EDP Sciences

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.

Vol. 585
In section 1. Letters

A non-glitch speed-up event in the Crab Pulsar

by M. Vivekanand A&A 597, L9


The Crab pulsar is one of the first radio pulsars ever detected, and the first one to be clearly linked to a supernova remnant. The neutron star spins at 33 ms and its rotation can be monitored both with radio telescopes and X-ray instruments, as the Crab is also a strong X-ray pulsar. The Crab pulsar is a precise clock. It is monitored monthly by radio telescope, and often by several X-ray instruments in orbit. Its spin period is known to an accuracy about 10 significant digits. As any other radio pulsar, the Crab pulsar slowly spins down due to rotational energy losses, which power the observed emission at all wavelengths. Sometimes it also shows glitches - sudden (less than a minute) increases of the spin frequency caused by some internal processes within the pulsar. In this Letter the discovery of a new type of speed-up event is reported. It is not impulsive, like a glitch, but it lasted for over two years. It is not easy to explain this new type of event. One possibility suggested by the author is in terms of an extra source of heat generated within the neutron star, resulting in a slowing down of the superfluid interior and speeding up the outer crust.