CiteULike: Group: Stellar Clusters - library

The Star Cluster System in the Local Group Starburst Galaxy IC 10

Sungsoon Lim — 2015-03-12T13:52:41-00:00

(11 Mar 2015)

We present a survey of star clusters in the halo of IC 10, a starburst galaxy in the Local Group based on Subaru R band images and NOAO Local Group Survey UBVRI images. We find five new star clusters. All these star clusters are located far from the center of IC 10, while previously known star clusters are mostly in the main body. Interestingly the distribution of these star clusters shows an asymmetrical structure elongated along the east and south-west direction. We derive UBVRI photometry of 66 star clusters including these new star clusters as well as previously known star clusters. Ages of the star clusters are estimated from the comparison of their UBVRI spectral energy distribution with the simple stellar population models. We find that the star clusters in the halo are all older than 1 Gyr, while those in the main body have various ages from very young (several Myr) to old (>1 Gyr). The young clusters (<10 Myr) are mostly located in the Hα emission regions and are concentrated on a small region at 2' in the south-east direction from the galaxy center, while the old clusters are distributed in a wider area than the disk. Intermediate-age clusters (~100 Myr) are found in two groups. One is close to the location of the young clusters and the other is at ~4' from the location of the young clusters. The latter may be related with past merger or tidal interaction.
Sungsoon Lim, Myung Lee

Detailed Abundances of 15 Stars in the Metal-Poor Globular Cluster NGC 4833

Ian Roederer — 2015-03-12T13:52:20-00:00

(10 Mar 2015)

We have observed 15 red giant stars in the relatively massive, metal-poor globular cluster NGC 4833 using the MIKE spectrograph at Magellan. We calculate stellar parameters for each star and perform a standard abundance analysis to derive abundances of 43 species of 39 elements, including 20 elements heavier than the iron group. We derive <[Fe/H]> = -2.25 +/- 0.02 from Fe I lines and <[Fe/H> = -2.19 +/- 0.013 from Fe II lines. We confirm earlier results that found no internal metallicity spread in NGC 4833, and there are no significant star-to-star abundance dispersions among any elements in the iron group (19 <= Z <= 30). We recover the usual abundance variations among the light elements C, N, O, Na, Mg, Al, and possibly Si. The heavy-element distribution reflects enrichment by r-process nucleosynthesis ([Eu/Fe] = +0.36 +/- 0.03), as found in many other metal-poor globular clusters. We investigate small star-to-star variations found among the neutron-capture elements, and we conclude that these are probably not real variations. Upper limits on the Th abundance, log epsilon (Th/Eu) < -0.47 +/- 0.09, indicate that NGC 4833, like other globular clusters where Th has been studied, did not experience a so-called "actinide boost."
Ian Roederer, Ian Thompson

The normal chemistry of multiple stellar populations in the dense globular cluster NGC 6093 (M 80)

E Carretta — 2015-03-12T13:51:51-00:00

(10 Mar 2015)

We present the abundance analysis of 82 red giant branch stars in the dense, metal-poor globular cluster NGC 6093 (M 80), the largest sample of stars analyzed in this way for this cluster. From high resolution UVES spectra of 14 stars and intermediate resolution GIRAFFE spectra for the other stars we derived abundances of O, Na, Mg, Al, Si, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Zr, Ba, La, Ce, Pr, Nd, Sm, Eu. On our UVES metallicity scale the mean metal abundance of M 80 is [Fe/H]=-1.791+/-0.006+/-0.076 (+/-statistical +/-systematic error) with rms=0.023 (14 stars). M 80 shows star to star variations in proton-capture elements, and the extension of the Na-O anticorrelation perfectly fit the relations with (i) total cluster mass, (ii) horizontal branch morphology, and (iii) cluster concentration previously found by our group. The chemistry of multiple stellar populations in M 80 does not look extreme. The cluster is also a typical representative of halo globular clusters for what concerns the pattern of alpha-capture and Fe-group elements. However we found that a significant contribution from the s-process is required to account for the distribution of neutron-capture elements. A minority of stars in M 80 seem to exhibit slightly enhanced abundances of s-process species, compatible with those observed in M 22 and NGC 1851, although further confirmation from larger samples is required.
E Carretta, A Bragaglia, RG Gratton, V D'Orazi, S Lucatello, A Sollima, Y Momany, G Catanzaro, F Leone

A Census of Variability in Globular Cluster M68 (NGC 4590)

N Kains — 2015-02-27T10:01:47-00:00

(25 Feb 2015)

We analyse 20 nights of CCD observations in the V and I bands of the globular cluster M68 (NGC 4590), using these to detect variable objects. We also obtained electron-multiplying CCD (EMCCD) observations for this cluster in order to explore its core with unprecedented spatial resolution from the ground. We reduced our data using difference image analysis, in order to achieve the best possible photometry in the crowded field of the cluster. In doing so, we showed that when dealing with identical networked telescopes, a reference image from any telescope may be used to reduce data from any other telescope, which facilitates the analysis significantly. We then used our light curves to estimate the properties of the RR Lyrae (RRL) stars in M68 through Fourier decomposition and empirical relations. The variable star properties then allowed us to derive the cluster's metallicity and distance. We determine new periods for the variable stars, and search for new variables, especially in the core of the cluster where our method performs particularly well. We detect an additional 4 SX Phe stars, and confirm the variability of another star, bringing the total number of confirmed variable stars in this cluster to 50. We also used archival data stretching back to 1951 in order to derive period changes for some of the single-mode RRL stars, and analyse the significant number of double-mode RRL stars in M68. Furthermore, we find evidence for double-mode pulsation in one of the SX Phe stars in this cluster. Using the different types of variables, we derived an estimate of the metallicity, [Fe/H]=$-2.07 ± 0.06$ on the ZW scale, and 4 independent estimates of the distance modulus ($μ_0 ∼$ 15.00 mag) for this cluster. Thanks to the first use of difference image analysis on time-series observations of M68, we are now confident that we have a complete census of the RRL stars in this cluster.
N Kains, Arellano Ferro, Figuera Jaimes, DM Bramich, J Skottfelt, UG Jørgensen, Y Tsapras, RA Street, P Browne, M Dominik, K Horne, M Hundertmark, S Ipatov, C Snodgrass, IA Steele, KA Alsubai, V Bozza, Calchi Novati, S Ciceri, G D'Ago, P Galianni, SH Gu, K Harpsøe, TC Hinse, D Juncher, H Korhonen, L Mancini, A Popovas, M Rabus, S Rahvar, J Southworth, J Surdej, C Vilela, XB Wang, O Wertz

MOCCA Code for Star Cluster Simulation: Comparison with Optical Observations using COCOA

Abbas Askar — 2015-01-07T08:35:27-00:00

(29 Dec 2014)

We introduce and present preliminary results from COCOA (Cluster simulatiOn Comparison with ObservAtions) code for a star cluster after 12 Gyrs of evolution simulated using the MOCCA code. The COCOA code is being developed to quickly compare results of numerical simulations of star clusters with observational data. We use COCOA to obtain parameters of the projected cluster model. For comparison, a FITS file of the projected cluster was provided to observers so that they could use their observational methods and techniques to obtain cluster parameters. The results show that the similarity of cluster parameters obtained through numerical simulations and observations depends significantly on the quality of observational data and photometric accuracy.
Abbas Askar, Mirek Giersz, Wojciech Pych, Arkadiusz Olech, Arkadiusz Hypki

Which young clusters/associations are we missing today?

