are theoretically predicted to exhibit the most distinctive CMRs from other preferred CMRs (e.g., for g-z). As a case study, we performed the HST/WFPC2 archival u-band photometry for the M87 GC system with confirmed color bimodality. We show that any weak yet discernible feature on CMRs is amplified appreciably on the color domain by the projection effect, and hence, under the assumption of the nonlinear CMRs, the u-band color distributions are significantly different and readily distinguishable from those under the assumption of the conventional linear CMRs. With more data, this method will support or rule out the nonlinear-CMR scenario for GC color bimodality with high confidence.

Wei, L. H.: Bound Cluster Formation in the Antennae

Observations by the Hubble Space Telescope have revealed a wealth of super star clusters (SSCs) in lower extinction areas between dust lanes of starburst galaxies. These SSCs may be present-day analogues of young globular clusters, and are thought to form directly from giant molecular clouds. The mode of formation (efficient compression from shocks vs. slow compression within super giant molecular clouds), however, is still not well-understood. We report on high-spatial resolution CO(2-1) observations of the Antennae Galaxies with the Submillimeter Array (SMA) and the IRAM Plateau de Bure Interferometer (PdBI). We discuss the implications of our results on the various formation scenarios of SSCs in starburst galaxies.

Sanna, N.: The Wfpc2 Uv Survey Of Globular Clusters: The Case Of Ngc 6229

One of the valedictory projects undertaken by WFPC2 was a survey of UV bright objects in 30 globular clusters. Eventually these results will be combined with similar results obtained by our group for 15 clusters. For most of these clusters observations were obtained with 4 or more filters. For a subset of clusters we also have observations from GALEX which will allow us see if the sort radial variations previously found in blue straggler stars (BSS) also exists in the hottest stellar populations. Here we present the case of NGC 6229. The data set has been obtained by combining high-resolution (HST/WFPC2 and ACS) and wide-field space (GALEX) observations and ground-based (MegaCam-CFHT) images. The photometric sample covers the entire cluster extension from the very central regions up to the tidal radius and beyond. We determine the radial density profile and we study the BSS population and its radial distribution.

Barnard, R.: Chandra Watches Over A Decade Of Variability In M31 Globular Clusters

The central region of M31 has been monitored with Chandra > 120 times over the last 11 years. In this region we find X-ray sources corresponding to 35 out of 420 globular clusters; these are highly likely to be X-ray binaries. We have created long-term, calibrated lightcurves for all 35 sources, and will present highlights of our variability survey. We have detected significant variability in all the sources with 0.3-10 keV luminosity > 2x10E+36 erg/s. Since the emission spectra of background active galaxies often resemble those of X-ray binaries, the long term variability will be a valuable tool for identifying X-ray binaries in the remaining 400 sources in our field. This work is funded by Chandra grant GO9-0100X and HST grant GO-1101.

Shull, J. M.: Astro2010 Science in the Galactic Neighborhood (Redshift z < 0.1)

The GAN (Galactic Neighborhood) Science Frontier Panel of the Astro2010 decadal study provided 4 science questions and 2 discovery areas ripe for investment. Defining our "neighborhood" out to redshift z = 0.1 (420 Mpc), we focused on: (1) Flows of matter and energy in and out of galaxies (2) Cycles of mass, energy, and chemistry within galaxies and their ISM (3) Fossil record of galaxy assembly, from the first stars to the present (4) Connections between dark and luminous matter in galaxies and black holes. Our panel was excited about the promise of two Discovery Areas: (A) Time-Domain Astronomy, to explore the transient sky and stellar populations, including spectroscopic follow-up; and (B) Astrometry of objects ranging from extra-solar planets to halo stars, masers, globular clusters, galaxies, and quasars (maser disks, bound stars).

