Contact Binaries with Additional Components. II. A Spectroscopic Search for Faint Tertiaries

Abstract

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 farther 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 Å (considering only contact and semidetached binaries, we find 20 out of 66). In a homogeneous subset of 59 contact binaries, we are fairly confident that the 15 tertiaries we have detected are all tertiaries present with mass ratios 0.28