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

TL;DR

This study uses Bayesian analysis of imaging data to characterize ultracool dwarf binary populations, revealing a high mass ratio concentration, low wide binary frequency, and the need for extensive spectroscopic surveys.

Contribution

It provides the first comprehensive Bayesian assessment of ultracool dwarf companion distributions, constraining binary frequencies and informing formation theories.

Findings

High mass ratio binary systems are common among ultracool dwarfs.

Wide binaries (>20 AU) are rare, comprising only 1-2% of the population.

A significant fraction (at least 30%) of ultracool binaries are spectroscopic.

Abstract

I have undertaken a comprehensive statistical investigation of the ultracool dwarf companion distribution (spectral type M6 and 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 frequency can be no more the 1-2%; 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; 5) Find that the population of spectroscopic binaries must be be at least 30% that of currently known ultracool binaries. The best fit value for the overall M6 and later binary frequency is ~20%-22%, of which only ~6% consists of currently undetected companions with separations less than 1 AU. 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 formation 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 surveys and improved high-resolution imaging techniques.