Discovery of a companion at the L/T transition with the wide-field infrared survey explorer

We report the discovery of a substellar companion to the nearby solar-type star HD46588 (F7V, 17.9pc, τ 3Gyr). HD46588 B was found through a survey for common proper motion companions to nearby stars using data from the Wide-field Infrared Survey Explorer (WISE) and the Two Micron All Sky Survey (2MASS). It has an angular separation of 792 from its primary, which corresponds to a projected physical separation of 1420 AU. We have measured a spectral type of L9 for this object based on near-infrared spectroscopy performed with TripleSpec at Palomar Observatory. We estimate a mass of 0.064^+0.008 _{- 0.019} M_⊙ from a comparison of its luminosity to the values predicted by theoretical evolutionary models for the age of the primary. Because of its companionship to a well-studied star, HD46588 B is one of the few known brown dwarfs at the L/T transition for which both age and distance estimates are available. Thus, it offers new constraints on the properties of brown dwarfs during this brief evolutionary phase. The discovery of HD46588 B also illustrates the value of the WISE for identifying brown dwarfs in the solar neighborhood via their proper motions.