Asteroseismology of White Dwarf Stars




Hall, Weston

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All single stars that are born with masses up to 8.5 - 10 solar masses will end their lives as a white dwarf (WD) star. Upon entering this evolutionary stage, WDs enter the cooling sequence, where the stars radiate away their thermal energy, and are basically cooling. As these stars cool, they reach temperatures and conditions that cause the stars to pulsate. Using only the light emitted from the surface of the WD, we can measure the periods of the pulsation from the WD. Analyzing these periods allows us to determine internal aspects about the WD such as temperature, mass, and chemical composition with a technique called asteroseismology. Since variable WDs are effectively the same as all other WDs (sans pulsation), this allows us to learn about the internal structure of all white dwarfs, and place important constraints on the WD cooling sequence, and stellar evolution of low mass stars as a whole. For this project, I used the White Dwarf Evolution Code (WDEC) to calculate a model grid of over 125,000 synthetic stars with various temperature, mass and atmospheric abundances of helium and hydrogen, and I calculated their theoretical pulsation periods. I then compared this grid to observed pulsating WDs by matching observed periods with the grid’s periods and using those solutions to ascertain information about the star. In this thesis, I describe my approach for this seismological study, and I compare my solutions to those obtained through spectroscopy, as well as previous asteroseismic studies in order to determine the effectiveness of my technique, and the effectiveness of an updated WDEC version.