EDP Sciences

Vol. 601
In section 7. Stellar structure and evolution

Importance of fingering convection for accreting white dwarfs in the framework of full evolutionary calculations: the case of the hydrogen-rich white dwarfs GD133 and G29-38

by F. C. Wachlin, G. Vauclair, S. Vauclair, L. G. Althaus, A&A, 601, A13


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The presence of heavy elements in hydrogen-layer white dwarfs (the most common variety) several decades ago led to a number of different hypotheses regarding the source of metals. One of the more popular ideas currently, is linked to the discovery of debris disks around some white dwarfs: a source of heavy elements could be the disruption of planetisimals that are close to the tidal radius of the white dwarf. In order to link the composition of disrupted planetesimals to the white dwarf surface composition, a number of processes need to be taken into consideration, especially those that influence the mixing of any accreted material. Wachlin and collaborators have carried out such a study, focussing on the most important of these: the double-diffusive instability that results from the accretion of heavy elements. They found that the accretion rates required to explain the abundances in two well-observed white dwarfs, G 29-38 and GD 133, are higher by 1.7 to 2 dex presenting a challenge to current observations, and implying the disruption of significantly more massive bodies than previously considered.

Vol. 601
In section 7. Stellar structure and evolution

Ap stars with resolved magnetically split lines: Magnetic field determinations from Stokes I and V spectra

by G. Mathys, A&A, 601, A14


Magnetic fields have been known in the chemically peculiar stars since the 1940s. Discovered and measured photographically, the fields were biased toward resolvable lines, and with relatrively low signal-to-noise in the strongest fields. This paper is the culmnination of a comphensive program of integrated and polarized light measurements over nearly a decade and analysis of a very large sample (43) of the strongest field stars. It is the definitive one-stop-study, presenting a detailed evaluation of field strengths and geometries, including quadratic Zeeman effect measurements, binarity, rotation, and secular evolution.

Vol. 600
In section 7. Stellar structure and evolution

A VLT-ULTRACAM study of the fast optical quasi-periodic oscillations in the polar V834 Centauri

by M. Mouchet, J.-M. Bonnet-Bidaud, L. Van Box Som, et al. A&A 600, A53


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In this paper Mouchet and collaborators observed the polar system V834 Cen with the ESO-VLT equipped with the ULTRACAM instrument in a fast timing mode. Observations were taken simultaneously in the r' and u' bands, as well as across the HeII (4686A) emission line, with a temporal resolution of 2 ms. The optical light curves show clear signs of quasi-periodic oscillations (QPO) at a period of ~2 s. The authors interpret the QPOs as originating from cyclotron emission of the accretion column created by the white dwarf magnetic field. The oscillations are not strictly periodic, probably due to the superposition of individual blobs of matter falling on the white dwarf along different magnetic field lines.

Vol. 600
In section 6. Interstellar and circumstellar matter

ALMA hints at the existence of an unseen reservoir of diffuse molecular gas in the Galactic bulge

by M. Gerin and H.Liszt, A&A 600, A48


While studying chemistry in local diffuse molecular gas with the ALMA array, the authors of this work serendipitously found mm-wave molecular absorption at -200 km/s < v −140 km/s towards the Galactic bulge. Combining their results with the near-infrared dust extinction in Shultheis et al. (2014), the authors argue that their finding suggests the existence of an unseen reservoir of diffuse molecular gas in the galactic bulge. The physical properties and total mass of these molecular gas in the bulge suggested by this work will need to be determined with future observations and re-analysis of existing archival near-infrared dust extinction and HI data.

Vol. 599
In section 6. Interstellar and circumstellar matter

Structure of Herbig AeBe disks at the milliarcsecond scale. A statistical survey in the H band using PIONIER-VLTI

by B. Lazareff, J.-P. Berger, J. Kluska, et al. A&A 599, A85


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The near-infrared excess of intermediate-mass pre-main sequence stars (Herbig AeBe stars) originates in the dust of a circumstellar disk. In this paper, the authors infer the radial and vertical structure of these disks at scales of the order of 1 au. The 51 program objects were observed in the H band (1.6 µm) with the PIONIER/VLTI interferometer and dedicated UBVRIJHK photometric measurements were also obtained. Spectral and geometrical parameters were extracted by fits of a few simple geometrical disk gmodels as well as fits of physical disk models. Sample statistics were evaluated against similar statistics for the physical disk models to derive properties of the sample objects as a group. The authors find that dust at the inner rim of the disk has a sublimation temperature of approx. 1800 K. A ring morphology is confirmed for approximately half the resolved objects; these rings are wide (delta r/r approx. 0.5). A wide ring favors a rim that, on the star-facing side, looks more like a knife edge than a doughnut. The data are also compatible with the combination of a narrow ring and an inner disk of unspecified nature inside the dust sublimation radius. The authors also find, in a significant fraction of the sample, a spatial component that is fully resolved at an angular scale larger than 40 mas. They also find that the presence and relative flux of that component are a property of the Herbig Ae/Be group. Diffusion of starlight off a flared disk is a possible cause of such extended components.

Vol. 599
In section 1. Letters to the Editor

Low-velocity collisions of chondrules: How a thin dust cover helps enhance the sticking probability

by N. Gunkelmann, A. Kataoka, C. P. Dullemond, and H. M. Urbassek A&A 599, L4


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To form planetesimals and then planets, grains must stick to each other to grow. The collision velocity at which chondrules, an essential component of the meteoritic sample, bounce rather than stick has until now been thought to be extremely low, in the order of mm/s. Gunkelmann et al. show that this critical velocity may be two orders of magnitude greater when accounting for the presence of a thin dust cover around the chondrules, a crucial finding that will help to understand how these objects were accreted into asteroids.