Pure Hydrogen Model Atmospheres for Very Cool White Dwarfs

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Abstract
Microlensing events observed in the line of sight toward the LMC indicate that a significant fraction of the mass of the dark halo of the Galaxy is probably composed of white dwarfs. In addition, white dwarf sequences have now be observed in the H-R diagrams of several globular clusters. Because of the unavailability of white dwarf atmospheres for Teff < 4000 K, the cooling timescales for white dwarfs older than ≈10 Gyr are very uncertain. Moreover, the identification of a MACHO white dwarf population by direct observation depends on a knowledge of the colors and bolometric corrections of very cool white dwarfs. In this Letter, we present the first detailed model atmospheres and spectra of very cool hydrogen white dwarfs for Teff < 4000 K. We include the latest description of the opacities of hydrogen, and, significantly, we introduce a nonideal 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, and we confirm that they become bluer in most color indices as they cool below Teff ≈ 3500 K.
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