Brüns, R. C.: A parametric study on the formation of extended star clusters and ultra-compact dwarf galaxies

Context. In the last decade, very extended old stellar clusters with masses in the range from a few 10⁴ to 10⁸ M_ȯ have been found in various types of galaxies in different environments. Objects with masses comparable to normal globular clusters (GCs) are called extended clusters (ECs), while objects with masses in the dwarf galaxy regime are called ultra-compact dwarf galaxies (UCDs). In heavily interacting galaxies star clusters tend to form in larger conglomerations called star cluster complexes (CCs). The individual star clusters in a CC can merge and form a variety of spheroidal stellar objects.
Aims: The parametric study aims to analyze how the structural parameters of the final merger objects correlate with the underlying CC parameter space.
Methods: In this work we systematically scan a suitable parameter space for CCs and perform numerical simulations to study their further fate. The varied sizes and masses of the CCs cover a matrix of 5 × 6 values with CC Plummer radii between 10-160 pc and CC masses between 10^5.5-10⁸ M_ȯ, which are consistent with observed CC parameters. The CCs of the parametric study are on orbits with galactocentric distances between 20 kpc and 60 kpc. In addition, we studied also the evolution of CCs on a circular orbit at a galactocentric distance of 60 kpc to verify that also extremely extended ECs and UCDs can be explained by our formation scenario.
Results: All 54 simulations end up with stable merger objects, wherein 26 to 97% of the initial CC mass is bound. The objects show a general trend of increasing effective radii with increasing mass. Despite the large range of input Plummer radii of the CCs (10 to 160 pc) the effective radii of the merger objects are constrained to values between 10 and 20 pc at the low mass end and to values between 15 and 55 pc at the high mass end. The structural parameters of the models are comparable to those of the observed ECs and UCDs. The results of the circular orbits demonstrate that even very extended objects like the M 31 ECs found by Huxor in 2005 and the very extended (r_eff > 80 pc), high-mass UCDs can be explained by merged cluster complexes in regions with low gravitational fields at large galactocentric radii.
Conclusions: We conclude that the observed ECs and UCDs can be well explained as evolved star cluster complexes.

Lebzelter, T.: Long period variables and mass loss in the globular clusters NGC 362 and NGC 2808

Context. The pulsation periods of long period variables (LPVs) depend on their mass and helium abundance as well as on their luminosity and metal abundance. Comparison of the observed periods of LPVs in globular clusters with models is capable of revealing the amount of mass lost on the giant branch and the helium abundance.
Aims: We aim to determine the amount of mass loss that has occurred on the giant branches of the low metallicity globular clusters NGC 362 and NGC 2808. We also aim to see if the LPVs in NGC 2808 can tell us about helium abundance variations in this cluster.
Methods: We have used optical monitoring of NGC 362 and NGC 2808 to determine periods for the LPVs in these clusters. We have made linear pulsation models for the pulsating stars in these clusters taking into account variations in mass and helium abundance.
Results: Reliable periods have been determined for 11 LPVs in NGC 362 and 15 LPVs in NGC 2808. Comparison of the observed variables with models in the log P-K diagram shows that mass loss of ~0.15-0.2 M_ȯ is required on the first giant branch in these clusters, in agreement with estimates from other methods. In NGC 2808, there is evidence that a high helium abundance of Y ~ 0.4 is required to explain the periods of several of the LPVs.
Conclusions: It would be interesting to determine periods for LPVs in other Galactic globular clusters where a helium abundance variation is suspected to see if the completely independent test for a high helium abundance provided by the LPVs can confirm the high helium abundance estimates.

Church, R. P.: Coordinates and 2MASS and OGLE identifications for all stars in Arp's 1965 finding chart for Baade's Window

Aims: We seek to provide 2MASS and OGLE identifications and coordinates for all stars in the finding chart published by Arp (1965, ApJ, 141, 43). This chart covers the low extinction area around NGC 6522, also known as Baade's window, at coordinates (l,b) = (1.02, -3.92).
Methods: A cross correlation, using numerical techniques, was performed between a scan of the original finding chart from Arp and 2MASS and OGLE-II images and stellar coordinates.
Results: We provide coordinates for all stars in Arp's finding chart and 2MASS and OGLE identifications wherever possible. Two identifications in quadrant II do not appear in the original finding chart. Table 1 is only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/529/A104

