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C. Proffitt (CUA/CSC/STScI)
HgMn stars show a wide variety of elemental abundance and ionization anomalies. Some of these anomalies are due to the effects of radiative acceleration and atmospheric stratification effects, while others may be simply be the result of non-LTE ionization equilibrium.
We discuss the physics that needs to be included to properly model these effects, and present detailed non-LTE models tailored to individual HgMn stars. The elements carbon, mercury, and gallium are singled out as examples, and predictions based on these non-LTE calculations are compared to observed line profiles. We find that the often observed underabundances of neutral species relative to singly ionized atoms of the same element can usually be explained by non-LTE effects. However, the overabundance of many doubly ionized species cannot be explain in this way, and we discuss whether or not these anomalies provide clear evidence for chemical stratification within the atmospheres of HgMn stars.
We wish to acknowledge support for this research from NASA grant NAG 5-8276.