NLTE Strontium and Barium in Metal‐poor Red Giant Stars

We present atmospheric models of red giant stars of various metallicities,including extremely metal poor (XMP, [Fe/H]<-3.5) models, with many chemicalspecies, including, significantly, the first two ionization stages of Strontium(Sr) and Barium (Ba), treated in Non-Local Thermodynamic Equilibrium (NLTE)with various degrees of realism. We conclude that 1) for all lines that areuseful Sr and Ba abundance diagnostics the magnitude and sense of the computedNLTE effect on the predicted line strength is metallicity dependent, 2) theindirect NLTE effect of overlap between Ba and Sr transitions and transitionsof other species that are also treated in NLTE non-negligibly enhances NLTEabundance corrections for some lines, 3) the indirect NLTE effect of NLTEopacity of other species on the equilibrium structure of the atmospheric modelis not significant, 4) the computed NLTE line strengths differ negligibly ifcollisional b-b and b-f rates are an order of magnitude smaller or larger thanthose calculated with standard analytic formulae, and 5) the effect of NLTEupon the resonance line of Ba II at 4554.03 AA is independent of whether thatline is treated with hyperfine splitting. As a result, the derivation ofabundances of Ba and Sr for metal-poor red giant stars with LTE modeling thatare in the literature should be treated with caution.Comment: 28 pages, 10 figures. Accepted for publication in April 2006 Astrophysical Journa