Survival of a brown dwarf after engulfment by a red giant star

Abstract

(Summary of paper) High resolution optical spectroscopy of the white dwarf WD0137-349 reveals a weak Halpha line in emission, due to a low mass companion in a close orbit. Using this emission feature, and the narrow NLTE core of the white dwarf's Halpha absorption line, we measure the orbital period at 116 minutes, the separation at 0.65Rsun, and the mass ratio m2/m1 = K1/K2 = 0.134+/-0.006. From the mass of the white dwarf (0.39+/-0.035Msun) we derive a mass for the companion of 0.053+/-0.006Msun. This is well below the limit of about 0.075Msun commonly used to distinguish stars from brown dwarfs. The observed infrared flux distribution of WD0137-349 is also consistent with a model of an old brown dwarf spectral type mid T or slightly earlier. The system is detached, and the brown dwarf must have survived a previous phase of common envelope evolution, providing a key observational test for models of this interaction.