A Spectroscopic Binary at the M/L Transition

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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, a1 sin i = 0.159 ± 0.003 AU, and M2 sin i = 0.2062(M1 + M2)2/3± 0.0034 M☉. Assuming a primary mass of M1 = 0.08 M☉ (based on spectral type), we estimate the secondary minimum mass to be M2 sin i = 0.054 M☉. 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.
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