Susanne Pfalzner — 2015-01-07T08:33:54-00:00

(5 Jan 2015)

Currently clusters/associations of stars are mainly detected as surface density enhancements relative to the background field. While clusters form, their surface density increases. It likely decreases again at the end of the star formation process when the system expands as a consequence of gas expulsion. Therefore the surface density of a single cluster can change considerably in young clusters/associations during the first 20 Myr of their development. We investigate the effect of the gas expulsion on the detectability of clusters/associations typical for the solar neighborhood, where the star formation efficiency is <35%. The main focus will be laid on the dependence on the initial cluster mass. Nbody methods are used to determine the cluster/association dynamics after gas expulsion. We find that, even for low background densities, only clusters/associations with initial central surface densities exceeding a few 5000 M(sun)/pc2 will be detected as clusters at ages ~5 Myr. Even the Orion Nebula cluster, one of the most massive nearby clusters, would only be categorized as a small co-moving group with current methods after 5 Myr of development. This means that cluster expansion leads to a selection effect - at ages of <1-2 Myr the full range of clusters/associations is observed whereas at ages > 4 Myr only the most massive clusters are identified, while systems with initially M_c < 3 000 M(sun) are missing. The temporal development of stellar properties is usually determined by observing clusters of different ages. The potentially strong inhomogeneity of the cluster sample makes this methods highly questionable. However, GAIA could provide the means to rectify this situation as it will be able to detect lower mass clusters.
Susanne Pfalzner, Kirsten Vincke, Mai Xiang

The emerging state of open clusters upon their violent relaxation

Mario Spera — 2015-01-07T08:31:03-00:00

(5 Jan 2015)

The state after virialization of a small-to-intermediate N-body system depends on its initial conditions; in particular, systems that are, initially, dynamically "cool" (virial ratios Q=2T/|Omega| below ~ 0.3) relax violently in few crossing times. This leads to a metastable system (virial ratio ~ 1) which carries a clear signature of mass segregation much before the gentle 2-body relaxation time scale. This result is obtained by a set of high precision N-body simulations of isolated clusters composed of stars of two different masses (in the ratio m_h/m_l=2), and is confirmed also in presence of a massive central object (simulating a black hole of stellar size). We point out that this (quick) mass segregation occurs in two phases: the first one shows up in clumps originated by sub-fragmentation before the deep overall collapse; this segregation is erased during the deep collapse to re-emerge, abruptly, during the second phase that occurs after the first bounce of the system. This way to segregate masses, actual result of a violent relaxation, is an interesting feature also on the astronomical-observational side. In those stellar systems that start their dynamical evolution from cool conditions, this kind of mass segregation adds to the sequent, slow, secular segregation as induced by 2- and 3- body encounters.
Mario Spera, Roberto Capuzzo-Dolcetta

Observing multiple stellar populations with FORS2@VLT - Main sequence photometry in outer regions of NGC 6752, NGC 6397, and NGC 6121 (M 4)

D Nardiello — 2014-10-29T08:11:31-00:00

(28 Oct 2014)

We present the photometric analysis of the external regions of three Galactic Globular Clusters: NGC 6121, NGC 6397 and NGC 6752. The main goal is the characterization of the multiple stellar populations along the main sequence (MS) and the study of the radial trend of the different populations hosted by the target clusters. The data have been collected using FORS2 mounted at the ESO/VLT@UT1 telescope in UBVI filters. From these data sets we extracted high-accuracy photometry and constructed color-magnitude diagrams. We exploit appropriate combination of colors and magnitudes which are powerful tools to identify multiple stellar populations, like B versus U-B and V versus c_U,B,I=(U-B)-(B-I) CMDs. We confirm previous findings of a split MS in NGC 6752 and NGC 6121. Apart from the extreme case of omega Centauri, this is the first detection of multiple MS from ground-based photometry. For NGC 6752 and NGC 6121 we compare the number ratio of the blue MS to the red MS in the cluster outskirts with the fraction of first and second generation stars measured in the central regions. There is no evidence for significant radial trend. The MS of NGC 6397 is consistent with a simple stellar population. We propose that the lack of multiple sequences is due both to observational errors and to the limited sensitivity of U,B,V,I photometry to multiple stellar populations in metal-poor GCs. Finally, we compute the helium abundance for the stellar populations hosted by NGC 6121 and NGC 6752, finding a mild (Delta Y ~ 0.02) difference between stars in the two sequences.
D Nardiello, AP Milone, G Piotto, AF Marino, A Bellini, S Cassisi

New Galactic embedded clusters and candidates from a WISE Survey

Denilso Camargo — 2014-06-26T09:28:25-00:00

New Astronomy (24 Jun 2014), doi:10.1016/j.newast.2014.05.007

We carried out a search for new infrared star clusters, stellar groups and candidates using WISE images, which are very sensitive to dust emission nebulae. We report the discovery of 437 embedded clusters and stellar groups that show a variety of structures, both in the stellar and nebular components. Pairs or small groupings of clusters are observed, suggesting multiple generations at the early formation stages. The resulting catalogue provides Galactic and equatorial coordinates, together with angular sizes for all objects. The nature of a representative test sub-sample of 14 clusters is investigated in detail by means of 2MASS photometry. The colour magnitude diagrams and radial density distributions characterize them as stellar clusters. The 437 new objects were found in the ranges $145^\circ\,≤\,\ell\,≤ 290^\circ$ and $-25^\circ\,≤\,b\,≤ 20^\circ$, and they appear to be a major object source for future studies of star cluster formation and their early evolution. WISE is a powerful tool to further probe for very young clusters throughout the disk.
Denilso Camargo, Eduardo Bica, Charles Bonatto

Global survey of star clusters in the Milky Way III. 139 new open clusters at high Galactic latitudes

S Schmeja — 2014-06-25T08:20:32-00:00

(24 Jun 2014)

An earlier analysis of the Milky Way Star Cluster (MWSC) catalogue revealed an apparent lack of old (> 1 Gyr) open clusters in the solar neighbourhood (< 1 kpc). To fill this gap we undertook a search for hitherto unknown star clusters, assuming that the missing old clusters reside at high Galactic latitudes |b|> 20\deg. We were looking for stellar density enhancements using a star count algorithm on the 2MASS point source catalogue. To increase the contrast between potential clusters and the field, we applied filters in colour-magnitude space according to typical colour-magnitude diagrams of nearby old open clusters. The subsequent comparison with lists of known objects allowed us to select thus far unknown cluster candidates. For verification they were processed with the standard pipeline used within the MWSC survey for computing cluster membership probabilities and for determining structural, kinematic, and astrophysical parameters. In total we discovered 782 density enhancements, 522 of which were classified as real objects. Among them 139 are new open clusters with ages 8.3 < log (t [yr]) < 9.7, distances d < 3 kpc, and distances from the Galactic plane 0.3 < Z < 1 kpc. This new sample has increased the total number of known high latitude open clusters by about 150%. Nevertheless, we still observe a lack of older nearby clusters up to 1 kpc from the Sun. This volume is expected to still contain about 60 unknown clusters that probably escaped our detection algorithm, which fails to detect sparse overdensities with large angular size.
S Schmeja, NV Kharchenko, AE Piskunov, S Röser, E Schilbach, D Froebrich, RD Scholz

Spectroscopy of Red Giants in the globular cluster Terzan 8: kinematics and evidence for the surrounding Sagittarius stream

A Sollima — 2014-06-25T08:17:53-00:00

(24 Jun 2014)

We present the results of a spectroscopic survey of Red Giants in the globular cluster Terzan 8 with the aim of studying its kinematics. We derived accurate radial velocities for 82 stars located in the innermost 7 arcmin from the cluster center identifying 48 bona fide cluster members. The kinematics of the cluster have been compared with a set of dynamical models accounting for the effect of mass segregation and a variable fraction of binaries. The derived velocity dispersion appears to be larger than that predicted for mass-segregated stellar systems without binaries, indicating that either the cluster is dynamically young or it contains a large fraction of binaries (>30%). We detected 7 stars with a radial velocity compatible with the cluster systemic velocity but with chemical patterns which stray from those of both the cluster and the Galactic field. These stars are likely members of the Sagittarius stream surrounding this stellar system.
A Sollima, E Carretta, V D'Orazi, RG Gratton, A Bragaglia, S Lucatello

The AIMSS Project I: Bridging the Star Cluster - Galaxy Divide

Mark Norris — 2014-06-25T08:15:24-00:00

(23 Jun 2014)

