Spectral modeling of gaseous metal disks around DAZ white dwarfs

We report on our attempt for the first non-LTE modeling of gaseous metal disks around single DAZ white dwarfs recently discovered by Gänsicke et al. and thought to originate from a disrupted asteroid. We assume a Keplerian rotating viscous disk ring composed of calcium and hydrogen and compute the detailed vertical structure and emergent spectrum. We find that the observed infrared Ca ii emission triplet can be modeled with a hydrogen-deficient gas ring located at R=1.2R⊙, inside of the tidal disruption radius, with Teff≈ 6000K and a low surface mass density of ≈ 0.3 g/cm. A disk having this density and reaching from the central white dwarf out to R = 1.2R⊙ would have a total mass of 7 ·10 21 g, corresponding to an asteroid with ≈160 km diameter.