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.
Highlights
As the end products of the lives of the vast majority of stars, a thorough understanding of white dwarfs is crucial to our understanding of stellar evolution
Barstow et al (2003) stated that preliminary calculations suggested that the metals in the photosphere of the star was stratified and, in light of the investigation into the photospheric nitrogen in REJ 1032+532, REJ 1614-085 and GD 659, PG 0948+534 is investigated to see whether its enigmatic absorption line profiles can be modelled using stratified metal configurations
A degeneracy was seen in the modelling of photospheric nitrogen in REJ 1032+532, REJ 1614-085 and GD 659
Summary
As the end products of the lives of the vast majority of stars, a thorough understanding of white dwarfs is crucial to our understanding of stellar evolution. Dreizler (1999) developed a method of modelling the EUV spectral region of hot white dwarfs, where the chemical abundance at each depth point was that produced by the equilibrium between radiative levitation and downward diffusion (with depth dependent radiation intensity). Distributed nitrogen with an abundance of N(N)/N(H)=5x10−5 gave model line profiles with a depth similar to those observed, though they were pressure broadened beyond the observed profiles (figure 1, lower curve). Barstow et al (2003) stated that preliminary calculations suggested that the metals in the photosphere of the star was stratified and, in light of the investigation into the photospheric nitrogen in REJ 1032+532, REJ 1614-085 and GD 659, PG 0948+534 is investigated to see whether its enigmatic absorption line profiles can be modelled using stratified metal configurations
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