We describe the structural and kinematic properties of the first compact stellar systems discovered by the AIMSS project. These spectroscopically confirmed objects have sizes ($∼$6 $<$ R$_ e$ [pc] $<$ 500) and masses ($∼$2$×$10$^6$ $<$ M$_*$/M$_odot$ $<$ 6$×$10$^9$) spanning the range of massive globular clusters (GCs), ultra compact dwarfs (UCDs) and compact elliptical galaxies (cEs), completely filling the gap between star clusters and galaxies. Several objects are close analogues to the prototypical cE, M32. These objects, which are more massive than previously discovered UCDs of the same size, further call into question the existence of a tight mass-size trend for compact stellar systems, while simultaneously strengthening the case for a universal "zone of avoidance" for dynamically hot stellar systems in the mass-size plane. Overall, we argue that there are two classes of compact stellar systems: 1) massive star clusters and 2) a population closely related to galaxies. Our data provide indications for a further division of the galaxy-type UCD/cE population into two groups, one population that we associate with objects formed by the stripping of nucleated dwarf galaxies, and a second population that formed through the stripping of bulged galaxies or are lower-mass analogues of classical ellipticals. We find compact stellar systems around galaxies in low to high density environments, demonstrating that the physical processes responsible for forming them do not only operate in the densest clusters.
Mark Norris, Sheila Kannappan, Duncan Forbes, Aaron Romanowsky, Jean Brodie, Favio Faifer, Avon Huxor, Claudia Maraston, Amanda Moffett, Samantha Penny, Vincenzo Pota, Analia Smith-Castelli, Jay Strader, David Bradley, Kathleen Eckert, Dora Fohring, JoEllen McBride David, Ovidiu Vaduvescu

Clues on the Missing Sources of Reionization from Self-consistent Modeling of Milky Way and Dwarf Galaxy Globular Clusters

Harley Katz — 2014-06-25T08:14:13-00:00

(23 Jun 2014)

Globular clusters are unique tracers of ancient star formation. We determine the formation efficiencies of globular clusters across cosmic time by modeling the formation and dynamical evolution of the globular cluster population of a Milky Way type galaxy in hierarchical cosmology, using the merger tree from the Via Lactea II simulation. All of the models are constrained to reproduce the observed specific frequency and initial mass function of globular clusters in isolated dwarfs. Globular cluster orbits are then computed in a time varying gravitational potential after they are either accreted from a satellite halo or formed in situ, within the Milky Way halo. We find that the Galactocentric distances and metallicity distribution of globular clusters are very sensitive to the formation efficiencies of globular clusters as a function of redshift and halo mass. Our most accurate models reveal two distinct peaks in the globular cluster formation efficiency at z~2 and z~7-12 and prefer a formation efficiency that is mildly increasing with decreasing halo mass, the opposite of what expected for feedback-regulated star formation. This model accurately reproduces the positions, velocities, mass function, metallicity distribution, and age distribution of globular clusters in the Milky Way and predicts that ~ 40% formed in situ, within the Milky Way halo, while the other ~ 60% were accreted from about 20 satellite dwarf galaxies with Vc > 30 km/s, and about 29% or all globular clusters formed at redshifts z > 7. These results further strengthen the notion that globular cluster formation was an important mode of star formation in high-redshift galaxies and likely played a significant role in the reionization of the intergalactic medium
Harley Katz, Massimo Ricotti

An improved bound on axion-photon coupling from Globular Clusters

Adrian Ayala — 2014-06-25T08:13:37-00:00

(23 Jun 2014)

We derive a strong bound on the axion-photon coupling g_aγ from the analysis of a sample of 39 Galactic Globular Clusters. As recognized long ago, the R parameter, i.e. the number ratio of stars in horizontal over red giant branch of sufficiently old stellar clusters, would be reduced by the axion production from photon conversions occurring in stellar cores. In this regard we have compared the measured R with state-of-the-art stellar models obtained under different assumptions for g_aγ. We show that the estimated value of g_aγ substantially depends on the adopted He mass fraction Y, an effect often neglected in previous investigations. Taking as benchmark for our study the most recent determination of the primordial He abundance, we obtain an upper bound g_aγ<0.66 X 10^-10 GeV^-1 at 95 % confidence level. This result significantly improves the constraints from the previous analyses and is currently the strongest limit on the axion-photon coupling in a wide mass range.
Adrian Ayala, Inma Dominguez, Maurizio Giannotti, Alessandro Mirizzi, Oscar Straniero

Age Determination of Fifteen Old to Intermediate-Age Small Magellanic Cloud Star Clusters

MC Parisi — 2014-02-10T08:21:03-00:00

(7 Feb 2014)

We present CMDs in the V and I bands for fifteen star clusters in the Small Magellanic Cloud (SMC) based on data taken with the Very Large Telescope (VLT, Chile). We selected these clusters from our previous work, wherein we derived cluster radial velocities and metallicities from Calcium II infrared triplet (CaT) spectra also taken with the VLT. We discovered that the ages of six of our clusters have been appreciably underestimated by previous studies, which used comparatively small telescopes, graphically illustrating the need for large apertures to obtain reliable ages of old and intermediate-age SMC star clusters. In particular, three of these clusters, L4, L6 and L110, turn out to be amongst the oldest SMC clusters known, with ages of 7.9 +- 1.1, 8.7 +- 1.2 and 7.6 +- 1.0 Gyr, respectively, helping to fill a possible "SMC cluster age gap" (Glatt et al. 2008). Using the present ages and metallicities from Parisi et al. (2009), we analyze the age distribution, age gradient and age metallicity relation (AMR) of a sample of SMC clusters measured homogeneously. There is a suggestion of bimodality in the age distribution but it does not show a constant slope for the first 4 Gyr (Piatti 2011), and we find no evidence for an age gradient. Due to the improved ages of our cluster sample, we find that our AMR is now better represented in the intermediate/old period than that we derived in Parisi et al. (2009), where we simply took ages available in the literature. Additionally, clusters younger than aprox. 4 Gyr now show better agreement with the bursting model, but we confirm that this model is not a good representation of the AMR during the intermediate-age/old period. A more complicated model is needed to explain the SMC chemical evolution in that period.
MC Parisi, D Geisler, G Carraro, JJ Clariá, E Costa, AJ Grocholski, A Sarajedini, R Leiton, AE Piatti

Terzan 5: a pristine fragment of the Galactic Bulge?

Davide Massari — 2014-01-20T17:29:47-00:00

(16 Jan 2014)

Terzan 5 is a stellar system located in the inner Bulge of the Galaxy and has been historically catalogued as a globular cluster. However, recent photometric (Ferraro et al. 2009) and spectroscopic (Origlia et al. 2011; Origlia et al. 2013) investigations have shown that it hosts at least three stellar populations with different iron abundances (with a total spread of Delta[Fe/H]>1 dex) thus demonstrating that Terzan 5 is not a genuine globular cluster. In addition, the striking similarity between the chemical patterns of this system and those of its surrounding environment, the Galactic Bulge, from the point of view of both the metallicity distribution and the alpha-element enrichment, suggests that Terzan 5 could be a pristine fragment of the Bulge itself.
Davide Massari, FR Ferraro, A Mucciarelli, L Origlia, E Dalessandro, B Lanzoni

Observing Open Clusters with a Sequence of Ages with Kepler

Joyce Guzik — 2013-10-03T12:41:37-00:00

(2 Oct 2013)

We propose to observe with Kepler an age sequence of nearby uncrowded open clusters. The current Kepler field contains very few (only 4) somewhat distant and/or old clusters. Nearby open clusters are already well characterized from ground- and many space-based observations. Our proposal focuses on mid and upper main-sequence variables (main sequence and pre-main sequence gamma Dor or delta Scuti stars, SPB/beta Cep stars, Cepheids, or yellow supergiants), having periods of hours to days accessible by longer cadence observations. Asteroseismology for these objects to date is limited by the number of modes observable from the ground, difficulty in obtaining spectroscopic or photometric mode ID for the fainter targets that have been observed by Kepler, uncertainties in interior and initial abundances, especially for stars with abundance anomalies, uncertainties in distance/luminosity, and lack of knowledge of prior evolution history. The additional constraints of common age, distance and initial abundances in clusters will place these variables in their evolutionary context, and help unlock some of the science and reduce uncertainties for understanding star formation and stellar evolution.
Joyce Guzik, Paul Bradley, Robert Szabo, Laszlo Molnar, Andrzej Pigulski, Konstanze Zwintz, Chow-Choong Ngeow, Jesper Schou, Gerald Handler

