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Preprint 01/11

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

S. Hartmann, T. Nagel, T. Rauch, K. Werner

Institute for Astronomy and Astrophysics, University of Tuebingen, Germany

To be published in: A&A

Abstract. Context:
Gaseous metal discs around single white dwarfs were recently discovered. They are thought to originate from disrupted planetary bodies.

Aims:
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).

Methods:
We perform non-LTE modeling 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 modeled by assuming spiral-arm and excentric disc structures as suggested by hydrodynamical simulations.

Results:
The observed infrared Ca II emission triplet can be modeled with a hydrogen-deficient metal gas disc located inside of the tidal disruption radius, with T_eff~6000 K and a surface mass density of ~0.3 g/cm³. The inner radius is well constrained at about 0.64 R_sun The line profile asymmetry can be reproduced by either a spiral-arm structure or an excentric disc, the latter being favored by its time variability behaviour. Such structures, reaching from 0.64 to 1.5 R_sun contain a mass of about 3--6 10²¹ g, the latter corresponding to the mass of asteroid (8) Flora.
 

Preprint (589 kb PDF file including figures)

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