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 <100AU, 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%, for systems with at least one companion within 100AU. 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 (<100Myr) 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.

Cone search capability for table J/MNRAS/422/2765/table1 (Basic properties of sample members)