Institut für Astronomie und Astrophysik Abteilung Astronomie Sand 1, D-72076 Tübingen, Germany
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Dissertation Jens Adamczak

X-ray spectroscopy of hot white dwarfs

Summary.
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.
White dwarfs are the final stage of evolution of low and intermediate mass stars ( 8-10M ). They consist of a carbon/oxygen core with surrounding layers of helium and hydrogen. Their atmospheres are characterized by a quasi monoele- mental composition that results from gravitational settling of heavy elements. In the case of the DA spectral class the lightest element, hydrogen, floats on top of the others. In hot DAs the temperature is high enough (>30 000 K) to almost completely ionize the hydrogen, thus, the opacity in the atmosphere is strongly reduced. Soft X-ray radiation coming from hot, deep, photospheric layers can be emitted from these objects and it was detected in the X-ray survey of the ROSAT telescope. The small number of detected WDs, however, lead to the conclusion that the atmospheres can be polluted by the opacity of heavy elements by radiative levitation. The strong dependence of this mechanism on the effective temperature results in only a few detected DA white dwarfs with temperatures above 60 000 K in the X-ray range. In order to analyze the spectra of these stars model atmo- spheres are used that calculate the abundances of the elements self-consistently at each depth of the atmosphere. These models are based on the assumption of an equilibrium of radiative and gravitational forces at each depth point. The result is a chemical stratification that is defined by only two free parameters, the effective temperature T eff and the surface gravity g. The spectral energy distribution (SED) of extreme ultraviolet (EUV) spectra of most DA white dwarfs can be described by such a stratification. The resolution of available EUV spectra, however, is too low to identify individual lines. In order to determine the metallicity of an object, therefore, models assuming a homogeneous mixture of elements in the atmosphere are used with an abundance pattern similar to that of the well studied white dwarf G 191-B2B. The actual metallicity of the object is then derived from scaling this specific pattern. An observed metallicity larger than predicted can for example be explained by accretion. The reasons for the lower metallicities in some objects are still unclear and are investigated in this thesis.
The hot DA white dwarf LB 1919 shows an untypically low abundance of heavy elements. A metallicity scaled relative to the one of G 191-B2B results in a too high opacity to reproduce the Extreme Ultraviolet Explorer (EUVE) spectrum of LB 1919 with a homogeneously mixed atmosphere and the stratified models fail as well. Many spectral lines of high ionization stages of the heavy elements iron and nickel are located in the X-ray wavelength range. These lines are the key to the explanation of the unusual metal poorness. X-ray observations of single white dwarfs are scarce and no detailed investigations of these objects exist. Therefore, the analysis presented in this work is breaking new ground.
The investigated Chandra spectrum of LB 1919 shows no evidence for the oc- currence of Fe and Ni in the atmosphere of this star. For a further analysis addi- tional observations in different wavelength ranges were considered. Using spectra of the Far Ultraviolet Spectroscopic Explorer (FUSE) various elements were iden- tified and their abundances were determined with models that assume a homoge- neously mixture of elements in the atmosphere. A temperature determination with model fits on the Lyman hydrogen lines in the ultraviolet shows that LB 1919 is around 10 000 K cooler than assumed. Considering only those elements that have been identified in the FUSE spectra of LB 1919 the analysis was extended to the EUV, the optical, and again the soft X-ray range. The results from the investigation with the homogeneously mixed models are always compared to calculations with stratified atmospheres with the same elements in order to make a statement about the suitability of both kinds of models for these type of stars. The restriction to the few elements that were found in the UV spectra turns out to be the crucial point in order to achieve a good fit quality. Obviously, some heavy elements like Fe and Ni are completely missing in the atmosphere of LB 1919. The greater success of the stratified model atmospheres in reproducing the SED of the observed spectra, especially in the EUV range, indicates that the equilibrium between gravitational and radiative forces is in good order. Therefore, unexpected and disturbing mech- anisms like convection or mass-loss can be excluded. This suggests that the cause for the metal deficiency of LB 1919 and comparable stars has to be found in an earlier evolutionary state and is subject of speculation.
Contrary to the DA white dwarfs, the PG 1159 stars undergo an unconventional development. Their evolution is characterized by a late thermal pulse that defines their unusual chemical composition. This pulse causes matter to be dredged up from the interior and results in a burning or dilution of the residual hydrogen. PG 1159 stars are also capable of emitting detectable amounts of X-ray radiation. Some of these stars pulsate and define the GW Virginis (GW Vir) instability strip in the Hertzsprung Russell Diagram (HRD). The position of the blue edge of this strip can be constrained by a pulsator/non-pulsator pair of spectroscopically very similar stars. The investigation of the temperature sensitive soft X-ray range of the non-pulsating star PG 1520+525 makes it possible to determine the effective temperature with a precision that cannot be achieved by the analysis of observations in the UV alone. The result is especially helpful for the refinement of pulsational models that predict the position of the instability strip in dependence of T_eff and log g.

