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

TL;DR

This paper models the gaseous metal discs around white dwarfs using non-LTE techniques, revealing their composition, structure, and dynamics, and explaining observed spectral features and asymmetries.

Contribution

It presents the first detailed non-LTE models of gas discs around white dwarfs, incorporating complex structures like spirals and eccentricities to match observations.

Findings

Inner radius constrained at 0.64 solar radii

Model reproduces infrared Ca II emission triplet

Discs contain mass comparable to a 135-km asteroid

Abstract

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 (SDSS J122859.93+104032.9). Therefor 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 Ca II emission triplet can be modelled with a hydrogen-deficient metal gas disc located inside of the tidal disruption radius, with an effective temperature of about 6000 K and a surface mass density of 0.3 g/cm^2. The inner radius is well constrained at about 0.64 Solar radii. 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 1.5 Solar radii, contain a mass of about 3 to 6*10^21 g, the latter equivalent to the mass of a 135-km diameter Solar System asteroid.