Highlighted papers

X-ray image of SNR Puppis A (G. Dubner et al.)

aanda.paris@obspm.fr (Astronomy & Astrophysics) — Fri, 14 Jun 2013 12:11:40 +0000

Vol. 554
In section 6. Interstellar and circumstellar matter

The most complete and detailed X-ray view of the SNR Puppis A

by G.M. Dubner, N. Loiseau, P. Rodríguez Pascual, et al. A&A 555, A9

The southern supernova remnant, Puppis A, has for decades been a benchmark object for studies of the interaction between the expanding ejecta and the interstellar medium. Its expansion age is about 4500 years, making it a potentially historically recorded event, at a distance of about 2 kpc. The remnant has been studied across the entire spectrum, from GeV to radio. This paper presents the most complete and definitive reconstruction of the X-ray emission from the remnant using new and archival XMM/Newton and archival Chandra observations (covering the interval 0.3-8 keV with a spatial resolution of 10 arcsec. These observations also present the first complete spectral energy distribution covering the entire panchromatic data. The spectacular view of the barrel-shaped remnant details the interaction with the interstellar medium, a part of which contains the disintegrated debris of a previous encounter with an ambient molecular cloud and another part of which shows the effects of a still-continuing collision. The cellular structure of the surface of the remnant recalls similar multidimensional structures observed in detonation fronts and is different from younger remnants (e.g. Cas A, Tycho, and Kepler) that also lack internal pulsar winds. The present study is a major contribution to a small but growing number of deep multiwavelength-imaged Galactic SNRs. There is more to come, the authors promise a more extensive modeling study.

Detection of the Galactic haze with Planck (Planck Collaboration)

aanda.paris@obspm.fr (Astronomy & Astrophysics) — Tue, 18 Jun 2013 08:31:41 +0000

Vol. 554
In section 5. Galactic structure, stellar clusters and populations

Planck intermediate results. IX. Detection of the Galactic haze with Planck

by Planck Collaboration A&A 554, A139

In the course of its all-sky survey, the final results of which will be reported in an upcoming special issue of A&A, the Planck collaboration studied the Galactic haze between 20 and 100 GHz. There are two components to the diffuse emission, one that is consistent with a cosmic-ray component of synchrotron-emitting electrons and another much more energetic population that is responsible for the structures discovered by the Fermi/LAT observations at high Galactic latitude toward the Galactic center. This more confined - yet still diffuse - emission, which correlates spatially with the Fermi bubbles at GeV energies discovered by the Fermi/LAT, appears to be caused by synchrotron emission from electrons with a harder spectral energy distribution than anywhere else in the Milky Way. Other possible sources, rapidly spinning dust grains or thermal bremsstrahlung, are ruled out by the spectrum. Perhaps the most remarkable result is that the 30 GHz emission from the Galactic southern hemisphere appears to coincide precisely with the extent of the Fermi 2-5 GHz haze, even to the extent of showing the same edge at a latitude of about -50 degrees, about 17 degrees from the bubble's center as determined by Fermi.

Semiconvection: numerical simulations (F. Zaussinger and H. C. Spruit)

aanda.paris@obspm.fr (Astronomy & Astrophysics) — Fri, 14 Jun 2013 14:30:10 +0000

Vol. 554
In section 2. Astrophysical processes

Semiconvection: numerical simulations

by F. Zaussinger and H. Spruit A&A 554, A119

The processes that mix stellar interiors are central to their evolution, especially during advanced stages. While classical turbulent convective transport is reasonably under control, with a number of approximations available for homogeneous interiors, the process of chemical transport not understood as well. In semiconvection, the process is governed by molecular weight gradients that develop during advanced stages of nuclear burning. Different than the now popular thermohaline ("salt finger") modes of double diffusive instability, semiconvection is less well understood. This paper presents a direct numerical simulation of single and multiple layers with density stratification. The authors show that the layers are thin and that the Boussinesq approximation yields reliable results in two dimensions that correspond to those obtained from fully compressible computations. While opacity feedback on the radiative properties of the mixing medium have not yet been included, these calculations show that such models are now feasible. The resulting mixing is minor, but thin density stratification barriers, which may be almost a full scale height in extension, produce din models of intermediate-mass stars. This will be important for dynamo, asteroseimological, and rotational studies of evolved stars, and it points the way to a fully generalized theory of inhomogeneous convection.

