Star Formation via the Little Guy: A Bayesian Study of Ultracool Dwarf Imaging Surveys for Companions

Abstract

I have undertaken a comprehensive statistical investigation of the ultracool dwarf companion distribution (spectral typeM6and later). Utilizing a Bayesian algorithm, I tested models of the companion distribution against data from an extensive set of space and ground-based imaging observations of nearby ultracool dwarfs. My main conclusions are fivefold: (1) confirm that the concentration of high mass ratio ultracool binary systems is a fundamental feature of the companion distribution, not an observational or selection bias; (2) determine that the wide (>20 AU) binary fre- quency can be no more than 1%Y2%; (3) show that the decreasing binary frequency with later spectral types is a real trend; (4) demonstrate that a large population of currently undetected low mass ratio systems are not consistent with the current data; and (5) find that the population of spectroscopic binaries must be at least 30%that of currently known ultracool binaries. The best-fit value for the overall M6 and later binary frequency is20%Y22%, of which only6% consists of currently undetected companionswith separations less than 1AU. If this is correct, then the upper limit of the ultracool binary population discovered to date is75%. I find that the numerical simulation results of the ejection for- mation method are inconsistent with the outcome of this analysis. However, dynamics do seem to play an important role, as simulations of small-N clusters and triple system decays produce results similar to those of this work. The observational efforts required to improve these constraints are shown to be primarily large spectroscopic binary sur- veys and improved high-resolution imaging techniques.