Long Period Variable Stars in the Globular Cluster M5

Michael Wehrung — 2013-10-03T12:41:04-00:00

(2 Oct 2013)

We report results of VI time series photometry for the bright globular cluster M5 taken between 2007-2011 using the 0.5-m telescope at Bowling Green State Univ. and the PROMPT #4 telescope at Cerro Tololo. We used DAOPHOT to obtain photometry to a limiting magnitude of V~19 mag. A search for variable stars using the DAOMASTER variability index enabled us to recover the three known bright variables (two Cepheids and one red, long period variable, or LPV) and many known RR Lyrae stars, and to discover as many as thirteen new, low-amplitude LPVs. We present light curves of several stars and analyze periods and amplitudes where applicable.
Michael Wehrung, Andrew Layden

Lithium abundance evolution in open clusters: Hyades, NGC752, and M67

Matthieu Castro — 2013-10-01T08:14:16-00:00

(30 Sep 2013)

Mixing mechanisms bring the Li from the base of the convective zone to deeper and warmer layers where it is destroyed. These mechanisms are investigated by comparing observations of Li abundances in stellar atmospheres to models of stellar evolution. Observations in open cluster are especially suitable for this comparison, since their age and metallicity are homogeneous among their members and better determined than in field stars. In this work, we compare the evolution of Li abundances in three different clusters: the Hyades, NGC752, and M67. Our models are calculated with microscopic diffusion and transport of chemicals by meridional circulation, and calibrated on the Sun. These comparisons allow us to follow the evolution of Li abundance as a function of stellar mass in each cluster and as a function of the age by comparing this evolution in each cluster. We evaluate the efficiency of the mixing mechanisms used in the models, and we try to identify the lacking mechanisms to reproduce the observed evolution of Li abundance.
Matthieu Castro, Giancarlo Pace, José Dias do Nascimento

Evolution of star clusters in a cosmological tidal field

Steven Rieder — 2013-10-01T08:13:56-00:00

(27 Sep 2013)

We present a method to couple N-body star cluster simulations to a cosmological tidal field, using the Astrophysical Multipurpose Software Environment. We apply this method to star clusters embedded in the CosmoGrid dark matter-only LambdaCDM simulation. Our star clusters are born at z = 10 (corresponding to an age of the Universe of about 500 Myr) by selecting a dark matter particle and initializing a star cluster with 32,000 stars on its location. We then follow the dynamical evolution of the star cluster within the cosmological environment. We compare the evolution of star clusters in two Milky-Way size haloes with a different accretion history. The mass loss of the star clusters is continuous irrespective of the tidal history of the host halo, but major merger events tend to increase the rate of mass loss. From the selected two dark matter haloes, the halo that experienced the larger number of mergers tends to drive a smaller mass loss rate from the embedded star clusters, even though the final masses of both haloes are similar. We identify two families of star clusters: native clusters, which become part of the main halo before its final major merger event, and the immigrant clusters, which are accreted upon or after this event; native clusters tend to evaporate more quickly than immigrant clusters. Accounting for the evolution of the dark matter halo causes immigrant star clusters to retain more mass than when the z=0 tidal field is taken as a static potential. The reason for this is the weaker tidal field experienced by immigrant star clusters before merging with the larger dark matter halo.
Steven Rieder, Tomoaki Ishiyama, Paul Langelaan, Junichiro Makino, Stephen McMillan, Simon Zwart

The Environment around the Young Massive Star Cluster RSGC 1 and HESS J1837-069

Yutaka Fujita — 2013-09-30T07:58:10-00:00

(26 Sep 2013)

We report on Mopra observations toward the young massive star cluster RSGC 1, adjoined by, and possibly associated with the gamma-ray source HESS J1837-069. We measure the CO (J=1-0) distribution around the cluster and gamma-ray source, and find that the cluster is slightly higher than the velocity ranges associated with the Crux-Scutum arm. We reveal the cluster is associated with much less molecular gas compared with other young massive clusters in the Galaxy, Westerlund 1 (Wd 1) and 2 (Wd 2), which also radiate gamma-rays. We find no other structures that would otherwise indicate the action of supernova remnants, and due to the lack of material which may form gamma-rays by hadronic interaction, we conclude that the gamma-rays detected from HESS J1837-069 are not created through proton-proton interactions, and may more plausibly originate from the pulsar that was recently found near RSGC 1.
Yutaka Fujita, Hiroyuki Nakanishi, Erik Muller, Naoto Kobayashi, Masao Saito, Chikako Yasui, Hiroki Kikuchi, Keigo Yoshinaga

The state of globular clusters at birth: emergence from the gas-embedded phase

Nathan Leigh — 2013-09-30T07:57:46-00:00

(26 Sep 2013)

In this paper, we discuss the origin of the observed correlation between cluster concentration c and present-day mass function (PDMF) slope α reported by De Marchi, Paresce & Pulone. This relation can either be reproduced from universal initial conditions combined with some dynamical mechanism(s) that alter(s) the cluster structure and mass function over time, or it must arise early on in the cluster lifetime, such as during the gas-embedded phase of cluster formation. Using a combination of Monte Carlo and N-body models for globular cluster evolution performed with the MOCCA and NBODY6 codes, respectively, we explore a number of dynamical mechanisms that could affect the observed relation. For the range of initial conditions considered here, our results are consistent with an universal initial binary fraction ≈ 10% (which does not, however, preclude 100%) and an universal initial stellar mass function resembling the standard Kroupa distribution. Most of the dispersion observed in the c-α relation can be attributed to two-body relaxation and Galactic tides. However, dynamical processes alone could not have reproduced the dispersion in concentration, and we require at least some correlation between the initial concentration and the total cluster mass. We argue that the origin of this trend could be connected to the gas-embedded phase of cluster evolution.
Nathan Leigh, Mirek Giersz, Jeremy Webb, Arkadiusz Hypki, Guido de Marchi, Pavel Kroupa, Alison Sills

On the nature of the brightest globular cluster in M81

YD Mayya — 2013-09-20T15:12:47-00:00

(18 Sep 2013)

We analyse the photometric, chemical, star formation history and structural properties of the brightest globular cluster (GC) in M81, referred as GC1 in this work, with the intention of establishing its nature and origin. We find that it is a metal-rich ([Fe/H]=-0.60+/-0.10), alpha-enhanced ([Alpha/Fe]=0.20+/0.05), core-collapsed (core radius r_c=1.2 pc, tidal radius r_t = 76r_c), old (>13 Gyr) cluster. It has an ultraviolet excess equivalent of ~2500 blue horizontal branch stars. It is detected in X-rays indicative of the presence of low-mass binaries. With a mass of 10 million solar masses, the cluster is comparable in mass to M31-G1 and is four times more massive than Omega Cen. The values of r_c, absolute magnitude and mean surface brightness of GC1 suggest that it could be, like massive GCs in other giant galaxies, the left-over nucleus of a dissolved dwarf galaxy.
YD Mayya, D Rosa-Gonzalez, M Santiago-Cortes, LH Rodriguez-Merino, O Vega, JP Torres-Papaqui, A Bressan, L Carrasco

An Age Difference of 2 Gyr between a Metal-Rich and a Metal-Poor Globular Cluster

BMS Hansen — 2013-08-02T09:07:43-00:00

(31 Jul 2013)

Globular clusters trace the formation history of the spheroidal components of both our Galaxy and others, which represent the bulk of star formation over the history of the universe. They also exhibit a range of metallicities, with metal-poor clusters dominating the stellar halo of the Galaxy, and higher metallicity clusters found within the inner Galaxy, associated with the stellar bulge, or the thick disk. Age differences between these clusters can indicate the sequence in which the components of the Galaxy formed, and in particular which clusters were formed outside the Galaxy and later swallowed along with their original host galaxies, and which were formed in situ. Here we present an age determination of the metal-rich globular cluster 47 Tucanae by fitting the properties of the cluster white dwarf population, which implies an absolute age of 9.9 (0.7) Gyr at 95% confidence. This is about 2.0 Gyr younger than inferred for the metal-poor cluster NGC 6397 from the same models, and provides quantitative evidence that metal-rich clusters like 47 Tucanae formed later than the metal-poor halo clusters like NGC 6397.
BMS Hansen, JS Kalirai, J Anderson, A Dotter, HB Richer, RM Rich, MM Shara, GG Fahlman, JR Hurley, IR King, D Reitzel, PB Stetson

