Additional Ultracool White Dwarfs Found in the Sloan Digital Sky Survey

We identify seven new ultracool white dwarfs discovered in the Sloan Digital Sky Survey (SDSS). The SDSS photometry, spectra, and proper motions are presented, and additional BVRI data are given for these and other previously discovered ultracool white dwarfs. The observed colors span a remarkably wide range, qualitatively similar to colors predicted by models for very cool white dwarfs. One of the new stars (SDSS J1251+44) exhibits strong collision-induced absorption (CIA) in its spectra, while the spectra and colors of the other six are consistent with mild CIA. Another of the new discoveries (SDSS J2239+00A) is part of a binary system – its companion is also a cool white dwarf, and other data indicate that the companion exhibits an infrared flux deficiency, making this the first binary system composed of two CIA white dwarfs. A third discovery (SDSS J0310−00) has weak Balmer emission lines. The proper motions of all seven stars are consistent with membership in the disk or thick disk. U.S. Naval Observatory, Flagstaff Station, Flagstaff, AZ 86001; [email protected] The Univ. of Chicago, Department of Astronomy & Astrophysics, 5640 S. Ellis Ave., Chicago, IL 60637 Adler Planetarium and Astronomy Museum, Chicago, IL 60605 Department of Astronomy, University of Washington, Seattle, WA 98195 York University, North York, ON Steward Observatory, Univ. of Arizona, Tucson, AZ 85721 Princeton Univ. Observatory, Peyton Hall, Princeton, NJ 08544 Apache Point Observatory, PO Box 59, Sunspot, NM 88349-0059 The Pennsylvania State Univ., University Park, PA 16802 Austin Peay State Univ., Department of Physics and Astronomy, Clarksville, TN 37044