The binary fraction and mass ratio of Be and B stars: a comparative Very Large Telescope/NACO study★

Abstract

In order to understand the formation mechanism of the disks around Be stars it is imperative to have a good overview of both the differences and similarities between normal B stars and the Be stars. Here we investigate a previous report that there may be a large population of sub-arcsecond companions to Be stars. We present the first systematic, comparative imaging study of the binary properties of matched samples of B and Be stars observed using the same equipment. We obtained high angular resolution (0.07-0.1 arcsec) K band Adaptive Optics data of 40 B stars and 39 Be stars. The separations that can be probed range from 0.1 to 8 arcsec (corresponding to 20-1000 AU), and magnitude differences up to 10 magnitudes can in principle be covered. We detect 11 binaries out of 37 Be targets (corresponding to a binary fraction of 30 +/- 8%) and 10 binaries out of 36 B targets (29 +/- 8%). Further tests demonstrate that the B and Be binary systems are not only similar in frequency but also remarkably similar in terms of binary separations, flux differences and mass ratios. We find that any hypotheses invoking binary companions as responsible for the formation of a disk need the companions to be closer than 20 AU. Close companions are known to affect the circumstellar disks of Be stars, but as not all Be stars have been found to be close binaries, the data suggest that binarity can not be responsible for the Be phenomenon in all Be stars. Finally, the similarities of the binary parameters themselves also shed light on the Be formation mechanism. They strongly suggest that the initial conditions that gave rise to B and Be star formation must, to all intents and purposes, be similar. This in turn indicates that the Be phenomenon is not the result of a different star formation mechanism.