Radiative Levitation in Hot White Dwarfs: Equilibrium Theory

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Abstract

We present the results of detailed calculations of radiative levitation in hot white dwarfs using the extensive and homogeneous atomic data given in TOPBASE. Radiative accelerations and equilibrium abundances have been computed for C, N, O, Ne, Na, Mg, Al, Si, S, Ar, Ca, and Fe on grids of pure hydrogen and pure helium stellar envelope models. The DA model grid has log g = 7.0, 7.5, 8.0, and 8.5, and spans the range of effective temperature 100,000 ≥ T_eff ≥ 20,000 K in steps of 2500 K. The DO/DB grid is similar but extends to T_eff = 130,000 K. We discuss at some length the input physics used in order to provide a good physical understanding of radiative levitation under white dwarf conditions. We also discuss the depth dependence and the morphology of the reservoirs of levitating elements created by an equilibrium between the radiative acceleration and the local effective gravity in various stellar envelopes. The important role played in the morphology of the reservoirs by dominant ionization states in closed-shell electronic configurations is emphasized. Our central results are presented in the form of figures showing the behavior of the expected photospheric abundance of each element as a function of effective temperature and surface gravity. While only a handful of abundances are available from the few analyses of observations that have been carried out, we are nevertheless able to infer through a detailed comparison that equilibrium radiative levitation theory fails to explain quantitatively the observed abundance patterns of heavy elements in hot white dwarfs. At least one other mechanism must be competing with radiative levitation and gravitational settling in the atmospheres/envelopes of hot white dwarfs. This basic conclusion is not affected by the inherent uncertainties and limitations of our calculations. Among those, we single out our neglect of the effects of trace pollutants in the background plasma, our use of a model envelope approach to estimate photospheric abundances, our relatively crude description of the momentum redistribution that an ion experiences following a photoexcitation, and our neglect of the atomic fine structure. These represent areas where improvements may be called upon in future calculations of radiative levitation in white dwarfs. We finally indicate promising avenues for further progress in this chapter of the theory of spectral evolution of white dwarfs.

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