The first modern theory of white dwarf cooling was established by Mestel in 1952. In his famous paper, Mestel pictured the cooling as a gradual process powered by the energy stored in the thermal motions of the ions. He also assumed that the energy transport is totally due to radiation diffusion with Kramer’s opacity in the atmosphere. Although his picture is basically correct, it is apparent that the details aren’t very realistic. More realistic elements such as an opacity given by the surface chemical composition, detailed treatment in the equation of state, consideration of the advent of convection zone, crystallization and neutrino energy loss, have been used to improve the solution since 1952.
I am especially interested in giving a more detailed analysis regarding the evolution of its spectrum and surface chemical composition. The surface chemical composition affects energy outflow directly because it determines the opacity in the atmosphere, which in turn determines the size of convection zone and effective temperature. (given a specific core) However, as shown by spectroscopic observations (Bergeron, Leggett & Ruiz 2001), the surface chemical composition of white dwarf stars is not a constant but is evolving in a very complicated way. It was shown that during a Hubble time, the atmosphere of a white dwarf star can switch from hydrogen rich to helium rich, and back to hydrogen rich and then back to helium rich again. My advisor and I both feel that this behavior is related to the interplay between the size of convection zone(and consequent mixing of hydrogen and helium) and the accretion from interstellar medium.
As mentioned in the first paragraph, the knowledge on white dwarf cooling can help us estimate the age of stellar population. If our hypothesis on the effects of accretion turns out to be correct, the evolution of white dwarfs in the Galactic disk, halo and globular clusters will be significantly different due to their different accretion histories. The research result will give us a more accurate estimation of age on these stellar populations.
Recent observational work on white dwarf stars is showing unprecedented quality. In a couple of months, observations of the globular cluster NGC 6397 at the Hubble Space Telescope will be made. (HST Program GO-10424, PI: Harvey Richer) In addition, the Sloan Digital Sky Survey will be increasing the size of Galactic Disk white dwarf sample in the next few years. It is expected that within the next few years, more and more high quality observational results will be published and provide valuable input to the proposed theoretical work.
Click here to view a more thorough description of my research. (Actually my thesis proposal)