The VAST Survey - I. Companions and the unexpected X-ray detection of B6-A7 stars

TL;DR

This study uses adaptive optics imaging to investigate whether unresolved lower-mass companions explain unexpected X-ray detections in B6-A7 stars, revealing a higher multiplicity rate in X-ray detected stars.

Contribution

It provides the first systematic comparison of companion frequencies in X-ray detected and non-detected A-type stars, highlighting the role of companions in X-ray emission.

Findings

X-ray detected stars have a 43% companion rate within ROSAT error ellipses.

Control stars have a 12% companion rate, significantly lower.

Results support the hypothesis that companions contribute to X-ray emission in these stars.

Abstract

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", 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)%. The corresponding control sample multiple star frequency is three times lower at 12 (+4,-3)% -- a difference of 31\pm7%. 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.