Binarity and beyond in A stars – I. Survey description and first results of VLTI/GRAVITY observations of VAST targets with highGaia–Hipparcosaccelerations

Abstract

ABSTRACTThe connection between the multiplicity of A stars and that of their white dwarf (WD) descendants is not known and the observational mappings of both multiplicities are far from complete. We are conducting an interferometric survey with VLTI/GRAVITY of 20 out of 108 southern A stars within the VAST sample which show large Gaia–Hipparcos proper motion changes suggestive of a $M \sim 1 \, {\rm M}_{\odot }$ companion at separations of 1–20 au. In this paper, we detail our sample selection and report the results on a partial sample of 10 targets. We (i) interferometrically detected five new stars (two of them in 3 + systems); (ii) spatially resolved one astrometric and one spectroscopic binary; and (iii) did not detect the acceleration culprit down to a main-sequence (MS) mass $M \lesssim 0.4\!-\!0.5 \, {\rm M}_{\odot }$ for two targets, which are therefore candidates for having a WD companion (Sirius analogues). We discuss our preliminary results in the context of the multiplicity of A stars and implications for WDs. In particular, we find that (i) the apparent suppression of companions below 30–50 au in VAST is at least partly due to an observational bias; (ii) the fact that four of the six closest WDs to the Sun have an MS companion within a few tens of au is a statistical fluke but more than 10 such binaries are likely still missing within 20 pc; (iii) a large fraction of such systems had high multiplicity (3 +) progenitors with very close (<1 au) companions and therefore underwent non-trivial evolution.