EDP Sciences

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.

Vol. 598
In section 10. Planets and planetary systems

The radial velocity fitting challenge. II. First results of the analysis of the data set

by X. Dumusque, F. Borsa, M. Damasso, R. Diaz, P. C. Gregory, et al. A&A 598, A133


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Radial velocimetry has become an essential method for the discovery and characterization of exoplanets. However, its efficiency and reliability have remain unclear for small signals (i.e., small-mass planets, or planets on long-period orbits). This paper shows the results of a blind test run by eight different teams. The analysis of these results will be crucial in determining the reliability of the detections and can be used to improve detection algorithms.

Vol. 598
In section 3. Cosmology

Searching for galaxy clusters in the Kilo-Degree Survey

by M. Radovich, E. Puddu, F. Bellagamba, et al. A&A 598, A107

This newly published article presents a cluster-finding technique that is applied to the released (DR2) Kilo-Degree Survey (KiDS) data to produce a large, new cluster catalogue with an estimated purity of 80% and completeness of 85%. A total of 1858 candidate clusters with redshift 0 < zc < 0.7 and mass 1013.5 < M500 < 1015 Msol are found in an area of 114 sq. degrees. The catalogue is compared to the available SDSS-based cluster catalogues and shows that more than 50% of the candidate clusters are matched with previously extracted clusters from the same area of the sky. The catalogue is available at CDS and http://kids.strw.leidenuniv.nl/DR2/

Vol. 598
In section 1. Letters

Proxima’s orbit around Alpha Centauri

by P. Kervella, F. Thévenin, C. Lovis, A&A 598, L7


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Alpha Centauri A and B, and Proxima Centauri are our closest stars. In addition, Proxima Centauri has recently been shown to host a small-mass planet in its so-called habitable zone. It is thus important to become better aquainted with these very close neighbours. From statistical arguments, there could be little doubt that Proxima Centauri is gravitationally bound to the binary system formed by Alpha Centauri A and B. Kervella and Thévenin analyze high-precision radial velocity and astrometric data to show, with a high level of confidence, that it is the case. They also derive the orbit of Proxima which has a period of approximately 550,000 years, an eccentricity of 0.5 and a periastron of 4300 au. Planet (and comet) formation in this triple system should be investigated on the basis of these measurements.

Vol. 597
In section 2. Astrophysical processes

Power requirements for cosmic ray propagation models involving diffusive reacceleration; estimates and implications for the damping of interstellar turbulence

by L. O'C. Drury & A. W. Strong A&A 597, A117


This paper presents a surprisingly efficient physical process for diffusively accelerating cosmic ray particles . In this process, interstellar turbulence suffices to produce a substantial fraction of the cosmic ray energy in the Galaxy. Under very reasonable physical conditions, this acceleration may also be a previously unconsidered mechanism for damping the same turbulence on scales well above the diffusive subscale. This proposal is tested using GALPROP modeling, and can be easily incorporated in other particle propagation codes.