The behavior of hydrogen and helium at white dwarf photosphere conditions




Schaeuble, Marc-Andre

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White Dwarfs (WDs) represent the evolutionary endpoint for nearly all stars in the universe. Their atmospheres are usually dominated by either hydrogen, helium, or carbon. In astrophysics, WDs are used to determine the age of the universe, to test current stellar evolutionary models, and to constrain the physics of Type Ia supernovae as well as dark energy. Accurate WD masses are needed for all of these applications. In this thesis, I review the weaknesses of the currently available WD mass determination methods and show that for hydrogen-dominated WDs, such deficiencies can be traced to an incomplete understanding of H line shapes at the atmospheric temperaturesand densities (T [subscript e] ~ 1 eV, n [subscript e] ~ 1x10¹⁷ cm⁻³) of those stars. I further demonstrate that helium WD mass derivation techniques suffer from uncertain Stark shift and neutral broadening models, while C WD masses are suspect due to unverified electron broadening parameters. Addressing each of these atomic physics limitations requires both experimental data and theoretical developments. The White Dwarf Photosphere Experiment (WDPE) at Sandia National Laboratories Z-machine delivers laboratory data directed at solving the mass determination problems for WDs. For hydrogen line shapes, I find that fundamental assumptions regarding emission and absorption processes may not be valid. Helium WD mass derivations are hampered by the inability of current models to reproduce my experimental He shift and neutral broadening measurements. Since C WDs are at higher temperatures than H or He WDs, these investigations are limited to proof-of concept experiments. All my WDPE results have led to additional questions about fundamental physical assumptions that are relevant not only to stellar atmospheres and the experimental platform, but all of plasma physics. I discuss each of these aspects in the context of atomic, plasma, and WD physics.



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