Michaud, G.: Horizontal branch evolution, metallicity, and sdB stars

Context. Abundance anomalies have been observed in field sdB stars and in nearly all horizontal branch (HB) stars of globular clusters with T_eff > 11 000 K, whatever be the cluster metallicity.
Aims: We aim to determine the abundance variations that are expected in sdB stars and in HB stars of metallicities Z ≥ 10^-4 and investigate what the observed abundances teach us about hydrodynamical processes competing with atomic diffusion.
Methods: Complete stellar evolution models, including the effects of atomic diffusion and radiative acceleration, have been computed from the zero age main-sequence for metallicities of Z₀ = 0.0001, 0.001, 0.004 and 0.02. On the HB the masses were selected to cover the T_eff interval from 7000 to 37 000 K. Some 60 evolutionary HB models were calculated. The calculations of surface abundance anomalies during the horizontal branch depend on one parameter, the surface mixed mass.
Results: For sdB stars with T_eff < 37 000 K and for HB stars with T_eff > 11 000 K in all observed clusters, independent of metallicity, we found that most observed abundance anomalies (even up to ~ × 200) were compatible, within error bars, with expected abundances. A mixed mass of was determined by comparison with observations.
Conclusions: Observations of globular cluster HB stars with T_eff > 11 000 K and of sdB stars with T_eff < 37 000 K suggest that most observed abundance anomalies can be explained by element separation driven by radiative acceleration occuring at a mass fraction of . Mass loss or turbulence appear to limit the separation between and the surface. Appendices are only available in electronic form at http://www.aanda.org

Cenarro, A. J.: MgI and sTiO index definitions (Cenarro+, 2009)

Using the near-infrared spectral stellar library of Cenarro et al. (J/MNRAS/326/959), the behaviour of the MgI line at 8807Å and nearby TiO bands is analyzed in terms of the effective temperature, surface gravity and metallicity of the library stars. New spectroscopic indices for both spectral features - namely MgI and sTiO - are defined, and their sensitivities to different signal-to-noise ratios, spectral resolutions, flux calibrations and sky emission-line residuals are characterized. The two new indices exhibit interesting properties. In particular, MgI is a good indicator of the Mg abundance, whereas sTiO is a powerful dwarf-to-giant discriminator for cold spectral types. Empirical fitting polynomials that reproduce the strength of the new indices as a function of the stellar atmospheric parameters are computed, and a fortran routine with the fitting function predictions is made available. A thorough study of several error sources, non-solar [Mg/Fe] ratios and their influence on the fitting function residuals is also presented. From this analysis, an [Mg/Fe] underabundance of ~-0.04 is derived for the Galactic open cluster M67. (2 data files).

Lane, R. R.: Stellar parameters from 10 Galactic GC fields (Lane+, 2011)

Various stellar parameters were extracted, using two different reduction pipelines, from individual stellar spectra obtained with AAOmega on the Anglo-Australia Telescope. These spectra were obtained from ten fields centered on Galactic globular clusters, namely M4, M12, M22, M30, M53, M55, M68, NGC 288, NGC 6752 and 47 Tuc. Stellar parameters provided include the radial velocity, sum of the equivalent widths of the calcium triplet lines, equatorial coordinates, I magnitude, V magnitude, effective temperature, surface gravity, metallicity, rotational velocity, J magnitude, H magnitude, K magnitude, distance from the cluster centre, position angle with respect to the cluster centre and whether each star was determined to be a member of the cluster. (3 data files).

Kouvaris, C.: Constraining asymmetric dark matter through observations of compact stars

We put constraints on asymmetric dark matter candidates with spin-dependent interactions based on the simple existence of white dwarfs and neutron stars in globular clusters. For a wide range of the parameters (WIMP mass and WIMP-nucleon cross section), weakly interacting massive particles (WIMPs) can be trapped in progenitors in large numbers and once the original star collapses to a white dwarf or a neutron star, these WIMPs might self-gravitate and eventually collapse forming a mini-black hole that eventually destroys the star. We impose constraints competitive to direct dark matter search experiments, for WIMPs with masses down to the TeV scale.

Saha, A.: Calibration of B V R I Photometry for the Wide Field Channel of the HST Advanced Camera for Surveys

We present new observations of two Galactic globular clusters, PAL4 and PAL14, using the Wide Field Channel of the Advanced Camera for Surveys (ACS) on board the Hubble Space Telescope (HST) and reanalyze archival data from a third, NGC2419. We matched our photometry of hundreds of stars in these fields from the ACS images to existing ground-based photometry of faint sequences that were calibrated on the standard B V R I system of Landolt. These stars are significantly fainter than those generally used for HST calibration purposes and therefore are much better matched to supporting precision photometry of ACS science targets. We were able to derive more accurate photometric transformation coefficients for the commonly used ACS broadband filters, compared with those published by Sirianni et al., due to the use of a factor of several more calibration stars that span a greater range of color. We find that the inferred transformations from each cluster individually do not vary significantly from the average, except for a small offset of the photometric zero point in the F850LP filter. Our results suggest that the published prescriptions for the time-dependent correction of CCD charge transfer efficiency appear to work very well over the ˜ 3.5 yr interval that spans our observations of PAL4 and PAL14 and the archived images of NGC2419.