Co-Evolution of Galactic Nuclei and Globular Cluster Systems

Oleg Gnedin — 2013-08-02T09:06:36-00:00

(13 Jan 2014)

We revisit the hypothesis that dense galactic nuclei are formed from inspiraling globular clusters. Recent advances in understanding of the continuous formation of globular clusters over cosmic time and the concurrent evolution of the galaxy stellar distribution allow us to construct a simple model that matches the observed spatial and mass distributions of clusters in the Galaxy and the giant elliptical galaxy M87. In order to compare with observations, we model the effects of dynamical friction and dynamical evolution, including stellar mass loss, tidal stripping of stars, and tidal disruption of clusters by the growing galactic nucleus. We find that inspiraling globular clusters form a dense central structure, with mass and radius comparable to the typical values in observed nuclear star clusters (NSCs) in late-type and low-mass early-type galaxies. The density contrast associated with the NSC is less pronounced in giant elliptical galaxies. Our results indicate that the NSC mass as a fraction of mass of the galaxy stellar spheroid scales as M_NSC/M_* = 0.0025 M_*,11^-0.5. Thus disrupted globular clusters could contribute most of the mass of NSCs in galaxies with stellar mass below 10^11 Msun. The inner part of the accumulated cluster may seed the growth of a central black hole via stellar dynamical core collapse, thereby relieving the problem of how to form luminous quasars at high redshift. The seed black hole may reach about 10^5 Msun within 1 Gyr of the beginning of globular cluster formation.
Oleg Gnedin, Jeremiah Ostriker, Scott Tremaine

Stars on the run: escaping from stellar clusters

Moyano Loyola — 2013-07-08T09:30:10-00:00

(4 Jul 2013), doi:10.1093/mnras/stt1190

A significant proportion of Milky Way stars are born in stellar clusters, which dissolve over time so that the members become part of the disc and halo populations of the Galaxy. In the present work we will assume that these young stellar clusters live mainly within the disc of the Galaxy and that they can have primordial binary percentages ranging from 0% to as high as 70%. We have evolved models of such clusters to an age of 4 Gyr through N-body simulations, paying attention to the stars and binaries that escape in the process. We have quantified the contribution of these escaping stars to the Galaxy population by analysing their escape velocity and evolutionary stage at the moment of escape. In this way we could analyse the mechanisms that produced these escapers, whether evaporation through weak two- body encounters, energetic close encounters or stellar evolution events, e.g. supernovae. In our models we found that the percentage of primordial binaries in a star cluster does not produce significant variations in the velocities of the stars that escape in the velocity range of 0-20 km/s. However, in the high-velocity 20-100 km/s range the number of escapers increased markedly as the primordial binary percentage increased. We could also infer that dissolving stellar clusters such as those that we have modelled can populate the Galactic halo with giant stars for which the progenitors were stars of up to 2.4 Msun. Furthermore, choices made for the velocity kicks of remnants do influence the production of hyper-velocity stars - and to a lesser extent stars in the high-velocity range - but once again the difference for the 99% of stars in the 0-20 km/s range is not significant.
Moyano Loyola, JR Hurley

Rotating Globular Clusters

P Bianchini — 2013-05-28T08:25:24-00:00

(26 May 2013)

Internal rotation is considered to play a major role in the dynamics of some globular clusters. However, in only few cases it has been studied by quantitative application of realistic and physically justified global models. Here we present a dynamical analysis of the photometry and three-dimensional kinematics of omega Cen, 47 Tuc, and M15, by means of a recently introduced family of self-consistent axisymmetric rotating models. The three clusters, characterized by different relaxation conditions, show evidence of differential rotation and deviations from sphericity. The combination of line-of-sight velocities and proper motions allows us to determine their internal dynamics, predict their morphology, and estimate their dynamical distance. The well-relaxed cluster 47 Tuc is very well interpreted by our model; internal rotation is found to explain the observed morphology. For M15, we provide a global model in good agreement with the data, including the central behavior of the rotation profile and the shape of the ellipticity profile. For the partially relaxed cluster omega Cen, the selected model reproduces the complex three-dimensional kinematics; in particular the observed anisotropy profile, characterized by a transition from isotropy, to weakly-radial anisotropy, and then to tangential anisotropy in the outer parts. The discrepancy found for the steep central gradient in the observed line-of-sight velocity dispersion profile and for the ellipticity profile is ascribed to the condition of only partial relaxation of this cluster and the interplay between rotation and radial anisotropy.
P Bianchini, AL Varri, G Bertin, A Zocchi

Super-Massive Stars as a Source of Abundance Anomalies of Proton-Capture Elements in Globular Clusters

Pavel Denissenkov — 2013-05-28T08:24:59-00:00

(26 May 2013)

We propose that the abundance anomalies of proton-capture elements in globular clusters, such as the C-N, Na-O, Mg-Al and Na-F anti-correlations, were produced by super-massive stars with M ~ 10,000 Msun. Such stars could form in the runaway collisions of massive stars that sank to the cluster center as a result of dynamical friction, or via the direct collapse of the low-metallicity gas cloud from which the cluster formed. To explain the observed abundance anomalies, we assume that the super-massive stars had lost significant fractions of their initial masses when only a small mass fraction of hydrogen, Delta X ~ 0.15, was transformed into helium. We speculate that the required mass loss might be caused by the super-Eddington radiation continuum-driven stellar wind or by the diffusive mode of the Jeans instability.
Pavel Denissenkov, FDA Hartwick

Photometric studies of open star clusters Haffner 11 and Czernik 31

D Bisht — 2013-05-01T18:23:41-00:00

(30 Apr 2013)

We present the broad band UBVI CCD photometric investigations in the region of the two open clusters Haffner 11 and Czernik 31. The radii of the clusters are determined as 3.5 arcmin and 3.0 arcmin for Haffner 11 and Czernik 31 respectively. Using two colour (U-B) versus (B-V) diagram we determine the reddening E(B-V) = 0.50+/-0.05 mag and 0.48+/-0.05 mag for the cluster Haffner 11 and Czernik 31 respectively. Using 2MASS JHKs and optical data, we determined E(J-K) = 0.27+/-0.06 mag and E(V-K) = 1.37+/-0.06 for Haffner 11 and E(J-K) = 0.26+/-0.08 mag and E(V-K) = 1.32+/-0.08 mag for Czernik 31. Our analysis indicate normal interstellar extinction law in the direction of both the clusters. Distance of the clusters is determined as 5.8+/-0.5 Kpc for Haffner 11 and 3.2+/-0.3 Kpc for Czernik 31 by comparing the ZAMS with the CM diagram of the clusters. The age of the cluster has been estimated as 800+/-100 Myr for Haffner 11 and 160+/-40 Myr for Czernik 31 using the stellar isochrones of metallicity Z = 0.019.
D Bisht, RKS Yadav, AK Durgapal

On the internal pollution mechanisms in the globular cluster NGC 6121 (M4): heavy-element abundances and AGB models

Valentina D'Orazi — 2013-04-29T08:19:05-00:00

(25 Apr 2013)

