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The extreme abundance and isotopic anomalies found in HgMn stars are usually assumed to be due to radiative levitation. Diffusion theory makes some qualitative predictions as to how the radiative force should vary as a function of depth in the atmosphere in order to explain the observations, but the lack of atomic data for the elements with the largest anomalies, such as mercury, platinum, and gold, has prevented the detailed calculations of the line opacity needed to check this hypothesis.
We have begun to collect the existing atomic data on relevant elements, and will extend this data base with our own atomic calculations. The high atomic charge of the elements of interest requires methods that include a careful treatment of both relativistic effects and correlation. For the bound-bound transitions that dominate the radiative force, this will require large-scale multiconfiguration Dirac-Fock calculations.
We are also beginning to develop techniques for using this new atomic data to perform time-dependant diffusion calculations in stellar atmospheres. We discuss the observed abundance and isotopic anomalies, the numerical difficulties which are likely to be encountered, and the use of opacity distribution functions to simplify the radiative force calculations.
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