Chiappini, C.: Imprints of fast-rotating massive stars in the Galactic Bulge

The first stars that formed after the Big Bang were probably massive, and they provided the Universe with the first elements heavier than helium (`metals'), which were incorporated into low-mass stars that have survived to the present. Eight stars in the oldest globular cluster in the Galaxy, NGC6522, were found to have surface abundances consistent with the gas from which they formed being enriched by massive stars (that is, with higher α-element/Fe and Eu/Fe ratios than those of the Sun). However, the same stars have anomalously high abundances of Ba and La with respect to Fe, which usually arises through nucleosynthesis in low-mass stars (via the slow-neutron-capture process, or s-process). Recent theory suggests that metal-poor fast-rotating massive stars are able to boost the s-process yields by up to four orders of magnitude, which might provide a solution to this contradiction. Here we report a reanalysis of the earlier spectra, which reveals that Y and Sr are also overabundant with respect to Fe, showing a large scatter similar to that observed in extremely metal-poor stars, whereas C abundances are not enhanced. This pattern is best explained as originating in metal-poor fast-rotating massive stars, which might point to a common property of the first stellar generations and even of the `first stars'.

Nipoti, C.: Radial-orbit instability in modified Newtonian dynamics

The stability of radially anisotropic spherical stellar systems in modified Newtonian dynamics (MOND) is explored by means of numerical simulations performed with the N-body code N-MODY. We find that Osipkov-Merritt MOND models require for stability larger minimum anisotropy radii than equivalent Newtonian systems (ENSs) with the dark matter, and also than purely baryonic Newtonian models with the same density profile. The maximum value for stability of the Fridman-Polyachenko-Shukhman parameter in MOND models is lower than in ENSs, but higher than in Newtonian models with no dark matter. We conclude that MOND systems are substantially more prone to radial-orbit instability than ENSs with dark matter, while they are able to support a larger amount of kinetic energy stored in radial orbits than purely baryonic Newtonian systems. An explanation of these results is attempted and their relevance to the MOND interpretation of the observed kinematics of globular clusters, dwarf spheroidal and elliptical galaxies is briefly discussed.

Freire, P. C. C.: On the nature and evolution of the unique binary pulsar J1903+0327

PSR J1903+0327, a millisecond pulsar in an eccentric (e= 0.44) 95-d orbit with an ˜1 M_ȯ companion poses a challenge to our understanding of stellar evolution in binary and multiple-star systems. Here we describe optical and radio observations which rule out most of the scenarios proposed to explain formation of this system. Radio timing measurements of three post-Keplerian effects yield the most precise measurement of the mass of a millisecond pulsar to date: 1.667 ± 0.021 solar masses (99.7 per cent confidence limit). This rules out some equations of state for superdense matter; furthermore, it is consistent with the spin-up of the pulsar by mass accretion, as suggested by its short spin period and low magnetic field. Optical spectroscopy of a proposed main-sequence counterpart shows that its orbital motion mirrors the pulsar's 95-d orbit; being therefore its binary companion. This finding rules out a previously suggested scenario which proposes that the system is presently a hierarchical triple. Conventional binary evolution scenarios predict that, after recycling a neutron star into a millisecond pulsar, the binary companion should become a white dwarf and its orbit should be nearly circular. This suggests that if PSR J1903+0327 was recycled, its present companion was not responsible for it. The optical detection also provides a measurement of the systemic radial velocity of the binary; this and the proper motion measured from pulsar timing allow the determination of the systemic 3D velocity in the Galaxy. We find that the system is always within 270 pc of the plane of the Galaxy, but always more than 3 kpc away from the Galactic Centre. Thus an exchange interaction in a dense stellar environment (like a globular cluster or the Galactic Centre) is not likely to be the origin of this system. We suggest that after the supernova that formed it, the neutron star was in a tight orbit with a main-sequence star and the present companion was a tertiary farther out. The neutron star then accreted matter from its evolving inner companion, forming a millisecond pulsar. The inner companion then disappeared, either due to a chaotic three-body interaction with the outer star (caused by the expansion of the inner orbit that necessarily results from mass transfer), or in the case of a very compact inner system, due to ablation/accretion by the newly formed millisecond pulsar. We discuss in detail the possible evolution of such a system before the supernova.

