An improved optical spectrum and new model fits of the likely brown dwarf GD 165B

Long thought by some researchers to be an oddity, GD 165B has instead proved to be the first example of a class of very cool objects (the L dwarfs) that, because of dust formation in their photospheres, lack the dominant bands of TiO seen in warmer M dwarfs. Here we present an improved optical (6200-10300 Angstrom) spectrum of GD 165B and identify its prominent spectral features. Among these are newly identified bands of FeH and the first identification of CrH bands in a dwarf. We use the latest generation of model atmospheres, which include the effects of condensation and dust opacities, to derive values of T-eff = 1900 +/- 100 K and log g = 5.0 +/- 0.5 for GD 165B. We also derive a crude age of 1.2-5.5 Gyr for the GD 165 system. A comparison of the temperature and age of GD 165B with evolutionary models predicts GD 165B to be an object in the transition zone between stars and brown dwarfs. Further observational evidence-the discovery of lithium in other dwarfs spectroscopically similar to GD 165B and the scarcity of GD 165B-like companions found by imaging surveys-favors a substellar interpretation for this object. We argue that the weight of this observational evidence, together with the known shortcomings of the evolutionary models (which do not yet include opacity by grains), indicate that GD 165B is probably a brown dwarf.