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Title:
Evolutionary Calculations of Phase Separation in Crystallizing White Dwarf Stars
Authors:
Montgomery, M. H.; Klumpe, E. W.; Winget, D. E.; Wood, M. A.
Affiliation:
AA(McDonald Observatory and Department of Astronomy, University of Texas, Austin, TX 78712.; Institut für Astronomie, Universität Wien, Türkenschanzstraße 17, A-1180 Wien, Austria.), AB(McDonald Observatory and Department of Astronomy, University of Texas, Austin, TX 78712.), AC(McDonald Observatory and Department of Astronomy, University of Texas, Austin, TX 78712.), AD(Department of Physics and Space Sciences and SARA Observatory, Florida Institute of Technology, Melbourne, FL 32901-6988.)
Publication:
The Astrophysical Journal, Volume 525, Issue 1, pp. 482-491. (ApJ Homepage)
Publication Date:
11/1999
Origin:
UCP
Astronomy Keywords:
DENSE MATTER, EQUATION OF STATE, STARS: EVOLUTION, STARS: WHITE DWARFS, Dense Matter, Equation of State, Stars: Evolution, Stars: White Dwarfs
Abstract Copyright:
(c) 1999: The American Astronomical Society
DOI:
10.1086/307871
Bibliographic Code:
1999ApJ...525..482M

Abstract

We present an exploration of the significance of carbon/oxygen phase separation in white dwarf stars in the context of self-consistent evolutionary calculations. Because phase separation can potentially increase the calculated ages of the oldest white dwarfs, it can affect the age of the Galactic disk as derived from the downturn in the white dwarf luminosity function. We find that the largest possible increase in ages due to phase separation is ~1.5 Gyr, with a most likely value of approximately 0.6 Gyr, depending on the parameters of our white dwarf models. The most important factors influencing the size of this delay are the total stellar mass, the initial composition profile, and the phase diagram assumed for crystallization. We find a maximum age delay in models with masses of ~0.6 Msolar, which is near the peak in the observed white dwarf mass distribution. In addition, we note that the prescription that we have adopted for the mixing during crystallization provides an upper bound for the efficiency of this process, and hence a maximum for the age delays. More realistic treatments of the mixing process may reduce the size of this effect. We find that varying the opacities (via the metallicity) has little effect on the calculated age delays. In the context of Galactic evolution, age estimates for the oldest Galactic globular clusters range from 11.5 to 16 Gyr and depend on a variety of parameters. In addition, a 4-6 Gyr delay is expected between the formation of the globular clusters and the formation of the Galactic thin disk, while the observed white dwarf luminosity function gives an age estimate for the thin disk of 9.5+1.1-0.8 Gyr, without including the effect of phase separation. Using the above numbers, we see that phase separation could add between 0 and 3 Gyr to the white dwarf ages and still be consistent with the overall picture of Galaxy formation. Our calculated maximum value of <~1.5 Gyr fits within these bounds, as does our best-guess value of ~0.6 Gyr.
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