Inside Pulsating White Dwarfs: Clues from time-resolved spectroscopy

University dissertation from Lund Observatory

Abstract: Several analyses using time-resolved optical spectrocopy of pulsating white dwarfs are presented. The data have been put to a number of uses. First, line-of-sight velocities associated with the pulsations have been measured in three hydrogen-atmosphere white dwarfs (DAVs), bringing the total number of such measurements up to five. They have also been measured for the first time in a pulsating helium-atmosphere white dwarf (DBV). The relative velocity to flux amplitude ratios are compared with theoretical expectations based on theories of mode driving via convection. Secondly, the fractional, wavelength-dependent pulsation amplitudes (``chromatic amplitudes'') have been used to identify the spherical degree of the pulsation modes. Consistent results are obtained for objects with mode identifications based solely on photometry. This shows that the technique works. The new mode identifications can be used in pulsation models to constrain the asteroseismological solution of the white dwarf in question. The chromatic amplitudes have also been used to understand the shortcomings of currently used model atmospheres. These are most likely due to the lack of a realistic treatement of convection. Combining all the available information from data such as ours is a first step towards constraining atmospheric properties in a convectionally unstable environment from an observational perspective. In the final chapter an attempt is made to explain the observed quasi-periodic variations in the rotationally split modes of certain pulsators.

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