The metal content of hot DA white dwarf spectra

TL;DR

This study analyzes high ion metal absorption features in hot DA white dwarf spectra, resolving previous conflicting results on nitrogen content and searching for circumstellar gas, but finds no definitive evidence of circumstellar material.

Contribution

It introduces a more advanced technique for measuring column densities of non-photospheric material and clarifies the nature and origin of circumstellar features in hot DA white dwarfs.

Findings

Nitrogen abundances are consistent with higher Teff DAs.

No circumstellar gas discs were detected in the sample.

Column densities align with predictions for white dwarf Stromgren Spheres.

Abstract

A study of high ion metal absorption features present in the spectra of hot DA white dwarfs is presented. An analysis of three DAs is performed, where previous studies came to conflicting conclusions as to the stars' nitrogen configurations. The nitrogen abundances were found to be in keeping with DAs of higher Teff, with a homogeneous distribution. A search for circumstellar gas discs was performed on eight stars, where circumstellar pollution may explain the differences between predicted and observed metal abundances. No positive detections were made. Already the subject of previous studies, the circumstellar absorption features seen at many hot DAs were again analysed, using a more advanced technique than those implemented in previous studies. This allowed, for the first time, column density measurements for all non-photospheric absorbing material. The derived column density measurements are consistent with those predicted to exist in white dwarf Stromgren Spheres, and the velocities of the absorbing material are not far from the velocities of either the observed ISM or predicted LISM clouds along the stars' sight lines. However, given the distances to some of the stars, it is unlikely that the ionised material resides in the LISM in all cases; it may however be loosely related to it. The observations here could not conclusively rule out the ionisation of circumstellar material about the stars, though no evidence for such material has yet been found. The velocity of the circumstellar material at WD2218+706 is inconsistent with the expansion velocity of the PN at the star, implying that the circumstellar material does not reside in the PN, though it may have originated there. Once though to be related to these circumstellar features, mass loss at the DAs has been ruled out, since the high log g of these stars prohibits the loss of significant mass in a stellar wind.