Zusammenfassung
Spektroskopische Röntgenbeobachtungen von zwei heißen weißen Zwergen wurden vom Chandra Satelliten aufgenommen und ausgewertet. Das erste Objekt ist ein weißer Zwerg der Spektralklasse DA, dessen Atmosphäre fast ausschließlich aus Wasserstoff besteht. Im Gegensatz dazu finden sich in der Atmosphäre des zweiten Objekts, eines PG 1159 Sterns, keine signifikanten Mengen an Wasserstoff. Die Gegensätzlichkeit der beiden Objekte liegt in ihrer unterschiedlichen Entwicklungsgeschichte begründet.
Einige DA weiße Zwerge weisen eine Metallizität auf, die deutlich geringer ist als durch den Mechanismus des radiativen Auftriebs vorhergesagt. Viele Spektrallinien schwerer Elemente, die der Schlüssel zu einer Erklärung dieser ungewöhnlichen Metallarmut sind, lassen sich nur im Röntgen-Wellenlängenbereich finden. Einige PG 1159 Sterne pulsieren. Die Pulsationen hängen unter anderem von der Häufigkeit der Elemente in der Atmosphäre, log g und T eff ab. Der weiche Röntgenbereich ist besonders temperaturempfindlich und ermöglicht es, die Temperatur eines nicht pulsierenden PG 1159 Sterns präzise zu bestimmen. Dieses Objekt kann dann mit einem pulsierenden Stern mit ähnlichen spektroskopischen Eigenschaften verglichen werden. Bis jetzt gibt es keine detaillierten Analysen von Röntgenspektren einzelstehender weißer Zwerge.
Das Ziel dieser Dissertation war es, Spektren der DA weißen Zwerge LB 1919 und GD 246 in unterschiedlichen Wellenlängenbereichen zu analysieren, um herauszufinden, ob die Metalle in der Atmosphäre homogen verteilt oder chemisch geschichtet sind. Das liefert Erkenntnisse, um festzustellen, ob in der Atmosphäre unerwartete Prozesse auftreten, die das Gleichgewicht zwischen gravitativen und radiativen Kräften stören könnten. Ein zusätzliches Ziel war es, zum ersten Mal die chemische Zusammensetzung von LB 1919 zu bestimmen. Ein weiteres Ziel war die exakte Temperaturbestimmung des nicht pulsierenden PG 1159 Sterns PG 1520+525.
Die in unterschiedlichen Wellenlängenbereichen aufgenommenen Spektren von LB 1919 und GD 246 wurden mit homogenen und geschichteten Non-LTE Modellatmosphären analysiert. Das Chandra Spektrum des PG 1159 Sterns PG 1520+525 wurde ausschließlich mit homogenen Non-LTE Atmosphären untersucht, da eine Schichtung in diesem Falle nicht zu erwarten ist.
Die Ergebnisse zeigen, dass die Atmosphären von LB 1919 und GD 246 besser durch eine geschichtete Atmosphäre reproduziert werden können. Das deutet darauf hin, dass das Gleichgewicht zwischen gravitativen und radiativen Kräften intakt ist und dass der Grund für den Metallmangel von LB 1919 in einem früheren Entwicklungsstadium zu suchen ist. Die Temperatur von PG 1520+525 kann auf T_eff = 150 000 ± 5000 K eingeschränkt werden, das log g beträgt 7.5 ± 0.5.
 
Key words: X-ray spectroscopy, white dwarfs, PG1159-type stars, Chandra

Online-Publikation: http://nbn-resolving.de/urn:nbn:de:bsz:21-opus-51917

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