Non-LTE spectral models for the gaseous debris-disk component of Ton 345

TL;DR

This study models the non-LTE spectra of gaseous debris disks around white dwarfs, specifically analyzing the gas disk of Ton 345 to understand its composition, structure, and dynamics through spectral line profiles.

Contribution

It provides the first detailed non-LTE spectral models of a helium-dominated white dwarf's gas disk, revealing its narrow ring structure and challenging assumptions about chemical compositions.

Findings

Gas disk is a narrow ring with specific radius and temperature.

Standard chemical abundance assumptions produce unobserved spectral lines.

No short-term line-profile variability detected, but long-term asymmetry confirmed.

Abstract

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.