Percival, S. M.: Modelling realistic horizontal branch morphologies and their impact on spectroscopic ages of unresolved stellar systems

The presence of an extended blue horizontal branch (HB) in a stellar population is known to affect the age inferred from spectral fitting to stellar population synthesis models. This is due to the hot blue component which increases the strength of the Balmer lines and can make an old population look spuriously young. However, most population synthesis models still rely on theoretical isochrones, which do not include realistic modelling of extended HBs. In this work, we create detailed models for a range of old simple stellar populations (SSPs), with metallicities ranging from [Fe/H]=-1.3 to solar, to create a variety of realistic HB morphologies, from extended red clumps, to extreme blue HBs. We achieve this by utilizing stellar tracks from the BaSTI data base and implementing a different mass-loss prescription for each SSP created. This includes setting an average mass and a Gaussian spread in masses of individual stars coming on to the zero-age HB for each model, and hence resulting in different HB morphologies. We find that, for each metallicity, there is some HB morphology which maximizes Hβ, making an underlying 14-Gyr population look ˜5-6 Gyr old for the low- and intermediate-metallicity cases, and as young as 2 Gyr in the case of the solar metallicity SSP. We explore whether there are any spectral indices capable of breaking the degeneracy between an old SSP with extended blue HB and a truly young or intermediate-age SSP, and find that the Ca II index of Rose and the strength of the Mg II doublet at 2800 Å are promising candidates, in combination with Hβ and other metallicity indicators, such as Mgb and Fe5406. We also run Monte Carlo simulations to investigate the level of statistical fluctuations in the spectra of typical stellar clusters. We find that fluctuations in spectral indices are significant even for average to large globular clusters and that various spectral indices are affected in different ways, which has implications for full-spectrum fitting methods. Hence, we urge caution if these types of stellar clusters are to be used as empirical calibrating objects for various aspects of stellar population synthesis models.

Bekki, K.: Secondary star formation within massive star clusters: origin of multiple stellar populations in globular clusters

We numerically investigate whether and how gaseous ejecta from AGB stars can be converted into new stars within originally massive star clusters (MSCs) in order to understand the origin of multiple stellar populations in globular clusters (GCs). We adopt a scenario in which (i) MSCs with masses of M_s can be formed from high-mass, high-density giant molecular clouds (GMCs) in their host galactic building blocks embedded in dark matter haloes at high redshifts, and (ii) their evolution therefore can be significantly influenced by M_s, their initial locations and physical properties of their hosts. Our 3D hydrodynamical simulations show that gaseous ejecta from AGB stars can be retained within MSCs and consequently converted into new stars very efficiently in the central regions of MSCs, only if M_s exceeds a threshold mass (M_th) of ≈10⁶ M_ȯ. The new stars can correspond to the 'second generation (SG)' of stars with higher Na and lower O abundances observed in GCs. Star formation efficiencies during the formation of SG stars within MSCs with M_s≥M_th can be rather high (0.3-0.9) so that very compact new clusters within original MSCs can be formed. M_s should be as large as 10⁶-10⁷ M_ȯ to explain the observed large fraction of SG stars in the present ordinary Galactic GCs, because new stars can consist of only 1-4 per cent among all stars for the standard initial mass function. Nuclear MSCs are found to retain much more effectively the AGB ejecta and convert more efficiently the gas into new stars, owing to the much deeper gravitational potential of their hosts. Capture and accretion of cold molecular gas (or small GMCs) by forming MSCs themselves can be mechanisms for mixing (i.e., dilution) of AGB ejecta with cold pristine gas. We suggest that both M_s and their locations within their hosts can determine whether abundance spread can be seen only in light elements or even in heavy ones. We discuss how and in what time-scale MSCs preferentially lose old stars owing to tidal stripping by their host galactic building blocks. We also suggest that the origin of the intermediate-age GCs with possible age spread of ˜100 Myr yet apparently no/little abundance spread in light elements in the LMC is closely associated with their incapability to retain the AGB ejecta owing to their low masses.

