An Improved Optical Spectrum and New Model Fits of

Long thought by some researchers to be an oddity, GD 165B has instead proven to be the rst example of a class of very cool objects (the L dwarfs) which, due to dust formation in their photospheres, lack the dominant bands of TiO seen in warmer M dwarfs. Here we present an improved optical (6200{10300 A) spectrum of GD 165B and identify its prominent spectral features. Among these are newly identiied bands of FeH and the rst identiication of CrH bands in a dwarf. We use the latest generation of model atmospheres, which include the eeects of condensation and dust opacities, to derive values of T ee = 1900100K and log(g) = 5.00.5 for GD 165B. We also derive a crude age of 1.2 to 5.5 Gyr for the GD 165 system. A comparison of the temperature and age of GD 165B to 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.