Abstract

Near-UV absorption in very cool DA white dwarfs

Didier Saumon (Los Alamos National Laboratory)

J. Holberg (U. Arizona) P.M. Kowalski (Inst. of Energy & Climate Research, Germany)

The atmospheres of very cool, hydrogen-rich white dwarfs (Teff<6000K) are challenging to model because of the increased complexity of the equation of state, chemical equilibrium, and opacity sources in a low-temperature, dense gas that is mostly neutral. In particular, many models overestimate the flux in the blue part of the spectrum. A solution to this problem that has met with some success is that additional opacity at short wavelengths comes for the extreme broadening of the Lyman alpha line of atomic H by collisions primarily with H2 molecules. We present a more rigorous test of this model and derive the basic atmospheric parameters of several very cool white dwarfs. For this purpose, we acquired Hubble Space Telescope STIS spectra of 8 very cool white dwarfs (5 DA and 3 DC stars). Combined with their known parallaxes, BVRIJHK and Spitzer IRAC photometry, we analyze their entire spectral energy distribution (0.24 to 9.3 micron) with a large grid of model atmospheres and synthetic spectra. We find that the red wing of the Lyman alpha line reproduces the rapidly decreasing near-UV flux of these very cool stars very well. The analysis of the full SED of these stars also gives upper limits to the helium abundance in their atmospheres.

Mode of presentation: oral