Astronomy and Astrophysics – Astronomy
Scientific paper
Mar 2005
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2005phdt.........1l&link_type=abstract
Dissertation, Eberhard-Karls-Universitaet Tuebingen, 2005.
Astronomy and Astrophysics
Astronomy
1
Stellar Astronomy, White Dwarf, Stellar Atmosphere Modelling, Diffusion, Element Abundance
Scientific paper
The atmospheres of white dwarfs exhibit a quasi-mono-elemental composition. Due to high surface gravities, element segregation by gravitational settling is of great importance so that a large fraction of all heavy elements in these stars has disappeared from the outer layers. This gravitational sedimentation happens on much smaller time scales than the evolution along the white dwarf cooling sequence. Observations of young white dwarfs in the X-ray and extreme ultra-violet spectral ranges have revealed that the settling cannot act undisturbed there because additional opacity is observed which must be attributed to metals remaining in their atmospheres: It was one interesting result of the all-sky survey performed by ROSAT that essentially no hydrogen-rich white dwarfs above effective temperatures of about 65000 K were found. This is being explained by radiative levitation in hot white dwarfs that can efficiently counteract the downward diffusion of heavy elements. The interplay between these forces governs the atmospheric chemical composition. Traces of metals may be sustained by radiative levitation provided the radiation field is intense enough to supply substantial momentum transfer against gravity's downward pull. This is the case in most objects hotter than T_eff 40000 K.
Incorporating the competition of these processes into stellar atmosphere model calculations provides predictions for the vertical stratification and absolute abundances of metals. The radiative acceleration is exerted on trace elements by a NLTE radiation field through the element's local opacity and therefore can vary strongly with depth, which results in a chemically stratified atmospheric structure. An overview of the modelling work done in the field of such diffusion calculations, with a focus on white dwarfs, is given, with the relevant references where these ideas were first mentioned as well as recent papers which describe current implementations and their applications. Within these latest approaches, the equilibrium formulation is of special interest since it permits a relatively simple description of the balance between gravitational settling and radiative levitation.
Such models are presented in this work. They take into account the interplay between gravitational settling and radiative acceleration to predict the chemical stratification from an equilibrium between the two forces while self-consistently solving for the atmospheric structure. In contrast to atmospheric models with the assumption of chemical homogeneity, the number of free parameters in the new models is reduced to the effective temperature and surface gravity alone. Their superiority over other diffusion calculations is the full self-consistency, calculated under NLTE conditions. The models are also fully line-blanketed and incorporate detailed atomic data up to the iron group elements.
The results from various theoretical diffusion calculations are compared. A grid of earlier, not self-consistent models as well as variations of the new models that differ in the numerical treatment of the radiative transfer and the temperature correction scheme as well as in the complexity of atomic input data are considered and found to yield different solutions when examined in detail. Based on a large diffusion model grid, a EUV selected sample of hot DA white dwarfs is analysed using this new type of atmospheric models. The overall good agreement with observed EUVE spectra reveals that these models are on the whole able to describe the physical conditions in hot DA white dwarf atmospheres.
The EUVE spectra are dominated by the opacity due to iron, so the real iron abundances appear to be relatively well matched by the predictions of radiative levitation theory. In a comparison of abundance measurements from individual lines in a comprehensive set of IUE and HST-STIS UV spectra with equilibrium abundance predictions, a similar conclusion is reached for iron, and to some extent also for oxygen and silicon, while the abundance levels for the elements carbon, nitrogen and nickel are not equally well reproduced over the full photospheric parameter range. To take into account effects such as known variability or accretion, all sample objects are discussed individually. The case of REJ1032+535 is given special consideration to demonstrate the potential difficulties in the analysis of stratified abundance profiles in an exemplary way.
One of the main constraints for the models seems to be that the theoretically unlimited reservoir of any element implied by the equilibrium approach is not available in reality. This should not be a problem when the metal abundances drawn from that reservoir correspond to almost negligible pollutants, but the assumption breaks down in those cases where the equilibrium abundance of an element starts to be of the same order of magnitude as its cosmic abundance.
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