A Spectroscopic Binary at the M/L Transition

Abstract

We report the discovery of a single-lined spectroscopic binary with an ultracool dwarf (UCD) primary with a spectral type between M8 and L0.5.This system was discovered during the course of an ongoing survey to monitor L dwarfs for radial velocity variations and is the first known small-separation (a<1 AU) spectroscopic binary among dwarfs at the M/L transition. Based on radial velocity measurements with a typical precision of 300 m s -1, we estimate the orbital parameters of this system to be P=246.73+/-0.49 days, a 1 sini=0.159+/-0.003 AU, and M 2 sini=0.2062(M 1 +M 2) 2/3 +/-0.0034 M solar. Assuming a primary mass of M 1 =0.08 M solar (based on spectral type), we estimate the secondary minimum mass to be M 2 sini=0.054 M solar. With future photometric, spectroscopic, and interferometric observations it may be possible to determine the dynamical masses of both components directly, making this system one of the best-characterized UCD binaries known.