The VAST Survey – III. The multiplicity of A-type stars within 75 pc

Abstract

With a combination of adaptive optics imaging and a multi-epoch common proper\nmotion search, we have conducted a large volume-limited (D $\\le$ 75 pc)\nmultiplicity survey of A-type stars, sensitive to companions beyond 30 au. The\nsample for the Volume-limited A-STar (VAST) survey consists of 435 A-type\nstars: 363 stars were observed with adaptive optics, 228 stars were searched\nfor wide common proper motion companions and 156 stars were measured with both\ntechniques. The projected separation coverage of the VAST survey extends from\n30 to 45,000 au. A total of 137 stellar companions were resolved, including 64\nnew detections from the VAST survey, and the companion star fraction, projected\nseparation distribution and mass ratio distribution were measured. The\nseparation distribution forms a log-normal distribution similar to the\nsolar-type binary distribution, but with a peak shifted to a significantly\nwider value of 387 (+132,-98) au. Integrating the fit to the distribution over\nthe 30 to 10,000 au observed range, the companion star fraction for A-type\nstars is estimated as 33.8%+-2.6%. The mass ratio distribution of closer (<125\nau) binaries is distinct from that of wider systems, with a flat distribution\nfor close systems and a distribution that tends towards smaller mass ratios for\nwider binaries. Combining this result with previous spectroscopic surveys of\nA-type stars gives an estimate of the total companion star fraction of\n68.9%+-7.0%. The most complete assessment of higher order multiples was\nestimated from the 156-star subset of the VAST sample with both adaptive optics\nand common proper motion measurements, combined with a literature search for\ncompanions, yielding a lower limit on the frequency of single, binary, triple,\nquadruple and quintuple A-type star systems of 56.4 (-4.0,+3.8), 32.1\n(-3.5,+3.9), 9.0 (-1.8,+2.8), 1.9 (-0.6,+1.8) and 0.6 (-0.2,+1.4) per cent,\nrespectively.\n