Globular clusters display significant variations in their light-element content, pointing to the existence of a second stellar generation formed from the ejecta of an earlier generation. The nature of these internal polluters is still a matter of debate: the two most popular scenarios indicate intermediate-mass AGB stars and fast rotating massive stars. Abundances determination for some key elements can help distinguish between these competitor candidates. We present in this paper Y abundances for a sample of 103 red giant branch stars in NGC 6121. Within measurement errors, we find that the [Y/Fe] is constant in this cluster contrary to a recent suggestion. For a subsample of six stars we also find [Rb/Fe] to be constant, consistent with previous studies showing no variation in other s-process elements. We also present a new set of stellar yields for intermediate-mass AGB stellar models of 5 and 6 solar masses, including heavy element s-process abundances. The uncertainties on the mass-loss rate, the mixing-length parameter, and the nuclear reaction rates have a major impact on the stellar abundances. Within the IM-AGB pollution scenario, the constant abundance of heavy elements inside the cluster requires a marginal s-process efficiency in IM-AGB stars. Such a constrain could still be satisfied by the present models assuming a stronger mass-loss rate. The uncertainties mentioned above are limiting the predictive power of intermediate-mass AGB models. For these reasons, at the moment we are not able to clearly rule out their role as main polluters of the second population stars in globular clusters.
Valentina D'Orazi, Simon Campbell, Maria Lugaro, John Lattanzio, Marco Pignatari, Eugenio Carretta

Evolution of second generation stars in stellar disks of globular and nuclear clusters: Omega Centauri as a test case

Alessandra Mastrobuono-Battisti — 2013-04-24T13:00:48-00:00

(22 Apr 2013)

Globular clusters (GCs) and many nuclear clusters (NCs) show evidence for hosting multiple generations of stellar populations. Younger stellar populations in NCs appear to reside in disk like structures, including the nuclear cluster in our own Galactic center as well as in M31. Kinematic studies of the anomalous globular cluster Omega Centauri, thought to possibly be a former dwarf galaxy (or a galactic nucleus), show evidence for its hosting of a central, kinematically cold disk component. These observations suggest that formation of second (or multiple) generation stars may occur in flattened disk like structures. Here we use detailed N-body simulations to explore the possible evolution of such stellar disks, embedded in globular clusters. We follow the long term evolution of a disk like structure similar to that observed in Omega Centauri and study its properties. we find that a stellar-disk like origin for second generation stellar populations can leave behind kinematic signatures in properties of the clusters, including an anisotropic distribution, and lower velocity dispersions, and we discuss their implications in constraining the origin of second generations stars and their dynamical age.
Alessandra Mastrobuono-Battisti, Hagai Perets

Properties of young star cluster systems: the age signature from near-infrared integrated colours

JFC Santos — 2013-04-15T08:27:22-00:00

(11 Apr 2013)

A recent JHKs study of several grand-design spiral galaxies shows a bimodal distribution of their system of star clusters and star forming complexes in colour-magnitude and colour-colour diagrams. In a comparison with stellar population models including gas, the (J-H) vs (H-Ks) diagram reveals that embedded clusters, still immersed in their parental clouds of gas and dust, generally have a redder (H-Ks) colour than older clusters, whose gas and dust have already been ejected. This bimodal behaviour is also evident in the colour-magnitude diagram MK vs (J-Ks), where the brightest clusters split into two sequences separating younger from older clusters. In addition, the reddening-free index Qd = (H-Ks) - 0.884 (J-H) has been shown to correlate with age for the young clusters and thus provided an effective way to differentiate the embedded clusters from the older ones. We aim to study the behaviour of these photometric indices for star cluster systems in the Local Group. We investigate the effectiveness of the Qd index in sorting out clusters of different ages at their early evolutionary stages. Surface photometry was carried out for 2MASS images of populous clusters younger than ~100Myr whose ages were available. Some clusters, particularly the embedded ones, were studied for the first time using this method. The integrated magnitudes and colours extracted from the surface photometry of the most populous clusters/complexes in the Local Group shows the expected bimodal distribution in the colour-colour and colour-magnitude diagrams. In particular, we confirm the index Qd as a powerful tool for distinguishing clusters younger than about 7Myr from older clusters. (abridged)
JFC Santos, H Dottori, P Grosbøl

EMCCD photometry reveals two new variable stars in the crowded central region of the globular cluster NGC 6981

Jesper Skottfelt — 2013-04-09T15:22:20-00:00

(8 Apr 2013)

Two previously unknown variable stars in the crowded central region of the globular cluster NGC 6981 are presented. The observations were made using the Electron Multiplying CCD (EMCCD) camera at the Danish 1.54m Telescope at La Silla, Chile.The two variables were not previously detected by conventional CCD imaging because of their proximity to a bright star. This discovery demonstrates that EMCCDs are a powerful tool for performing high-precision time-series photometry in crowded fields and near bright stars, especially when combined with difference image analysis (DIA).
Jesper Skottfelt, DM Bramich, Figuera Jaimes, UG Jørgensen, N Kains, KBW Harpsoe, C Liebig, MT Penny, KA Alsubai, JM Andersen, V Bozza, P Browne, Calchi Novati, Y Damerdji, C Diehl, M Dominik, A Elyiv, E Giannini, F Hessman, TC Hinse, M Hundertmark, D Juncher, E Kerins, H Korhonen, L Mancini, R Martin, M Rabus, S Rahvar, Scarpetta, C Snodgrass, RA Street, J Surdej, J Tregloan-Reed, C Vilela, A Williams

Deep and Wide Photometry of Two Open Clusters NGC 1245 and NGC 2506: Dynamical Evolution and Halo

SH Lee — 2013-04-09T15:21:52-00:00

(7 Apr 2013)

We studied the structure of two old open clusters, NGC 1245 and NGC 2506, from a wide and deep VI photometry data acquired using the CFH12K CCD camera at CFHT. We devised a new method for assigning cluster membership probability to individual stars using both spatial positions and positions in the colour-magnitude diagram. From analyses of the luminosity functions at several cluster-centric radii and the radial surface density profiles derived from stars with different luminosity ranges, we found that the two clusters are dynamically relaxed to drive significant mass segregation and evaporation of some fraction of low-mass stars. There seems to be a signature of tidal tail in NGC 1245 but the signal is too low to be confirmed.
SH Lee, YW Kang, HB Ann

Are Super-Luminous supernovae and Long GRBs produced exclusively in young dense star clusters?

Edward van den Heuvel — 2013-03-29T12:38:35-00:00

(27 Mar 2013)

In the last few years new classes of extremely bright supernovae have been discovered, but their rates are so small that models either fail to produce any or dramatically over-produce the event rates. These super-luminal supernovae tend to occur almost exclusively is relatively low-mass galaxies that undergo active star formation. In the same type of galaxies another high energy phenomenon occurs, which are the long-duration gamma ray bursts, which are associated with another type of very energetic supernovae, the SN Ic-peculiar. We argue that both the super luminal supernovae and the long-duration GRBs are exclusive products of dynamical interactions and collisions in young dense star clusters, which are abundant in dwarf galaxies with active star formation. We present a model that explains how these different types of explosive events can be produced and show that this model can explain their observed rates. In our model the different types of super luminal supernovae and the long-duration gamma-ray bursts are related, in being a natural consequence of the dynamical evolution of dense star clusters.
Edward van den Heuvel, Simon Zwart

Comprehensive abundance analysis of red giants in the open clusters NGC 2527, 2682, 2482, 2539, 2335, 2251 and 2266

Arumalla Reddy — 2013-03-06T10:02:22-00:00

(5 Mar 2013)

We have analyzed high-resolution echelle spectra of red giant members for seven open clusters in the Galactic anticentre direction to explore their chemical positions. Cluster membership has been confirmed by radial velocity. The spread in temperatures and gravities being very small among the red giants, nearly the same stellar lines were employed for all stars thereby reducing the abundance errors: the errors of the average abundance for a cluster were generally in the 0.02 to 0.05 dex range. Our present sample covers Galactocentric distances of 8.3 to 11.3 kpc and an age range of 0.2 to 4.3 Gyr. A careful comparison of our results for the cluster NGC 2682 (M 67) to other high-resolution abundance studies in the literature shows general good agreement for almost all elements in common.
Arumalla Reddy, Sunetra Giridhar, David Lambert

The Distance to the Massive Galactic Cluster Westerlund 2 from a Spectroscopic and HST Photometric Study

Carlos Vargas Alvarez — 2013-02-06T09:42:11-00:00

(4 Feb 2013)

