LHS 2924 - A uniquely cool low-luminosity star with a peculiar energy distribution

Spectroscopic and photometric surveys of the faint red stars in the LHS Catalogue show LP 271-25 = LHS 2924 to be a uniquely cool proper motion star with a peculiar spectrum and energy distribution. The best fit blackbody gives Te = 1950 + or - 150 K, but with stronger V flux and much weaker TiO and CaOH bands than are seen in known very late M dwarfs. Several alternative explanations for these properties are considered. The possibilities of a very cool white dwarf, late M subdwarf, or dusty M dwarf are eliminated. A super-metal-rich late M dwarf seems very unlikely. A white dwarf-red dwarf composite can satisfy photometric and spectroscopic requirements but requires the combination of two kinds of rare objects, each uniquely cool, and with an implied conflict in their ages. A single very late M dwarf or substellar-mass brown dwarf is favored, despite difficulties in explaining the spectrum and colors.