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Daytona Beach


Physical Sciences

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I have undertaken a literature search through 1997 July 31 of white dwarfs (WDs) in open and globular clusters. I have tried to make a careful evaluation in each case of the likelihood that the object is a WD and that it is a cluster member. The results are presented for 13 open clusters and 11 globular clusters. Currently there are 36 single WDs and Ðve WDs in binaries known among the open clusters, and 340 single WDs and 11 WDs in binaries known among the globular clusters. From these data, I have calculated WD mass fractions for four open clusters (the Pleiades, NGC 2168, NGC 3532, and the Hyades) and one globular cluster (NGC 6121). I develop a simple model of cluster evolution that incorporates stellar evolution but not dynamical evolution to interpret the WD mass fractions. I augment the results of my simple model by turning to sophisticated N-body simulations incorporating stellar evolution. I Ðnd that even though these clusters undergo a range of degrees of kinematic evolution, from moderate (the Pleiades, NGC 2168, and NGC 3532) to strong (the Hyades and NGC 6121), the WD mass fraction is relatively insensitive to kinematic evolution and little changed from a model incorporating only stellar evolution with a Salpeter-like initial mass function. By comparing the cluster mass functions with that of the Galactic disk, and incorporating plausibility arguments for the mass function of the Galactic halo, I estimate the WD mass fraction in these two Ðeld populations. I assume the Galactic disk is D10 Gyr old and that the Galactic halo is D12 Gyr old, although the WD mass fraction is insensitive to age within this regime. I Ðnd that the Galactic halo should contain from 8%È9% (a\[2.35) to perhaps as much as 15%È17% (a\[2.0) of its stellar mass in the form of WDs. The Galactic disk WD mass fraction should be 6% to 7% (for a median stellar age of 5 to 7 Gyr and a\[2.35), consistent with the empirical estimates of 3% to 7%.

Publication Title

The Astronomical Journal


Institute of Physics Publishing, Inc.

Grant or Award Name

NASA grant GO-6424, Edgar P. and Nona B. McKinney Charitable Trust