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Post Info TOPIC: Hot DB White Dwarfs
Blobrana


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RE: Hot DB White Dwarfs
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Title: A photospheric metal line profile analysis of hot DA white dwarfs with circumstellar material
Authors: Nathan J. Dickinson, Martin A. Barstow, Barry Y. Welsh

Some hot DA white dwarfs have circumstellar high ion absorption features in their spectra, in addition to those originating in the photosphere. In many cases, the line profiles of these absorbing components are unresolved. Given the importance of the atmospheric composition of white dwarfs to studies of stellar evolution, extra-solar planetary systems and the interstellar medium, we examine the effect of including circumstellar line profiles in the abundance estimates of photospheric metals in six DA stars. The photospheric C and Si abundances are reduced in five cases where the circumstellar contamination is strong, though the relative weakness of the circumstellar Si IV absorption introduces minimal contamination, resulting in a small change in abundance. The inability of previous, approximate models to reproduce the photospheric line profiles here demonstrates the need for a technique that accounts for the physical line profiles of both the circumstellar and photospheric lines when modelling these blended absorption features.

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Blobrana


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Posts: 131433
Date:
Hot DA white dwarfs
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Title: The distribution of metals in hot DA white dwarfs
Authors: N.J. Dickinson, M.A. Barstow, I. Hubeny

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 satisfactorily.

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Blobrana


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Posts: 131433
Date:
RE: Hot DB White Dwarfs
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Title: The fraction of DA white dwarfs with kilo-Gauss magnetic fields
Authors: S. Jordan, R. Aznar Cuadrado, R. Napiwotzki, H. M. Schmid, S. K. Solanki

Weak magnetic field has been searched for on only a small number of white dwarfs. Current estimates for white dwarfs with fields in excess of 1MG are about 10%; according to previous studies this number increases up to about 25% in the kG regime.
Our aim is to improve the statistics by a new sample of ten white dwarfs in order to determine the ratio of magnetic to field-free white dwarfs.
Mean longitudinal magnetic fields strengths were determined by means of high-precision circular polarimetry of Hbeta and Hgamma with the FORS1 spectrograph of the VLT ''Kueyen'' 8 m telescope.
In one of our objects (LTT 7987) we detected a statistically significant (97% confidence level) longitudinal magnetic field varying between (-1±0.5$)kG and (+1±0.5)kG. This would be the weakest magnetic field ever found in a white dwarf, but at this level of accuracy, systematic errors cannot completely be ruled out. We also observed the sdO star EC11481-2303 but could not detect a magnetic field.
VLT observations with uncertainties of typically 1000 G or less suggest that 15-20% of WDs have kG fields. Together with previous investigations, the fraction of kG magnetic fields in white dwarfs amounts to about 11-15%, which is close to the current estimations for highly magnetic white dwarfs (> 1MG).

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Blobrana


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Posts: 131433
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Title: Hot DB White Dwarfs from the Sloan Digital Sky Survey
Authors: Daniel J. Eisenstein, James Liebert, Detlev Koester, S.J. Kleinmann, Atsuko Nitta, Paul S. Smith, J.C. Barentine, Howard J. Brewington, J. Brinkmann, Michael Harvanek, Jurek Krzesinski, Eric H. Neilsen Jr., Dan Long, Donald P. Schneider, Stephanie A. Snedden

Researchers present ugriz photometry and optical spectroscopy for 28 DB and DO white dwarfs with temperatures between 28,000K and 45,000K. About 10 of these are particularly well-observed; the remainder are candidates.
These are the hottest DB stars yet found, and they populate the "DB gap" between the hotter DO stars and the familiar DB stars cooler than 30,000K. Nevertheless, after carefully matching the survey volumes, they find that the ratio of DA stars to DB/DO stars is a factor of 2.5 larger at 30,000 K than at 20,000 K, suggesting that the "DB gap" is indeed deficient and that some kind of atmospheric transformation takes place in roughly 10% of DA stars as they cool from 30,000 K to 20,000 K.

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