We present a spectroscopic and photometric determination of the distance to the young Galactic open cluster Westerlund 2 using WFPC2 imaging from the Hubble Space Telescope and ground-based optical spectroscopy. HST imaging in the F336W, F439W, F555W, and F814W filters resolved many sources previously undetected in ground-based observations and yielded photometry for 1136 stars. We identified fifteen new O-type stars, along with two probable binary systems, including MSP 188 (O3 + O5.5). We fit reddened SEDs based on the Padova isochrones to the photometric data to determine individual reddening parameters $R_V$ and $A_V$ for O-type stars in Wd2. We find average values $\langle R_V \rangle = 3.77 ± 0.09$ and $\langle A_V \rangle = 6.51 ± 0.38$ mag, which result in a smaller distance than most other spectroscopic and photometric studies. After a statistical distance correction accounting for close unresolved binaries (factor of 1.08), our spectroscopic and photometric data on 29 O-type stars yield that Westerlund 2 has a distance $\langle d \rangle = 4.16 ± 0.07$ (random) $+0.26$ (systematic) kpc. The cluster's age remains poorly constrained, with an upper limit of 3 Myr. Finally, we report evidence of a faint mid-IR PAH ring surrounding the well-known binary candidate MSP~18, which appears to lie at the center of a secondary stellar grouping within Westerlund 2.
Carlos Vargas Alvarez, Henry Kobulnicky, David Bradley, Sheila Kannappan, Mark Norris, Richard Cool, Brendan Miller

The role of massive stars in the turbulent infancy of Galactic globular clusters: Feedback on the intracluster medium, and detailed timeline

C Charbonnel — 2013-01-23T10:00:17-00:00

(21 Jan 2013)

A major paradigm shift has recently revolutionized our picture of globular clusters (GC) that were long thought to be simple systems of coeval stars born out of homogeneous material. Indeed, detailed abundance studies of GC long-lived low-mass stars performed with 8-10m class telescopes, together with high-precision photometry of Galactic GCs obtained with HST,have brought compelling clues on the presence of multiple stellar populations in individual GCs. These stellar subgroups can be recognized thanks to their different chemical properties (more precisely by abundance differences in light elements from carbon to aluminium; see Bragaglia, this volume) and by the appearance of multimodal sequences in the colour-magnitude diagrams (see Piotto, this volume). This has a severe impact on our understanding of the early evolution of GCs, and in particular of the possible role that massive stars played in shaping the intra-cluster medium (ICM) and in inducing secondary star formation. Here we summarize the detailed timeline we have recently proposed for the first 40 Myrs in the lifetime of a typical GC following the general ideas of our so-called "Fast Rotating Massive stars scenario" (FRMS, Decressin et al. 2007b) and taking into account the dynamics of interstellar bubbles produced by stellar winds and supernovae. More details can be found in Krause et al. (2012, 2013).
C Charbonnel, M Krause, T Decressin, N Prantzos, G Meynet

Mass segregation in the diffuse outer-halo globular cluster Palomar 14

Matthias Frank — 2013-01-23T09:59:04-00:00

(21 Jan 2013)

We present an analysis of the radial dependence of the stellar mass function in the diffuse outer-halo globular cluster Palomar 14. Using archival HST/WFPC2 data of the cluster's central 39 pc (corresponding to ~0.85*r_h) we find that the mass function in the mass range of 0.55 to 0.85 solar masses is well approximated by a power-law at all radii. The mass function steepens with increasing radius, from a shallow power-law slope of 0.66+/-0.32 in the cluster's centre to a slope of 1.61+/-0.33 beyond the core radius, showing that the cluster is mass-segregated. This is seemingly in conflict with its long present-day half-mass relaxation time of ~20 Gyr, and with the recent finding by Beccari et al. (2011), who interpret the cluster's non-concentrated population of blue straggler stars as evidence that dynamical segregation has not affected the cluster yet. We discuss this apparent conflict and argue that the cluster must have either formed with primordial mass segregation, or that its relaxation time scale must have been much smaller in the past, i.e. that the cluster must have undergone a significant expansion.
Matthias Frank, Eva Grebel, Andreas Küpper

Panchromatic fits to the Globular Cluster NGC 6366

Fabíola Campos — 2013-01-21T20:45:56-00:00

(18 Jan 2013)

We present panchromatic isochrone fits to the color magnitude data of the globular cluster NGC 6366, based on HST ACS/WFC and SOAR photometric data. Before performing the isochrone fits, we corrected the photometric data for differential reddening and calculated the mean ridge line of the color magnitude diagrams. We compared the isochrones of Dartmouth Stellar Evolution Database and PAdova and TRieste Stellar Evolution Code (with microscopic diffusion starting on the main sequence). Based on previous determinations of the metallicity of this cluster we test it from [Fe/H]=-1.00 to [Fe/H]= -0.50, and the age from 9 to 13 Gyrs. The uncertainties do not decrease when we fit simultaneous colors. We also find that the Dartmouth Stellar Evolution Database isochrones have a better fit in the sub giant branch and low main sequence than the PAdova and TRieste Stellar Evolution Code. Considering the most recent spectroscopic determination of the metallicity ([Fe/H]= -0.67), we find E(B-V)=0.69+/-0.02, (m-M)_V=15.02+/-0.07 and 11+/-2 Gyr for NGC 6366.
Fabíola Campos, SO Kepler, C Bonatto

Ultraviolet properties of Galactic globular clusters with GALEX

E Dalessandro — 2013-01-21T20:44:12-00:00

(18 Jan 2013)

We present ultraviolet (UV) integrated colors of 44 Galactic globular clusters (GGCs) observed with the Galaxy Evolution Explorer (GALEX) in both FUV and NUV bands. We find for the first time that GCs associated with the Sagittarius dwarf galaxy have (FUV-V) colors systematically redder than GGCs with the same metallicity. M31 GCs show almost the same UV colors as GGCs, while M87 are systematically bluer. We speculate about the presence of an interesting trend, suggesting that the UV color of GCs may be correlated with the mass of the host galaxy, in the sense that more massive galaxies possess bluer clusters.
E Dalessandro, RP Schiavon, FR Ferraro, ST Sohn, B Lanzoni, O' Connell

Impact of metallicity on the evolution of young star clusters

M Mapelli — 2013-01-21T20:31:09-00:00

(17 Jan 2013)

We discuss the results of N-body simulations of intermediate-mass young star clusters (SCs) with three different metallicities (Z=0.01, 0.1 and 1 Zsun), including metallicity-dependent stellar evolution recipes and metallicity-dependent prescriptions for stellar winds and remnant formation. The initial half-mass relaxation time of the simulated young SCs (~10 Myr) is comparable to the lifetime of massive stars. We show that mass-loss by stellar winds influences the reversal of core collapse and the expansion of the half-mass radius. In particular, the post-collapse re-expansion of the core is weaker for metal-poor SCs than for metal-rich SCs, because the former lose less mass (through stellar winds) than the latter. As a consequence, the half-mass radius expands faster in metal-poor SCs. The difference in the half-light radius between metal-poor SCs and metal-rich SCs is (up to a factor of two) larger than the difference in the half-mass radius.
M Mapelli, A Bressan

Globular clusters seen by Gaia

E Pancino — 2013-01-21T19:34:43-00:00

(18 Jan 2013)

We present a simulation of twelve globular clusters with different concentration, distance, and background population, whose properties are transformed into Gaia observables with the help of the lates Gaia science performances prescriptions. We adopt simplified crowding receipts, based on five years of simulations performed by DPAC (Data Processing and Analysis Consortium) scientists, to explore the effect of crowding and to give a basic idea of what will be made possible by Gaia in the field of Galactic globular clusters observations.
E Pancino, M Bellazzini, S Marinoni

NGC5694: another extra-galactic globular cluster

A Mucciarelli — 2013-01-17T17:01:16-00:00

(16 Jan 2013)

We discuss the chemical composition of six giant stars of the outer Halo globular cluster NGC5694, through the analysis of UVES@FLAMES high-resolution spectra. The cluster has an average iron content [Fe/H]=--1.83+-0.01, solar-scaled [alpha/Fe] ratios and a very low Ba abundance ([Ba/Fe]=--0.71+-0.06). These anomalous abundance patterns are different from those observed in other Halo globular clusters but similar to those of the metal-poor stars in typical dwarf spheroidal galaxies. These findings suggest an extra-galactic origin for NGC5694, likely from a dwarf spheroidal galaxy.
A Mucciarelli, M Bellazzini, M Catelan, P Amigo, M Correnti, C Cortés, E Dalessandro, V D'Orazi

