2011A&A...530A...7H

Gaseous metal discs around single white dwarfs have been discovered recently. They are thought to develop from disrupted planetary bodies. Spectroscopic analyses will allow us to study the composition of extrasolar planetary material. We investigate in detail the first object for which a gas disc was discovered (SDSSJ122859.93+104032.9). We perform non-LTE modelling of viscous gas discs by computing the detailed vertical structure and line spectra. The models are composed of carbon, oxygen, magnesium, silicon, calcium, and hydrogen with chemical abundances typical for Solar System asteroids. Line asymmetries are modelled by assuming spiral-arm and eccentric disc structures as suggested by hydrodynamical simulations. The observed infrared Caii emission triplet can be modelled with a hydrogen-deficient metal gas disc located inside of the tidal disruption radius, with T_eff≃6000K and a surface mass density of Σ≃0.3g/cm². The inner radius is well constrained at about 0.64R_☉. The line profile asymmetry can be reproduced by either a spiral-arm structure or an eccentric disc, the latter being favoured by its time variability behaviour. Such structures, reaching from 0.64 to 1.5R_☉, contain a mass of about 3-6x10²¹g, the latter equivalent to the mass of a 135-km diameter Solar System asteroid.