Establishing the nature of companion candidates to X-ray emitting late B-type stars

TL;DR

This study investigates the nature of companions to X-ray emitting late B-type stars, finding that these companions are typically late-type dwarfs, supporting the idea that the X-ray emission originates from active pre-main sequence stars.

Contribution

The paper provides spectroscopic evidence that companions to X-ray emitting late B-type stars are late-type dwarfs, confirming their role as the source of X-ray emission.

Findings

All X-ray emitting systems have late-type dwarf companions.

The only system with an early-type companion does not emit X-rays.

Results support that X-ray emission is from active pre-main sequence companions.

Abstract

The most favored interpretation for the detection of X-ray emission from late B-type stars is that these stars have a yet undiscovered late-type companion (or an unbound nearby late-type star) that produces the X-rays. Several faint IR objects at (sub)-arcsecond separation from B-type stars have been uncovered in our earlier adaptive optics imaging observations, and some of them have been followed up with the high spatial resolution of the Chandra X-ray observatory, pinpointing the X-ray emitter. However, firm conclusions on their nature requires a search for spectroscopic signatures of youth. Here we report on our recent ISAAC observations carried out in low resolution spectroscopic mode. Equivalent widths have been used to obtain information on spectral types of the companions. All eight X-ray emitting systems with late B-type primaries studied contain dwarf like companions with spectral types later than A7. The only system in the sample where the companion turns out to be of early spectral type is not an X-ray source. These results are consistent with the assumption that the observed X-ray emission from late B-type stars is produced by an active pre-main sequence companion star.