The VAST Survey -- II. Orbital motion monitoring of A-type star multiples

TL;DR

This study monitors orbital motions of A-type star multiples, refining their orbital elements, estimating system masses, and revealing the prevalence of multiple systems and potential young companions, enhancing understanding of stellar multiplicity.

Contribution

The paper provides new orbital measurements for 26 binary systems, estimates the fraction of multiple systems within 100 AU, and identifies potential young M-dwarf companions, advancing knowledge of stellar multiplicity.

Findings

Refined orbital elements for 13 systems with historical data.

Estimated binary, triple, and quadruple fractions as 39%, 46%, and 15%.

Discovered potential young M-dwarf companions for model benchmarking.

Abstract

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.