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| Paper: |
Probing the Atmospheres of Hot DA White Dwarfs with O VI |
| Volume: |
348, Astrophysics in the Far Ultraviolet: Five Years of Discovery with FUSE |
| Page: |
209 |
| Authors: |
Chayer, P.; Oliveira, C.; Dupuis, J.; Moos, H.W.; Welsh, B.Y. |
| Abstract: |
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. |
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