AAS 205th Meeting, 9-13 January 2005
Session 52 Stellar Atmospheres, Abundances and Opacities
Poster, Tuesday, January 11, 2005, 9:20am-6:30pm, Exhibit Hall

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[52.13] New Opacities for Dense Helium and the Composition of Helium Rich, Very Cool White Dwarf Atmospheres

P. M. Kowalski (Vanderbilt Univ./LANL), S. Mazevet, D. Saumon (LANL)

Very cool white dwarfs (T\rm eff \rm \leq 4000K) are among the oldest stars in the Milky Way. They are of great interest as chronometers for understanding the history of star formation in our Galaxy. To realize the full potential of white dwarf cosmochronology, we need to understand better the physical processes that take place in the surface layers of cool white dwarfs. Strong surface gravity results in a compositionally stratified structure for those stars, with light elements "floating" to the surface. Accretion from the ISM over Gyrs should result in pure H atmosphere for all of them today, regardless of their initial composition. However, observations indicate that many very cool white dwarfs possess helium-rich atmospheres. Envelope models provide a possible explanation for this phenomenon, where He is transported to the atmosphere from the envelope by a convective zone which, for cool white dwarfs of T\rm eff \rm \leq 5000K, can extend from the surface down to the helium layer. However, an analysis based on current atmospheric models gives a He abundance that is much higher than can be explained by the convective mixing model. We think that one of the main reason for this discrepancy is an inadequate description of the opacity used in current atmosphere models. The very cool helium-rich atmospheres, with densities up to \rm 2 \ g/cm3, are fluid, not gaseous. The description of the opacity must be revised for this high density regime. Using quantum molecular dynamics simulations we calculated new opacities for dense helium that are much larger than previously thought. As a result, a much lower helium abundance is found in the coolest white dwarfs, which is in much better agreement with the predictions of the convective mixing model. This research was supported by the United States Department of Energy under contract W-7405-ENG-36.

The author(s) of this abstract have provided an email address for comments about the abstract: kowalski@lanl.gov

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