Terzan 5: a Fossil Remnant of the Galactic Bulge

D Massari — 2013-01-15T09:06:53-00:00

(11 Jan 2013)

Terzan 5 is a stellar system located in the Galactic Bulge, at a distance of 5.9 kpc. Recent discoveries show that it hosts two stellar populations with different iron abundance (Δ[Fe/H]=0.5). Such a large difference has been measured only in ω Centauri in the Galactic halo. Moreover no anticorrelation is observed in Terzan 5, hence it is not a genuine globular cluster. The observed chemical patterns are strikingly similar to those observed in the Bulge stars. This suggests that Terzan 5 is a remnant fragment of the Galactic bulge.
D Massari, FR Ferraro, E Dalessandro, B Lanzoni, A Mucciarelli, L Origlia

More Remote Globular Clusters in the Outer Halo of M31

Graziella di Tullio Zinn — 2013-01-11T10:08:24-00:00

(10 Jan 2013)

We searched the Sloan Digital Sky Survey for outer halo globular clusters (GCs) around M31. Our search of non-stellar objects, within the limits 0.3 < (g-i)o < 1.5 and 14.0 < ro < 19.0 concentrated in some remote areas of the extended halo, to a maximum projected distance of 240 kpc, for a total of approximately 200 square degrees. Another roughly 50 square degrees, approximately 5-75 kpc from M31, were surveyed as test areas. In these areas, we identified 39 GCs and 2 GC candidates, 84% of the previously known GCs (93% of the "classical GCs" and 40% of the "halo extended clusters", on the cluster classification scheme of Huxor et al.). For the entire survey, we visually inspected 78,516 objects for morphological evidence of cluster status, and we identified 18 new clusters, and 75 candidate clusters. The new clusters include 15 classical globulars and three clusters of lower density. Six of the clusters reside in the remote areas of the outer halo, beyond projected distances of 100 kpc. Previously, only MGC1 was found beyond this limit at 117 kpc. The farthest cluster discovered in this survey lies at a projected radius of 158 kpc from M31, assuming that the M31 distance is 780 kpc.
Graziella di Tullio Zinn, Robert Zinn

Chemical element abundances in the outer halo globular cluster M 75

Nikolay Kacharov — 2013-01-11T10:07:02-00:00

(10 Jan 2013)

We present the first comprehensive abundance study of the massive, outer halo globular cluster (GC) M 75 (NGC 6864). This unique system shows a very extended trimodal horizontal branch (HB), but no other clues for multiple populations have been detected in its colour-magnitude diagram (CMD). Based on high-resolution spectroscopic observations of 16 red giant stars, we derived the abundances of a large variety of alpha, p-capture, iron-peak, and n-capture elements. We found that the cluster is metal-rich ([Fe/H] = -1.16 +/- 0.02 dex, [alpha/Fe] = +0.30 +/- 0.02 dex), and shows a marginal spread in [Fe/H] of 0.07 dex, typical of most GCs of similar luminosity. We detected significant variations of O, Na, and Al among our sample, suggesting three different populations. Additionally, the two most Na-rich stars are also significantly Ba-enhanced, indicating a fourth population of stars. Curiously, most stars in M 75 (excluding the two Ba-rich stars) show a predominant r-process enrichment pattern, which is unusual at the cluster's high metallicity. We compare the abundance properties of M 75 and NGC 1851 (a GC very similar to M 75 in terms of age, metallicity, and HB morphology) and draw conclusions on M 75's possible formation scenarios.
Nikolay Kacharov, Andreas Koch

An intermediate-band photometric study of the "Globular Cluster" NGC 2419

Andreas Koch — 2013-01-09T09:08:18-00:00

(7 Jan 2013)

NGC 2419 is one of the remotest star clusters in the Milky Way halo and its exact nature is yet unclear: While it has traits reminiscent of a globular cluster (GC), its large radius and suggestions of an abundance spread have fueled the discussion about its origin in an extragalactic environment, possibly the remnants of the accretion of a dwarf galaxy. Here, we present first results from deep intermediate-band photometry of NGC 2419, which enables us to search for chemical (light element) abundance variations, metallicity spreads, and thus multiple stellar populations through well calibrated Stroemgren indices.
Andreas Koch, Matthias Frank, Sofia Feltzing, Daniel Aden, Nikolay Kacharov, Mark Wilkinson

Ruprecht 147: The oldest nearby open cluster as a new benchmark for stellar astrophysics

Jason Curtis — 2013-01-08T13:14:17-00:00

(5 Jan 2013)

Ruprecht 147 is a hitherto unappreciated open cluster that holds great promise as a standard in fundamental stellar astrophysics. We have conducted a radial velocity survey of astrometric candidates with Lick, Palomar, and MMT observatories and have identified over 100 members, including 5 blue stragglers, 11 red giants, and 5 double-lined spectroscopic binaries (SB2s). We estimate the cluster metallicity from spectroscopic analysis, using Spectroscopy Made Easy (SME), and find it to be [M/H] = +0.07 ± 0.03. We have obtained deep CFHT/MegaCam g'r'i' photometry and fit Padova isochrones to the (g' - i') and 2MASS (J - K) CMDs using the τ^2 maximum-likelihood procedure of Naylor (2009), and an alternative method using 2D cross-correlations developed in this work. We find best fits for isochrones at age t = 2.5 ± 0.25 Gyr, m - M = 7.35 ± 0.1, and A_V = 0.25 ± 0.05, with additional uncertainty from the unresolved binary population and possibility of differential extinction across this large cluster. The inferred age is heavily dependent by our choice of stellar evolution model: fitting Dartmouth and PARSEC models yield age parameters of 3 Gyr and 3.25 Gyr respectively. At approximately 300 pc and 3 Gyr, Ruprecht 147 is by far the oldest nearby star cluster.
Jason Curtis, Angie Wolfgang, Jason Wright, John Brewer, John Johnson

The S-Star Cluster at the Center of the Milky Way: On the nature of diffuse NIR emission in the inner tenth of a parsec

Nadeen Sabha — 2013-01-08T13:13:28-00:00

Astronomy & Astrophysics, Vol. 545 (4 Jan 2013), A70, doi:10.1051/0004-6361/201219203

Sagittarius A*, the super-massive black hole at the center of the Milky Way, is surrounded by a small cluster of high velocity stars, known as the S-stars. We aim to constrain the amount and nature of stellar and dark mass associated with the cluster in the immediate vicinity of Sagittarius A*. We use near-infrared imaging to determine the $K_\mathrms$-band luminosity function of the S-star cluster members, and the distribution of the diffuse background emission and the stellar number density counts around the central black hole. This allows us to determine the stellar light and mass contribution expected from the faint members of the cluster. We then use post-Newtonian N-body techniques to investigate the effect of stellar perturbations on the motion of S2, as a means of detecting the number and masses of the perturbers. We find that the stellar mass derived from the $K_\mathrms$-band luminosity extrapolation is much smaller than the amount of mass that might be present considering the uncertainties in the orbital motion of the star S2. Also the amount of light from the fainter S-cluster members is below the amount of residual light at the position of the S-star cluster after removing the bright cluster members. If the distribution of stars and stellar remnants is strongly enough peaked near Sagittarius A*, observed changes in the orbital elements of S2 can be used to constrain both their masses and numbers. Based on simulations of the cluster of high velocity stars we find that at a wavelength of 2.2 $μ$m close to the confusion level for 8 m class telescopes blend stars will occur (preferentially near the position of Sagittarius A*) that last for typically 3 years before they dissolve due to proper motions.
Nadeen Sabha, Andreas Eckart, David Merritt, Mohammad Zamaninasab, Gunther Witzel, Macarena García-Marín, Behrang Jalali, S Monica Valencia-, Senol Yazici, Rainer Buchholz, Banafsheh Shahzamanian, Christian Straubmeier