Radiative Levitation in Hot White Dwarfs

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Session 46 -- White Dwarfs
Display presentation, Tuesday, 10, 1995, 9:20am - 6:30pm

[46.04] Radiative Levitation in Hot White Dwarfs

P.Chayer (CEA/UCB), G.Fontaine, F.Wesemael (Univ.\ Montr\'eal)

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,$000 \ge T_{\rm{eff}} \ge 20$,000~K in steps of 2,500~K. The DO/DB grid is similar but extends to $T_{\rm{eff}} = 130$,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 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. Finally, we indicate promising avenues for further progress in spectral evolution theory for white dwarfs. This work has been supported by NASA contract NAS5-30180.

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