A Survey for Spectroscopic Binaries among Very Low Mass Stars

We report on the results of a survey for radial velocity variability in aheterogeneous sample of very low-mass stars and brown dwarfs. Onedistinguishing characteristic of the survey is its timespan, which allows anoverlap between spectroscopic binaries and those which can be found by highangular-resolution imaging. We are able to place a new constraint on the totalbinary fraction in these objects, which suggests that they are more likely theresult of extending the same processes at work at higher masses into this massrange, rather than a distinct mode of formation. Our basic result is that thereare $6 \pm 2$ out of 53, or $11^{+0.07}_{-0.04}$% spectroscopic binaries in theseparation range 0-6 AU, nearly as many as resolved binaries. This leads to anestimate of an upper limit of $26 \pm 10$% for the binary fraction of VLMobjects (it is an upper limit because of the possible overlap between thespectroscopic and resolved populations). A reasonable estimate for the verylow-mass binary fraction is $20 - 25$%. We consider several possible separationand frequency distributions, including the same one as found for GK stars, acompressed version of that, a version of the compressed distribution truncatedat 15 AU, and a theoretical distribution which considers the evaporation ofsmall-N clusters. We conclude that the latter two bracket the observations,which may mean that these systems form with intrinsically smaller separationsdue to their smaller mass, and then are truncated due to their smaller bindingenergy. We do not find support for the ``ejection hypothesis'' as theirdominant mode of formation, particularly in view of the similarity in the totalbinary fraction compared with slightly more massive stars, and the difficultythis mechanism has in producing numerous binary systems.Comment: 36 pages, accepted for publication in AJ, abstract shortened for arXiv.or