Thomas, D.: Chemical abundance ratios of galactic globular clusters from modelling integrated light spectroscopy

In a companion paper we present new, flux-calibrated stellar population models of Lick absorption-line indices with variable element abundance ratios. The model includes a large variety of individual element variations, which allows the derivation of the abundances for the elements C, N, O, Mg, Ca, Ti and Fe besides total metallicity and age. We use this model to obtain estimates of these quantities from integrated light spectroscopy of galactic globular clusters. We show that the model fits to a number of indices improve considerably when various variable element ratios are considered. The ages we derive agree well with the literature and are all consistent with the age of the Universe within the measurement errors. There is a considerable scatter in the ages, though, and we overestimate the ages preferentially for the metal-rich globular clusters. Our derived total metallicities agree generally very well with literature values on the Zinn & West scale once corrected for α enhancement, in particular for those cluster where the ages agree with the colour-magnitude diagram ages. We tend to slightly underestimate the metallicity for those clusters where we overestimate the age, in line with the age-metallicity degeneracy. It turns out that the derivation of individual element abundance ratios is not reliable at an iron abundance [Fe/H] < -1 dex where line strengths become weaker, while the [α/Fe] ratio is robust at all metallicities. The discussion of individual element ratios focuses therefore on globular clusters with [Fe/H] > -1 dex. We find general enhancement of light and α elements, as expected, with significant variations for some elements. The elements O and Mg follow the same general enhancement with almost identical distributions of [O/Fe] and [Mg/Fe]. We obtain slightly lower [C/Fe] and very high [N/Fe] ratios, instead. This chemical anomaly, commonly attributed to self-enrichment, is well known in globular clusters from individual stellar spectroscopy. It is the first time that this pattern is obtained also from the integrated light. The α elements follow a pattern such that the heavier elements Ca and Ti are less enhanced. More specifically, the [Ca/Fe] and [Ti/Fe] ratios are lower than [O/Fe] and [Mg/Fe] by about 0.2 dex. Most interestingly this trend of element abundance with atomic number is also seen in recent determinations of element abundances in globular cluster and field stars of the Milky Way. This suggests that Type Ia supernovae contribute significantly to the enrichment of the heavier α elements as predicted by nucleosynthesis calculations and galactic chemical evolution models.

Thomas, D.: Flux-calibrated stellar population models of Lick absorption-line indices with variable element abundance ratios

We present new stellar population models of Lick absorption-line indices with variable element abundance ratios. The models are based on our new calibrations of absorption-line indices with stellar parameters derived from the MILES stellar library. The key novelty compared to our previous models is that they are now available at the higher spectral resolution of MILES (2.5 Å full width at half-maximum) and flux calibrated, hence not tied anymore to the Lick/IDS system. This is essential for the interpretation of galaxy spectra where calibration stars are not available, such as large galaxy redshift surveys or other high-redshift observations. We note that the MILES resolution appears to be comparable to Sloan Digital Sky Survey (SDSS) resolution, so that our models can be applied to SDSS data without any corrections for instrumental spectral resolution. For the first time we provide random errors for the model predictions based on the uncertainties in the calibration functions and the underlying stellar parameter estimates. We show that random errors are small except at the edges of the parameter space (high/low metallicities and young ages ≲1 Gyr) where the stellar library is undersampled. We calibrate the base model for the parameters age, metallicity and α/Fe ratio with galactic globular cluster and galaxy gradient data. We discuss two model flavours with different input stellar evolutionary tracks from the Frascati and Padova groups. The new model release now includes abundance variations of the elements C, N, Mg, Na, Si, Ca, Ti, Cr and Fe. The individual elements that are best accessible with these models and the standard set of Lick absorption features are C, N, Mg, Ca, Ti and Fe. The model data are available at ˜thomasd.

Chilingarian, I. V.: Dynamical versus stellar masses of ultracompact dwarf galaxies in the Fornax cluster

