AAS Meeting #193 - Austin, Texas, January 1999
Session 108. Gravitational Lensing
Display, Saturday, January 9, 1999, 9:20am-4:00pm, Exhibit Hall 1

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[108.13] The Colors of Halo White Dwarfs

D. Saumon, S. B. Jacobson (Vanderbilt U.)

Microlensing events observed in the line of sight toward the LMC suggest that a significant fraction of the mass of the dark halo of the Galaxy is composed of white dwarfs. In addition, white dwarf sequences have now been observed in the HR diagrams of several globular clusters and hold the promise of an independent determination of their ages. This halo population of white dwarfs is more than 10 Gyr old and these stellar cinders may have effective temperatures as low as 2000 K.

The direct identification of a MACHO white dwarf population and the determination of globular cluster ages from their white dwarf sequences both require evolutionary sequences, colors and bolometric corrections for white dwarfs older than 10 Gyr or equivalently, Teff < 4000 K.

We present the first detailed model atmospheres and spectra for very-cool hydrogen-rich white dwarfs in the halo of the Galaxy. We include the latest description of the opacities of hydrogen and significantly, we introduce a non-ideal equation of state in the atmosphere calculation. We find that due to strong absorption from H2 in the infrared, very old white dwarfs are brightest in the V, R, and I bands. They also become bluer in most color indices as they cool below Teff \approx 3500 K. As a consequence, the color-color diagrams for hydrogen-rich white dwarfs are rather peculiar.

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