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Gaseous metal discs around single white dwarfs have been discovered recently.They are thought to develop from disrupted planetary bodies. Spectroscopicanalyses will allow us to study the composition of extrasolar planetarymaterial. We investigate in detail the first object for which a gas disc wasdiscovered (SDSS J122859.93+104032.9). Therefor we perform non-LTE modelling ofviscous gas discs by computing the detailed vertical structure and linespectra. The models are composed of carbon, oxygen, magnesium, silicon,calcium, and hydrogen with chemical abundances typical for Solar Systemasteroids. Line asymmetries are modelled by assuming spiral-arm and eccentricdisc structures as suggested by hydrodynamical simulations. The observedinfrared Ca II emission triplet can be modelled with a hydrogen-deficient metalgas disc located inside of the tidal disruption radius, with an effectivetemperature of about 6000 K and a surface mass density of 0.3 g/cm^2. The innerradius is well constrained at about 0.64 Solar radii. The line profileasymmetry can be reproduced by either a spiral-arm structure or an eccentricdisc, the latter being favoured by its time variability behaviour. Suchstructures, reaching from 0.64 to 1.5 Solar radii, contain a mass of about 3 to6*10^21 g, the latter equivalent to the mass of a 135-km diameter Solar Systemasteroid.

Non-LTE models for the gaseous metal component of circumstellar discs around white dwarfs