The atmospheres of very cool, hydrogen-rich white dwarfs (Teff <6000 K) are
challenging to models because of the increased complexity of the equation of
state, chemical equilibrium, and opacity sources in a low-temperature, weakly
ionized dense gas. In particular, many models that assume relatively simple
models for the broadening of atomic levels and mostly ideal gas physics
overestimate the flux in the blue part of their spectra. 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. For the purpose of validating this model
more rigorously, 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 (from 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. We determine better constrained values of Teff and gravity as well as
upper limits to the helium abundance in their atmospheres.Comment: 41 pages, 9 figures. Accepted for publication in the Ap
