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Institut für Astronomie und Astrophysik

Abteilung Astronomie

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Preprint 10/96

Observational and Computational Results for Magnetic White Dwarfs

Susanne Friedrich, Peter Faßbinder, Wolfgang Schweizer

Institut für Astronomie und Astrophysik,Universität Tübingen, D-72076 Tübingen, Germany

Contribution to the 3rd Conference on Faint Blue Stars, Schenectady USA, October 13-17, 1996

Abstract. In order to calculate flux and circular polarization spectra of magnetic white dwarfs we investigated the line components of the hydrogen atom in external magnetic and electric fields relevant to white dwarf stars, by calculating the wavelengths and the oscillator strengths of all Ly alpha and H alpha to H gamma components. We could show that an additional electric field can cause considerable shifts of the wavelengths of the Balmer transitions. Moreover, the electric field causes some transitions forbidden in the diamagnetic case to turn into allowed transitions, thus yielding further line components. These wavelengths and oscillator strengths were used in a radiative transfer code to calculate model spectra for the rotating magnetic white dwarf KPD0253+5052. The best fit model so far is an offset dipole with a dipole field strength of 800 T.

The calculation of continuum states poses additional problems because they are not square-integrable, which we solved by complex rotating the Hamiltonian. The spectrum of the rotated Hamiltonian consists of real eigenvalues, which are the same as the bound states of the unrotated Hamiltonian, and discrete complex eigenvalues which are associated with the resonances of the real system. Photoionization cross-sections are obtained from the complex eigenvalues and 'rotated' eigenfunctions of the Hamiltonian. The electric field has been found to have a strong influence on the spectra of the pure magnetic field.

Paper (283k gzip'ed Postscript including figures)

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