The distribution of metals in hot DA white dwarfs

TL;DR

This study re-analyzes metal abundances in hot DA white dwarfs, finding that homogeneous nitrogen models generally fit the observed data better, but some stars like PG 0948+534 present modeling challenges.

Contribution

It provides a detailed re-analysis of nitrogen line profiles in specific hot white dwarfs, clarifying model degeneracies and challenging previous assumptions about stratified atmospheres.

Findings

Homogeneous nitrogen models better fit most observed stars.

Degeneracy exists in modeling certain stars' nitrogen profiles.

Deep line profiles in PG 0948+534 remain unexplained.

Abstract

The importance to stellar evolution of understanding the metal abundances in hot white dwarfs is well known. Previous work has found the hot DA white dwarfs REJ 1032+532, REJ 1614-085 and GD 659 to have highly abundant, stratified photospheric nitrogen, due to the narrow absorption line profiles of the FUV N V doublet and the lack of EUV continuum absorption. A preliminary analysis of the extremely narrow, deep line profiles of the photospheric metal absorption features of PG 0948+534 suggested a similar photospheric metal configuration. However, other studies have found REJ 1032+532, REJ 1614-085 and GD 659 can be well described by homogeneous models, with nitrogen abundances more in keeping with those of white dwarfs with higher effective temperatures. Here, a re-analysis of the nitrogen absorption features seen in REJ 1032+532, REJ 1614-085 and GD 659 is presented, with the aim of better understanding the structure of these stars, to test which models better represent the observed data and apply the results to the line profiles seen in PG 0948+534. A degeneracy is seen in the modelling of the nitrogen absorption line profiles of REJ 1032+532, REJ 1614-085 and GD 659, with low abundance, homogeneously distributed nitrogen models most likely being a better representation of the observed data. In PG 0948+534, no such degeneracy is seen, and the enigmatically deep line profiles could not be modelled satisfactorially.