The Volume-limited A-Star (VAST) survey - I. Companions and the unexpected X-ray detection of B6-A7 stars

With an adaptive optics imaging survey of 148 B6-A7 stars, we have tested the hypothesis that unresolved lower mass companions are the source of the unexpected X-ray detections of stars in this spectral type range. The sample is composed of 63 stars detected in X-rays within the ROSAT All Sky Survey and 85 stars that form a control sample; both subsets have the same restricted distribution of spectral type, age, X-ray sensitivity and separation coverage. A total of 68 companion candidates are resolved with separations ranging from 0.3 to 26.2 arcsec, with 23 new detections. The multiple star frequency of the X-ray sample based on companions resolved within the ROSAT error ellipse is found to be 43(-6)(+6) per cent. The corresponding control sample multiple star frequency is three times lower at 12(-3)(+4) per cent - a difference of 31 +/- 7 per cent. These results are presented in the first of a series of papers based on our Volume-limited A-Star (VAST) survey - a comprehensive study of the multiplicity of A-type stars.