DISCOVERY OF A WIDE SUBSTELLAR COMPANION TO A NEARBY LOW-MASS STAR

We report the discovery of a wide (135 +/- 25 AU), unusually blue L5 companion, 2MASS J17114559+4028578, to the nearby M4.5 dwarf G203-50 as a result of a targeted search for common proper motion pairs in the Sloan Digital Sky Survey and the Two Micron All Sky Survey. Adaptive optics imaging with Subaru indicates that neither component is a nearly equal-mass binary with separation >0.18 '' and places limits on the existence of additional faint companions. An examination of TiO and CaH features in the primary's spectrum is consistent with solar metallicity and provides no evidence that G203-50 is metal-poor. We estimate an age for the primary of 1-5 Gyr based on activity. Assuming coevality of the companion, its age, gravity, and metallicity can be constrained from properties of the primary, making it a suitable benchmark object for the calibration of evolutionary models and for determining the atmospheric properties of peculiar blue L dwarfs. The low total mass (M(tot) = 0.21 +/- 0.03 M(circle dot)), intermediate mass ratio (q = 0.45 +/- 0.14), and wide separation of this system demonstrate that the star formation process is capable of forming wide, weakly bound binary systems with low-mass and brown dwarf components. Based on the sensitivity of our search we find that no more than 2.2% of the early-to-mid-M dwarfs (9.0 < M(V) < 13.0) have wide substellar companions with m greater than or similar to 0.06 M(circle dot).