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 directlyimaged 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 shownit to be an L7.5 brown dwarf and confirmed the spectral type of the primary tobe M8. We also derive a substellar mass of the companion of 0.068 Msun,although masses in the range (0.061-0.069) are possible, and the primary massas 0.090 Msun (0.089-0.094). Reanalysis of archival imaging from HST hasconfirmed the secondary as a common proper motion object. This binaryrepresents the first clear example of a brown dwarf companion within 4 AU of alow mass star and should be one of the first late L dwarfs to have a dynamicalmass. As part of a larger survey of M8-L0 stars, this object may indicate thatthere is no ``brown dwarf desert'' around low mass primaries.Comment: 17 pages, 3 figures, Accepted for publication in the Astrophysical Journa