AAS 204th Meeting, June 2004
Session 21 Moon, Mercury, Vesta and Late Stellar Evolutio
Oral, Monday, May 31, 2004, 10:00-11:30am, 610/612

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[21.05] Fluid Atmospheres of Cool White Dwarfs

P. Kowalski (Vanderbilt University, & Applied Physics Division, LANL)

We investigate quantitatively for the first time the dense fluid effects in the surface layers of very cool white dwarf stars. In general, the gas is so tenuous in stellar atmospheres that non-ideal gas effects are negligible. One important exception are the atmospheres of cool white dwarfs, especially those rich in helium, where temperature varies from \rm 1000K to \rm 10000K, the densities reach values as large as \rm 2 \ g/cm3, and pressure is as high as \rm 1 \ Mbar. Under such conditions, the atmosphere is no longer an ideal gas, but must be treated as a dense fluid. New physical effects occur. Helium atoms become strongly correlated and refraction effects are present. Opacity sources, such as \rm He- free-free absorption, require different treatment from diluted gases. The refractive index departs from unity and can be as large as 1.35. We present the first solution of the radiative transfer in refractive atmospheres of cool white dwarfs. The importance of total internal reflection is discussed. We find that through the constraint of the radiative equilibrium, the total internal reflection warms the white dwarf atmosphere in optically thin surface regions. Strong curvature of rays results in a much weakened limb darkening effect.

This preliminary result suggests that dense fluid effects may have a significant impact on studies of very cool white dwarf stars.

This research was supported by NSF grant AST97-31438, NASA grant NAG5-8906, and by the United States Department of Energy under contract W-7405-ENG-36.

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