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White Dwarf Cooling Curves and Searches For White Dwarfs

Published online by Cambridge University Press:  14 August 2015

Matt A. Wood
Matt A. Wood*
Affiliation:
Florida Institute of Technology Department of Physics and Space Sciences Melbourne, FL, 32901-6988, USAwood@kepler.pss.fit.edu

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White dwarf (WD) evolution is essentially a cooling problem in two parts. Degenerate electrons in the core provide the support while non-degenerate C and O ions provide the thermal reservoir (Koester & Chanmugam 1990, Kawaler 1996). The rate of energy loss through the non-degenerate envelope of H and He—segregated by gravitational settling—is meager enough that the vast majority of WDs have not had time to cool to undetectability in the ~10 Gyr age of the Galactic disk. Because WDs are a remarkably homogeneous class with a mass function sharply peaked about 0.6Mʘ, and because the cooling physics is relatively straightforward to calculate, the observed deficit of WDs below Mv = 16.5 [log(L/Lʘ) ≈ -4.5, Teff ʘ 4000 K; see Liebert et al. 1988 (LDM), Oswalt et al. 1996 (OSWH), and Leggett et al. 1997] provides constraints on the age and star-formation history of the Galactic disk (Winget et al. 1987 (WEA97), Wood 1992 (W92), Yuan 1992, Hernanz et al. 1994).

The maximum fractional surface layer masses that can remain following post-AGB evolution are log(qH) ~ —4 and log(qHe) ~ —2 (for a 0.6 Mʘ remnant—the values are inversely correlated with remnant mass), although we know from WD asteroseismology that surface H layer masses range from this value to smaller than logqH ~ —6 (e.g., Fontaine & Wesemael 1997). Some 75% ofWDs in the McCook & Sion (1987) catalog reveal pure-H photospheres (spectral type DA). The remainder are broadly classified as non-DAs, with most showing show pure-He spectra (DB) or continuous spectra (DC), and with ahandful that show mixed compositions (DAB), or C (DQ) or metal lines (DZ). See Bergeron et al. 1997 for a thorough discusion of the chemical evolutionof cool WDs.

Type
II. Joint Discussions
Information
Highlights of Astronomy , Volume 11 , Issue 1: Highlights of Astronomy. As presented at the XXIIIrd General Assembly of the IAU (Part A), 1997 , 1998 , pp. 427 - 429
Copyright
Copyright © Kluwer 1998

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