Searching for the M+T binary needle in the brown dwarf haystack

Abstract

Multiplicity is a key statistic for understanding the formation of very low mass (VLM) stars and brown dwarfs. Currently, the separation distribution of VLM binaries remains poorly constrained at small separations (< 1 AU), leading to uncertainty in the overall binary fraction. We approach this problem by searching for spectral binaries whose identification is independent of separation. The combined spectra of these systems exhibit traces of methane imposed on an earlier-type spectra. When the primary of such a system is a late-type M or early-type L dwarf, however, the relative faintness of the T dwarf secondary (up to 5 magnitudes at K-band) renders these features extremely subtle. We present a set of spectral indices newly designed to identify these systems, and a spectral fitting method to confirm and characterize them. We apply this method to a library of over 750 spectra from the SpeX Prism Spectral Libraries. We present new spectral binary candidates, compare them to recent discoveries, and describe ongoing followup to search for resolved companions and/or radial velocity variability.

arXiv comment3 pages, 3 figures, proceedings for the conference "Brown dwarfs come of age", 2013 May 20-24, to be published in Memorie della Societa Astronomica Italiana