The origin of ultracompact dwarf (UCD) galaxies, compact extragalactic stellar systems, is still a puzzle for present galaxy formation models. We present the comprehensive analysis of high-resolution multi-object spectroscopic data for a sample of 24 Fornax cluster UCDs obtained with VLT with Fibre Large Array Multi Element Spectrograph (FLAMES). It comprises previously published data for 19 objects which we re-analysed, including 13 with available Hubble Space Telescope (HST) photometric data. Using Virtual Observatory technologies we found archival HST images for two more UCDs and then determined their structural properties. For all objects we derived internal velocity dispersions, stellar population parameters and stellar mass-to-light ratios (M/L)_* by fitting individual simple stellar population (SSP) synthetic spectra convolved with a Gaussian against the observed spectra using the NBURSTS full spectral fitting technique. For 14 objects we estimated dynamical masses suggesting no dark matter (DM) in 12 of them and no more than 40 per cent DM mass fraction in the remaining two, in contrast to findings for several UCDs in the Virgo cluster. Some Fornax UCDs even have too high values of (M/L)_* estimated using the Kroupa stellar initial mass function (IMF) resulting in negative formally computed DM mass fractions. The objects with too high (M/L)_* ratios compared to the dynamical ones have relatively short dynamical relaxation time-scales, close to the Hubble time or below. We therefore suggest that their lower dynamical ratios (M/L)_dyn are caused by low-mass star depletion due to dynamical evolution. Overall, the observed UCD characteristics suggest at least two formation channels: tidal threshing of nucleated dwarf galaxies for massive UCDs (≃10⁸ M_ȯ), and a classical scenario of red globular cluster formation for lower-mass UCDs (≲10⁷ M_ȯ). Based on the archival ESO VLT with Fibre Large Array Multi Element Spectrograph (FLAMES) (programme 078.B-0496) available through and Hubble Space Telescope archival data (programmes 8090 and 10129) available through

Carraro, G.: An analysis of the blue straggler population in the Sgr dSph globular cluster Arp 2

We present and discuss new BVI CCD photometry in the field of the globular cluster Arp 2, which is considered a member of the Sagittarius dwarf spheroidal galaxy. The main goal of this investigation is to study the statistics and spatial distribution of blue straggler stars in the cluster. Blue stragglers are stars observed to be hotter and bluer than other stars with the same luminosity in their environments. As such, they appear to be much younger than the rest of the stellar population. Two main channels have been suggested to produce such stars: (1) collisions between stars in clusters; or (2) mass transfer between, or merger of, the components of primordial short-period binaries. The spatial distribution of these stars inside a star cluster, compared with the distribution of stars in different evolutionary stages, can cast light on the most efficient production mechanism at work. In the case of Arp 2, we found that blue straggler stars are significantly more concentrated than main-sequence stars, while they show the same degree of concentration as evolved stars (either red giants or horizontal branch stars). Since Arp 2 is not a very concentrated cluster, we suggest that this high central concentration is an indication that blue stragglers are mostly primordial binary stars. Based on observations carried out at ESO La Silla under programme 081.C-0087(A).

Weidner, C.: Top-heavy integrated galactic stellar initial mass functions in starbursts

Star formation rates (SFRs) larger than 1000 M_ȯ yr^-1 are observed in extreme starbursts. This leads to the formation of star clusters with masses >10⁶ M_ȯ in which crowding of the pre-stellar cores may lead to a change of the stellar initial mass function (IMF). Indeed, the large mass-to-light ratios of ultracompact dwarf galaxies and recent results on globular clusters suggest the IMF to become top-heavy with increasing star-forming density. We explore the implications of top-heavy IMFs in these very massive and compact systems for the integrated galactic initial mass function (IGIMF), which is the galaxy-wide IMF, in dependence of the SFR of galaxies. The resulting IGIMFs can have slopes, α₃, for stars more massive than about 1 M_ȯ between 1.5 and the Salpeter slope of 2.3 for an embedded cluster mass function (ECMF) slope (β) of 2.0, but only if the ECMF has no low-mass clusters in galaxies with major starbursts. Alternatively, β would have to decrease with increasing SFR > 10 M_ȯ yr^-1 such that galaxies with major starbursts have a top-heavy ECMF. The resulting IGIMFs are within the range of observationally deduced IMF variations with redshift.

Gnedin, O. Y.: Modeling Formation of Globular Clusters: Beacons of Galactic Star Formation

Modern hydrodynamic simulations of galaxy formation are able to predict accurately the rates and locations of the assembly of giant molecular clouds in early galaxies. These clouds could host star clusters with the masses and sizes of real globular clusters. I describe current state-of-the-art simulations aimed at understanding the origin of the cluster mass function and metallicity distribution. Metallicity bimodality of globular cluster systems appears to be a natural outcome of hierarchical formation and gradually declining fraction of cold gas in galaxies. Globular cluster formation was most prominent at redshifts z > 3, when massive star clusters may have contributed as much as 20% of all galactic star formation.