Visitors from the Halo: 11 Gyr Old White Dwarfs in the Solar Neighborhood

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Kilic, Mukremin
Munn, Jeffrey A.
Williams, Kurtis A.
Kowalski, P. M.
von Hippel, Ted
Harris, Hugh C.
Jeffery, Elizabeth J.
DeGennaro, Steven
Brown, Warren R.
McLeod, B.

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We report the discovery of three nearby old halo white dwarf (WD) candidates in the Sloan Digital Sky Survey (SDSS), including two stars in a common proper motion binary system. These candidates are selected from our 2800 deg(2) proper motion survey on the Bok and U. S. Naval Observatory Flagstaff Station 1.3 m telescopes, and they display proper motions of 0.'' 4-0 ''.5 yr(-1). Follow-up MMT spectroscopy and near-infrared photometry demonstrate that all three objects are hydrogen-dominated atmosphere WDs with T(eff) approximate to 3700-4100 K. For average mass WDs, these temperature estimates correspond to cooling ages of 9-10 Gyr, distances of 70-80 pc, and tangential velocities of 140-200 km s(-1). Based on the UVW space velocities, we conclude that they most likely belong to the halo. Furthermore, the combined main-sequence and WD cooling ages are 10-11 Gyr. Along with SDSS J1102+4113, they are the oldest field WDs currently known. These three stars represent only a small fraction of the halo WD candidates in our proper motion survey, and they demonstrate that deep imaging surveys like the Pan-STARRS and Large Synoptic Survey Telescope should find many old thick disk and halo WDs that can be used to constrain the age of the Galactic thick disk and halo.



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Kilic, Mukremin, Jeffrey A. Munn, Kurtis A. Williams, P. M. Kowalski, Ted von Hippel, Hugh C. Harris, Elizabeth J. Jeffery, Steven DeGennaro, Warren R. Brown, and B. McLeod. "VISITORS FROM THE HALO: 11 Gyr OLD WHITE DWARFS IN THE SOLAR NEIGHBORHOODBased on observations obtained at the MMT Observatory, a joint facility of the Smithsonian Institution and the University of Arizona." The Astrophysical Journal Letters, Vol. 715, No. 1 (May, 2010): L21.