Astronomy and Astrophysics – Astrophysics
Scientific paper
Jun 1997
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997phdt..........v&link_type=abstract
Ph.D. Thesis Universitaet Göttingen, Germany
Astronomy and Astrophysics
Astrophysics
6
Binaries: Eclipsing, Novae, Cataclysmic Variables, Accretion, Accretion Discs
Scientific paper
This work presents a new tomography algorithm, the Physical Parameter Eclipse Mapping method. It has been designed to reconstruct the structure of accretion disks in cataclysmic variables in terms of the basic physical parameters. Cataclysmic variables are close interacting binaries, in which mass transfer from one of the stars, typically a main sequence star, to the other star, a white dwarf, proceeds via an accretion disk around the white dwarf. Accretion disks are of general importance in astrophysics, since they occur in a number of objects with mass accretion, like active galactic nuclei and young stellar objects. The eclipsing cataclysmic variables are ideal systems to study such accretion process, since with the varying orbital phase different parts of the accretion disk can be viewed. The tomography method is based on the classical Eclipse Mapping algorithm which yields intensity distributions in the accretion disk by fitting the observed eclipse light curve. In order to avoid ambiguities this back-projection is using a maximum entropy algorithm, with selects the smoothest solution still compatible with the data. In my new method the intensity distributions are replaced by distributions of physical parameters, using a specific theoretical model spectrum to fit a set of eclipse light curves at various wavelengths. The spectrum is chosen according to the type of cataclysmic variable under investigation and its state at the time of observation. This work shows through application to synthetic data that with such an approach given distributions in physical parameters can be reproduced, as long as the parameters assume values in the parameter space outside of regions where ambiguities arise. Versions with two simple models are tested, but in principle this method can cope with any given model spectrum. The Physical Parameter Eclipse Mapping method is applied to two sets of real data of the dwarf nova IP Pegasi on decline from outburst and HT Cassiopeiae in quiescence. In spite of the simple assumptions regarding the applied models, good fits to the observations are achieved, leading to reconstructed distributions of the temperature and, for the optically thin accretion disk in HT Cassiopeiae, additionally the surface density. In addition, emission lines of HT Cassiopeiae are analysed with the classical Eclipse Mapping method to reconstruct intensity distributions. The comparison to continuum light distributions shows that the emission lines in the outer parts of the disk transform into absorption towards the disk centre.
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