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The Volume-limited A-Star (VAST) survey – II. Orbital motion monitoring of A-type star multiples



Based on observations obtained at the Canada–France–Hawaii Telescope (CFHT) under programme IDs 2001AF11, 2008AC22, 2009BC06, 2010AC14 and 2011AC11. Based on observations made with ESO telescopes at the La Silla Paranal Observatory under programme IDs 272.D-5068(A), 074.D-0180(A), 076.C-0270(A), 076.D-0108(A), 077.D-0147(A) and 080.D-0348(A). Based on observations obtained at the Gemini Observatory under programme IDs GN2008A-Q-74, GN-2008B-Q-119 and GN-2010A-Q-75.


As a part of our ongoing Volume-limited A-Star (VAST) adaptive optics survey, we have obtained observations of 26 binary systems with projected separations <100 au, 13 of which have sufficient historical measurements to allow for refinement of their orbital elements. For each system with an estimated orbit, the dynamical system mass obtained was compared with the system mass estimated from mass–magnitude relations. Discrepancies between the dynamical and theoretical system mass can be explained by the presence of a previously unresolved spectroscopic component or by a non-solar metallicity of the system. Using this approach to infer the presence of additional companions, a lower limit to the fraction of binaries, triples and quadruples can be estimated as 39, 46 and 15 per cent for systems with at least one companion within 100 au. The fraction of multiple systems with three or more components shows a relative increase compared to the fraction for solar-type primaries resolved in previous volume-limited surveys. The observations have also revealed a pair of potentially young (<100 Myr) M dwarf companions, which would make an ideal benchmark for the theoretical models during the pre-main-sequence contraction phase for M dwarfs. In addition to those systems with orbit fits, we report 13 systems for which further orbital monitoring observations are required, 11 of which are newly resolved as a part of the VAST survey.