  For a fraction of single white dwarfs with debris disks, an additional
gaseous disk was discovered. Both dust and gas are thought to be created by the
disruption of planetary bodies. The composition of the extrasolar planetary
material can directly be analyzed in the gaseous disk component, and the disk
dynamics might be accessible by investigating the temporal behavior of the Ca
II infrared emission triplet, hallmark of the gas disk.
  We obtained new optical spectra for the first helium-dominated white dwarf
for which a gas disk was discovered (Ton 345) and modeled the non-LTE spectra
of viscous gas disks composed of carbon, oxygen, magnesium, silicon, sulfur,
and calcium with chemical abundances typical for solar system asteroids. Iron
and its possible line-blanketing effects on the model structure and spectral
energy distribution was still neglected. A set of models with different radii,
effective temperatures, and surface densities as well as chondritic and
bulk-Earth abundances was computed and compared with the observed line profiles
of the Ca II infrared triplet.
  Our models suggest that the Ca II emission stems from a rather narrow gas
ring with a radial extent of R=0.44-0.94 Rsol, a uniform surface density
Sigma=0.3 g/cm2, and an effective temperature of Teff=6000 K. The often assumed
chemical mixtures derived from photospheric abundances in polluted white dwarfs
- similar to a chondritic or bulk-Earth composition - produce unobserved
emission lines in the model and therefore have to be altered. We do not detect
any line-profile variability on timescales of hours, but we confirm the
long-term trend over the past decade for the red-blue asymmetry of the
double-peaked lines.
