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Title: Heavy-element abundance patterns in hot DA white dwarfs
Authors: Barstow, Martin A.
Good, S. A.
Holberg, J. B.
Hubeny, I.
Bannister, N. P.
Bruhweiler, F. C.
Burleigh, Matthew R.
Napiwotzki, R.
First Published: 21-May-2003
Publisher: Oxford University Press (OUP), Royal Astronomical Society
Citation: Monthly Notices of the Royal Astronomical Society, 2003, 341 (3), pp.870-890
Abstract: We present a series of systematic abundance measurements for 25 hot DA white dwarfs in the temperature range ∼20 000–110 000 K, based on far-ultraviolet spectroscopy with the Space Telescope Imaging Spectrograph (STIS)/Goddard High Resolution Spectrograph (GHRS) on-board Hubble Space Telescope, IUE and FUSE. Using our latest heavy-element blanketed non-local thermodynamic equilibrium (non-LTE) stellar atmosphere calculations we have addressed the heavy-element abundance patterns, making completely objective measurements of abundance values and upper limits using a χ2 fitting technique to determine the uncertainties in the abundance measurements, which can be related to the formal upper limits in those stars where particular elements are not detected. We find that the presence or absence of heavy elements in the hot DA white dwarfs largely reflects what would be expected if radiative levitation is the supporting mechanism, although the measured abundances do not match the predicted values very well, as reported by other authors in the past. Almost all stars hotter than ∼50 000 K contain heavy elements. For most of these the spread in element abundances is quite narrow and similar to the abundances measured in G191-B2B. However, there is an unexplained dichotomy at lower temperatures with some stars having apparently pure H envelopes and others having detectable quantities of heavy elements. The heavy elements present in these cooler stars are often stratified, lying in the outermost layers of the envelope. A few strong temperature/evolutionary effects are seen in the abundance measurements. There is a decreasing Si abundance with temperature, the N abundance pattern splits into two groups at lower temperature and there is a sharp decline in Fe and Ni abundance to zero, below ∼50 000 K. When detected, the Fe and Ni abundances maintain an approximately constant ratio, close to the cosmic value of ∼20. For the hottest white dwarfs observed by STIS, the strongest determinant of abundance appears to be gravity.
DOI Link: 10.1046/j.1365-8711.2003.06462.x
ISSN: 0035-8711
Type: Article
Rights: This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society, Copyright 2003 RAS Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.
Appears in Collections:Published Articles, Dept. of Physics and Astronomy

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