Stellar variability in open clusters. I. [...] (Mowlavi et al.)

aanda.paris@obspm.fr (Astronomy & Astrophysics) — Wed, 12 Jun 2013 09:39:37 +0000

Vol. 554
In section 7. Stellar structure and evolution

Stellar variability in open clusters. I. A new class of variable stars in NGC 3766

by N. Mowlavi, F. Barblan, S. Saesen, and L. Eyer, A&A 554, A108

Pulsating stars have become important tools for studying the interior structure. Known instability strips for early-type main sequence stars comprise the delta Scu stars, beta Cep stars, and the slowly pulsating B stars (SPB). This article presents results of a seven-year photometric monitoring of the open cluster NGC 3766. Several new variables belonging to the known classes are found. Most significantly, detection is reported of 36 main sequence stars between the red edge of the SPB stars and the blue edge of the delta Scu stars. These variables appear to be members of a new class for which the authors propose the designation "low amplitude periodic A and late-B variables.

A Herschel [CII] Galactic plane survey I (Pineda et al.)

aanda.paris@obspm.fr (Astronomy & Astrophysics) — Tue, 11 Jun 2013 08:05:16 +0000

Vol. 554
In section 6. Interstellar and circumstellar matter

A Herschel [CII] Galactic plane survey I: the global distribution of ISM gas components

by J.L.P. Pineda, W. D. Langer, T. Velusamy, and P. F. Goldsmith A&A 554, A103

Our understanding of the distribution of hydrogen in the interstellar medium of our own and external galaxies has been based on the assumption that CO is a good tracer of molecular hydrogen. While it has always been known that this assumption was doubtful, there have been few quantitative studies aimed at understanding how much H2 is hidden in layers that are invisible in CO. In the article highlighted in this issue, Pineda et al. use the Herschel observations of the 157 micron line of ionized carbon to estimate the hidden H2 in the Galaxy, finding that the ratio of CO-dark H2 to total H2 varies from 20 percent in the inner Galaxy to 80 percent at a radius of 10 kpc.

The lithium isotopic ratio in very metal-poor stars (Lind et al.)

aanda.paris@obspm.fr (Astronomy & Astrophysics) — Mon, 10 Jun 2013 12:46:08 +0000

Vol. 554
In section 8. Stellar atmospheres

The lithium isotopic ratio in very metal-poor stars

by K. Lind, J. Melendez, M. Asplund, R. Collet, and Z. Magic, A&A 554, A96

Very metal-poor stars are ancient objects and thus the most important tracers of the cosmic abundance of lithium in the early universe. Big Bang nucleosynthesis predicts that they should show an undetectably low 6Li/7Li isotopic ratio. If this was not found to be the case, then an additional pregalactic production source of 6Li or a revision of the standard model of Big Bang nucleosynthesis would be required. It would also cast doubt on lithium depletion from stellar atmospheres as an explanation for the factor 35 discrepancy between the predicted primordial 7Li from the Big Bang and the observed value in metal-poor dwarf/turn-off stars. In this work, four Milky Way halo stars have been studied using improved model atmospheres and line formation treatment. For the first time, a combined 3D, nonlocal thermodynamic equilibrium (NLTE) modeling technique for Li, Na, and Ca lines is utilized to determine the lithium isotopic ratio. This allows superior modeling of the lithium resonance line and other neutral lines, leading to much more accurate determination of the 6Li/7Li isotopic ratio. Unlike previous less detailed modeling results from 1D and 3D LTE modeling, no star in this sample has any significant (2 sigma) detection of 6Li in NLTE. NLTE modeling can systematically reduce the best-fit lithium isotopic ratios by up to five percent. As a byproduct, the first ever 3D NLTE Ca and Na abundances of halo stars are presented, and they also reveal significant departures from LTE. In conclusion, the observational support for a significant and nonstandard 6Li production source in the early universe is substantially weakened by this new analysis, and thus it suggests that there is no need to revise standard Big Bang nucleosynthesis.

