Observations of Ultracool White Dwarfs

Creators: Oppenheimer, B. R.; Saumon, D.; Hodgkin, S. T.; Jameson, R. F.; Hambly, N. C.; Chabrier, G.; Filippenko, A. V.; Coil, A. L.; Brown, M. E.

Abstract

We present new spectroscopic and photometric measurements of the white dwarfs LHS 3250 and WD 0346+246. Along with F351-50, these white dwarfs are the coolest ones known, all with effective temperatures below 4000 K. Their membership in the Galactic halo population is discussed, and detailed comparisons of all three objects with new atmosphere models are presented. The new models consider the effects of mixed H/He atmospheres and indicate that WD 0346+246 and F351-50 have predominantly helium atmospheres with only traces of hydrogen. LHS 3250 may be a double degenerate whose average radiative temperature is between 2000 and 4000 K, but the new models fail to explain this object.

Additional Information

© 2001 American Astronomical Society. Received 2000 June 15; accepted 2000 November 17. We thank S. R. Kulkarni and J. S. Bloom for their support of this work, M. Irwin for providing us with his spectral data on F351-50, I. N. Reid for sharing the spectrum of LHS 3250, and J. Liebert for useful discussions and suggestions. We also graciously thank the two referees for a thorough job and for insisting that we check the technique in § 5. Financial assistance for this work was provided by NASA through Hubble Fellowship grant HF-01122.01-99A from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555. B. R. O. acknowledges FUTDI. D. S. and A. V. F. are supported by NSF grants AST 97-31438 and AST 99-87438, respectively. S. T. H. acknowledges the support of the Particle Physics and Astronomy Research Council. A. L. C. is grateful for a National Science Foundation Graduate Research Fellowship. Part of this work was done under the auspices of ALLIANCE project 00193 RL between the United Kingdom and France. The William Herschel Telescope is operated on the island of La Palma by the Isaac Newton Group in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofisica de Canarias. The W. M. Keck Observatory is operated as a scientific partnership among the California Institute of Technology, the University of California, and NASA; it was made possible by the generous financial support of the W. M. Keck Foundation.

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