Discovery of a Tight Brown Dwarf Companion to the Low Mass Star LHS 2397a

Using the adaptive optics system, Hokupa’a, at Gemini-North, we have directly imaged a companion around the UKIRT faint standard M8 star, LHS 2397a (FS 129) at a separation of 2.96 AU. Near-Infrared photometry of the companion has shown it to be an L7.5 brown dwarf and confirmed the spectral type of the primary to be M8. We also derive a substellar mass of the companion of 0.068M⊙, although masses in the range (0.061− 0.069) are possible, and the primary mass as 0.090M⊙(0.089− 0.094). Reanalysis of archival imaging from HST has confirmed the secondary as a common proper motion object. This binary represents the first clear example of a brown dwarf companion within 4 AU of a low mass star and should be one of the first late L dwarfs to have a dynamical mass. As part of a larger survey of M8-L0 stars, this object may indicate that there is no “brown dwarf desert” around low mass primaries. Subject headings: binaries: general — instrumentation: adaptive optics — stars: evolution — stars: formation — stars: individual(LHS 2397a) — stars: low-mass, brown dwarfs