Other
Scientific paper
Oct 2010
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010phdt.........2a&link_type=abstract
PhD Thesis, Eberhard Karls Universität Tübingen, 2010
Other
White Dwarfs, X-Ray Spectroscopy, Pg1159-Type Stars, Stellar Evolution
Scientific paper
X-ray spectra of two hot white dwarfs observed by the Chandra satellite have been analyzed. The first is a white dwarf of spectral class DA with an almost pure hydrogen atmosphere. Contrary to that, the atmosphere of the second object, a PG 1159 star, is basically hydrogen free. The reason for the different composition can be found in the differing evolution of these objects.
Some DA white dwarfs show much smaller metallicities than predicted by the mechanism of radiative levitation. Many spectral lines of the heavy elements that are the key to the explanation to the unusual metal poorness are located in the X-ray wavelength range. Some PG 1159 stars are non-radial g-mode pulsators. The pulsations depend amongst others on the abundances of the elements in the atmosphere, log g, and T eff. The soft X-ray range is particularly temperature sensitive and allows to constrain the temperature of a non-pulsating PG 1159 star with respect to its pulsating spectroscopic twin. Detailed analysis of X-ray spectra of single white dwarfs do not yet exist.
The aim of this thesis was to analyze spectra of the DA white dwarfs LB 1919 and GD 246 in different wavelength ranges in order to find out if the metals in the atmospheres of these objects are homogeneously mixed or chemically stratified. This helps to identify or exclude possible unexpected mechanisms that might disturb the equilibrium between gravitational and radiative forces in the atmosphere. For LB 1919 an additional aim was to identify photospheric features of several elements and determine their abundances for the first time. It was further intended to determine the temperature of the non-pulsating PG 1159 star PG 1520+525 precisely.
The spectra of LB 1919 and GD 246 ranging from X-ray to optical wavelengths were analyzed with advanced homogeneous and stratified Non-LTE model atmospheres. The Chandra spectrum of the PG 1159 star PG 1520+525 was analyzed with homogeneous Non-LTE model atmospheres only since no stratification is expected.
The results show that the atmospheres of LB 1919 and GD 246 can be better reproduced by stratified model atmospheres. This indicates that the equilibrium between radiative levitation and gravitational settling is in good order and that the cause for the metal poorness of LB 1919 has to be found in an earlier evolutionary stage. The temperature of PG 1520+525 can be constrained to T eff = 150 000 ± 5000 K, the log g is 7.5 ± 0.5.
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