Search

We present results of the CFHT adaptive optics search for companions of a homogeneous group of contact binary stars, as a contribution to our attempts to prove a hypothesis that these binaries require a third star to become so close as observed. In addition to companions directly discovered at separations of >=1", we introduced a new method of AO image analysis utilizing distortions of the AO diffraction ring pattern at separations of 0.07"-1". Very close companions, with separations in the latter range were discovered in systems HV Aqr, OO Aql, CK Boo, XY Leo, BE Scl, and RZ Tau. More distant companions were detected in V402 Aur, AO Cam, V2082 Cyg. Our results provide a contribution to the mounting evidence that the presence of close companions is a very common phenomenon for very close binaries with orbital periods

It is unclear how very close binary stars form, given that during the pre-main-sequence phase the component stars would have been inside each other. One hypothesis is that they formed further apart, but were brought in closer after formation by gravitational interaction with a third member of the system. If so, all close binaries should be members of triple (or higher-order) systems. As a test of this prediction, we present a search for the signature of third components in archival spectra of close binaries. In our sample of 75 objects, 23 show evidence for the presence of a third component, down to a detection limit of tertiary flux contributions of about 0.8% at 5200 \AA\ (considering only contact and semi-detached binaries, we find 20 out of 66). In a homogeneous subset of 58 contact binaries, we are fairly confident that the 15 tertiaries we have detected are all tertiaries present with mass ratios $0.28\lesssim M_3/M_{12}\lesssim0.75$ and implied outer periods $P\lesssim10^6{\rm d}$. We find that if the frequency of tertiaries was the same as that of binary companions to solar-type stars, we would have expected to detect about 12 tertiaries. In contrast, if all contact binaries were in triple systems, one would expect about 20. Thus, our results are not conclusive, but sufficiently suggestive to warrant further studies., Comment: 21 pages, 3 figures, 2 tables, submitted to the Astronomical Journal