A Very Low-luminosity, Very Cool, Dc White Dwarf

The star LHS 3250 is found to be a white dwarf at a distance of 30 pc. Its absolute magnitudes (MV = 15.72; Mbol = 16.2) put it among the least-luminous white dwarfs known. Its optical spectrum shows no features, indicating it has a DC classification, and it shows no detectable polarization, indicating it does not have a very strong magnetic field. However, its broadband colors show it to have a unique spectral energy distribution, and it stands out from all other stars in BV I and other broadband photometric surveys. We discuss these properties, and conclude that LHS 3250 must be an extremely cool white dwarf with strong collision-induced absorption at red-infrared wavelengths from molecular hydrogen, in accord with models for very cool white dwarf atmospheres. If so, it is the first such star known, and the first star to provide observational evidence supporting these models. It suggests that other very cool white dwarfs, both halo white dwarfs and the oldest disk white dwarfs, also may have colors affected by similar absorption. The atmospheric composition of LHS 3250 is not known, and therefore its temperature is poorly determined. It may be a helium-core star with a mass 0.3 < M < 0.45M⊙ and a product of mass-transfer in a close binary system. However, until its temperature is better known, its mass and age remain uncertain. Subject headings: Astrometry; Stars: Hertzsprung-Russell Diagram; Stars: Individual LHS 3250; Stars: White Dwarfs 1 Based partly on observations made at the Keck Observatory, the Steward Observatory, University of Arizona, and Kitt Peak National Observatory, National Optical Astronomical Observatories. NOAO is operated by the Association of Universities for Research in Astronomy, Inc., under cooperative agreement with the National Science Foundation. The W. M. Keck Observatory is operated by a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation. Universities Space Research Association