Long term variability of Cygnus X-1. V. (Grinberg et al.)

aanda.paris@obspm.fr (Astronomy & Astrophysics) — Thu, 06 Jun 2013 07:25:07 +0000

Vol. 554
In section 2. Astrophysical processes

Long term variability of Cygnus X-1. V. State definitions with all sky monitors

by V. Grinberg, N. Hell, K. Pottschmidt, et al., A&A 554, A88

The present "Golden Age" of multiwavelength all-sky monitoring satellites is exploited by the authors to define an empirical set of X-ray activity states for the prototypical black-hole, high-mass X-ray binary Cygnus X-1. They distinguish between hard, intermediate, and soft - depending on the spectral index in the 1 to 50 keV range - with transition and duration probabilities derived using nearly 16 years of X-ray observations. The two extremes show most probable durations longer than one week. The typical duration of the intermediate state, in contrast, is only a few days or less. This presents an interesting renewal problem, with a rapid, telegraph-like transition between the extreme spectral states and lag times between bands that depend on the spectral index in a nonlinear way. This paper will provide the basis for all future comparisons of the variations at high energy and, more important, for comparisons with any future changes in long-term behavior.

Detection of carbon monoxide in the high-resolution day-side spectrum [...] (de Kok et al.)

aanda.paris@obspm.fr (Astronomy & Astrophysics) — Thu, 06 Jun 2013 13:26:26 +0000

Vol. 554
In section 10. Planets and planetary systems

Detection of carbon monoxide in the high-resolution day-side spectrum of the exoplanet HD 189733b

by R.J. de Kok, M. Brogi, I.A.G. Snellen, J. Birkby, S. Albrecht, and E.J.W. de Mooij A&A 554, A82

Because it transits its very bright parent star, HD 189733b is one of the most studied exoplanets. Using the CRIRES infrared spectrograph on the VLT at 2.0 and 2.3 microns, de Kok et al. searched for the Doppler-shifted signatures of CO, H2O, CH4, and CO2 absorption in the tiny planetary contribution to the integrated light of the HD 189733 system. They did not find any sign of H2O, CH4, or CO2, but a significant 5-sigma absorption signal from CO. CO must be present above the haze that has been proposed to explain the visible and infrared spectrum of the planet, either because CO is abundant or because the haze is optically thin at normal incidence. The Doppler-shift of the CO line provides a model-independent measurement of the stellar and planetary masses. The technique thus powerfully characterizes the atmosphere of the planet and constrains the masses of the planet and its star.

Far-infrared-detected Lyman-break galaxies at z ~ 3 (Oteo et al.)

aanda.paris@obspm.fr (Astronomy & Astrophysics) — Tue, 04 Jun 2013 09:46:54 +0000

Vol. 554
In section 1. Letters

Far-infrared-detected Lyman-break galaxies at z ~ 3

by I. Oteo, J. Cepa, Á. Bongiovanni, A.M. Pérez-García, B. Cedrés, H. Domínguez Sánchez, A. Ederoclite, M. Sánchez-Portal, I. Pintos-Castro, and R. Pérez-Martínez A&A 554, L3

Lyman-break galaxies (LBGs) are high-z star-forming objects, selected in the optical domain. Their dust content was not well known until now. This work explores the far-infrared (FIR) emission of a sample of 16 LBGs at z =3 in the GOODS fields detected by PACS onboard Herschel. These detected LBGs have been identified as ultra (or hyper) luminous IR galaxies. They are affected by extinction to such an extreme that previous methods to determine their star formation rate (SFR) were in error by large factors. Compared to LBGs at z=1, these dusty objects have more FIR emission and have higher SFR for a given stellar mass than their dustier counterparts at z=1.

Hierarchical core formation in L1495/B213 (A. Hacar et al.)

aanda.paris@obspm.fr (Astronomy & Astrophysics) — Wed, 29 May 2013 12:14:11 +0000

Vol. 554
In section 6. Interstellar and circumstellar matter

Cores, filaments, and bundles: hierarchical core formation in the L1495/B213 Taurus region

by A. Hacar, M. Tafalla, J. Kauffmann, and A. Kovacs A&A 554, A55

Filaments in molecular clouds clearly have a close link with star formation, as has most recently been demonstrated by a series of publications making use of the far-infrared mapping capabilities of Herschel. The article by Hacar et al. highlighted in this issue shows, however, that maps in molecular spectral lines are crucial for understanding filament structure. They show that towards some nearby filaments in Taurus, one observes multiple velocity components in C_18O emission, suggesting that in some cases, at least, the filaments have different strands and that the individual strands have sonic velocity dispersion. Moreover, a fraction of the filaments appear to be responsible for most of the star formation.