The Physics of the Atmospheres of Cool White Dwarfs
The coolest white dwarfs known with effective temperatures below 5000K are among the oldest stars in the Milky Way. As they define the cutoff in the white dwarf luminosity function, they may be used as cosmochronometers. To achieve this goal we need reliable atmospheric models for these stars, as the atmosphere controls their cooling rate. The atmospheres of very cool white dwarfs, especially those rich in helium, reach densities typical of liquids (~1 g/cm3) at the photosphere. Under these physical conditions, the dilute gas physics typically used in atmosphere models is inadequate. The equation of state, radiative transfer, chemical equilibrium and opacities are all strongly affected by density effects. In this dissertation I have studied a number of these problems and incorporated their solutions in white dwarf atmosphere models: refractive radiative transfer, non-ideal dissociation/ionization equilibrium, the free-free opacity of He, pseudo-continuum bound-free absorption and extreme line broadening. I discuss these effects, especially their impact on the synthetic spectra and the photometry of cool white dwarfs. Using the new models for the analysis of the available observational data, including recent data release from the Sloan Digital Sky Survey (SDSS), I derive a new physical parameters (Teff, gravity, and He/H composition) of cool white dwarfs. I find that the coolest stars, previously thought to possess atmospheres highly enriched in helium in fact have hydrogen-rich atmospheres.