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X-ray image of SNR Puppis A (G. Dubner et al.) |
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Friday, 14 June 2013 12:11 |
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. |
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Semiconvection: numerical simulations (F. Zaussinger and H. C. Spruit) |
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Friday, 14 June 2013 14:30 |
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. |
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A Herschel [CII] Galactic plane survey I (Pineda et al.) |
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Tuesday, 11 June 2013 08:05 |
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. |
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Detection of the Galactic haze with Planck (Planck Collaboration) |
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Tuesday, 18 June 2013 08:31 |
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. |
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Stellar variability in open clusters. I. [...] (Mowlavi et al.) |
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Wednesday, 12 June 2013 09:39 |
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. |
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The lithium isotopic ratio in very metal-poor stars (Lind et al.) |
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Monday, 10 June 2013 12:46 |
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. |
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