Parameter-free Stark Broadening of Hydrogen Lines in DA White Dwarfs

We present new calculations for the Stark broadening of the hydrogen line profiles in the dense atmospheres of white dwarf stars. Our improved model is based on the unified theory of Stark broadening from Vidal, Cooper & Smith, but it also includes non-ideal gas effects from the Hummer & Mihalas occupation probability formalism directly inside the line profile calculations. This approach improves upon previous calculations that relied on the use of an ad-hoc free parameter to describe the dissolution of the line wing opacity in the presence of high electric microfields in the plasma. We present here the first grid of model spectra for hot Teff >~ 12,000 K DA white dwarfs that has no free parameters. The atmospheric parameters obtained from optical and UV spectroscopic observations using these improved models are shown to differ substantially from those published in previous studies.Comment: 8 pages, 8 figures, to appear in Journal of Physics Conference Proceedings for the 16th European White Dwarf Worksho

FUSE observations of intermediate temperature DA: Atmospheric parameters and metal abundances

We present results from our analysis of a sample of DA white dwarfs having effective temperature below 25,000 K observed with the FUSE satellite with the goals of better understanding the origin of metals detected in the atmosphere of these stars. When possible, we combine the FUSE spectra with the IUE spectra and determine atmospheric parameters by fitting the Lyman line profiles. In general we find a good agreement with published values based on fits of the Balmer series. We observe that the continuum in the blue wing of the Lyman α line profile is generally lower in comparison with model spectra and that the discrepancy appears to become less important at higher effective temperature. The agreement between models and observations is excellent at wavelengths shorter than 1100 , which gives us confidence in the determination of atmospheric parameters. Finally, using adopted atmospheric parameters; we have performed a detailed analysis of the composition of these stars. In several instances, we have observed the presence of silicon and in one case that of carbon. For each star in the sample we have either measured or set an upper limit on the presence of key species such as CII, CIII, SiIII, and SiIV. We then compare the measured abundances with equilibrium abundance predicted by radiative levitation theory for each star. In this limited sample, we find that when detected, the abundance of silicon is in good agreement with theory. However there are several cases where the upper limits are smaller than the predictions and one case where it is considerably larger. © 2009 IOP Publishing Ltd

Far ultraviolet spectroscopy of hot DA white dwarfs: The case of silicon

With its unprecedented sensitivity and spectral resolution, FUSE has been a source of important data that have helped us further our knowledge and understanding of white dwarf stars. In order to illustrate the crucial contribution of FUSE to the study of white dwarfs, we present an abundance analysis of Si based on FUSE observations of 83 hydrogen-rich (DA) white dwarfs. Studying the Si abundances in the atmospheres of white dwarfs is important for understanding the mechanisms that counteract the downward diffusion of elements heavier than hydrogen. The FUSE observations show that Si is present in about 25% of white dwarfs. The Si abundances show a large scatter that is difficult to interpret in terms of a strict equilibrium between the gravitation and the radiative levitation. © 2009 American Institute of Physics

FUSE observations of intermediate temperature DA: Atmospheric parameters and metal abundances

We present results from our analysis of a sample of DA white dwarfs having effective temperature below 25,000 K observed with the FUSE satellite with the goals of better understanding the origin of metals detected in the atmosphere of these stars. When possible, we combine the FUSE spectra with the IUE spectra and determine atmospheric parameters by fitting the Lyman line profiles. In general we find a good agreement with published values based on fits of the Balmer series. We observe that the continuum in the blue wing of the Lyman α line profile is generally lower in comparison with model spectra and that the discrepancy appears to become less important at higher effective temperature. The agreement between models and observations is excellent at wavelengths shorter than 1100 , which gives us confidence in the determination of atmospheric parameters. Finally, using adopted atmospheric parameters; we have performed a detailed analysis of the composition of these stars. In several instances, we have observed the presence of silicon and in one case that of carbon. For each star in the sample we have either measured or set an upper limit on the presence of key species such as CII, CIII, SiIII, and SiIV. We then compare the measured abundances with equilibrium abundance predicted by radiative levitation theory for each star. In this limited sample, we find that when detected, the abundance of silicon is in good agreement with theory. However there are several cases where the upper limits are smaller than the predictions and one case where it is considerably larger. © 2009 IOP Publishing Ltd

FUSE observations of PG1342+444 : new insights into the nature of the hottest DA white dwarfs

‘The definitive version is available at www.blackwell-synergy.com '. Copyright Blackwell Publishing. DOI: 10.1046/j.1365-8711.2002.05082.x [Full text of this article is not available in the UHRA]We present Far Ultraviolet Spectroscopic Explorer (FUSE) observations of the very hot (Teff≈60 000 K) DA white dwarf PG1342+444, describing our data reduction and analysis techniques. The spectrum reveals a number of photospheric absorption lines from high ionization species along with numerous interstellar features. The photospheric detections include the 1031.9- and 1037.0-Å O vi lines which are seen for the first time in a hot DA atmosphere and are usually associated with the much hotter PG1159 stars and so-called O vi central stars of planetary nebulae. Estimates of the stellar effective temperature made independently using both the Balmer and Lyman series lines are in disagreement (Teff≈67 000 and ≈54 000 K respectively), when taking into account just the statistical uncertainties in the analyses. However, the presence of weak absorption from the C iii multiplet near 1176 Å, which is predicted to be much stronger if the star were as cool as the Lyman measurement suggests, leads us to favour the higher temperature. PG1342+444 appears to have enhanced C, Fe and Ni abundances in its atmosphere compared with all the other G191−B2B-like DA white dwarfs, which might affect the temperature structure of the atmosphere if not homogeneously distributed, as assumed in this study.Peer reviewe