Astronomy and Astrophysics – Astrophysics
Scientific paper
Jun 2006
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006aspc..348..209c&link_type=abstract
Astrophysics in the Far Ultraviolet: Five Years of Discovery with FUSE ASP Conference Series, Vol. 348, Proceedings of the Confe
Astronomy and Astrophysics
Astrophysics
4
Scientific paper
We present an abundance analysis of the oxygen detected in the FUSE spectra of three hot white dwarfs with effective temperatures lower than 50,000 K: GD984, REJ1032+532, and REJ2156-543. The FUSE data show unexpectedly strong O VI λλ1032 and 1038 lines. We demonstrate that stellar atmosphere models with homogeneous oxygen abundances neither reproduce the O VI doublet nor the Extreme Ultraviolet Explorer data. Our models show that some O VI ions are formed by photoionization in the upper layers of the atmosphere and are sensitive to the EUV opacity of O III and O IV at lower depths. Based on these findings we compute models that include an oxygen abundance that varies as a function of depth. We show that the oxygen abundance should be lower at depths where the O III and O IV ions predominate in order to reduce the opacity in the EUV range, and it should be higher in the upper layers to allow the production of strong O VI lines. Our results provide one of the best evidence that the abundance of an element heavier than hydrogen is stratified in the atmospheres of white dwarfs.
Chayer Pierre
Dupuis Jean
Moos Warren H.
Oliveira Cristina
Welsh Barry Y.
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