New Level Resolved Ground and Excited State Pb III, IV, V & VI Photoionization Cross Sections for Heavy Metal Subdwarf Modeling

Abstract

High abundances of various lead (Pb) species have been identified in the spectra of many Asymptotic Giant Branch (AGB) stars and O- and B-type subdwarfs (sdO/B). Additional atomic data relating to Pb, and in particular photoionization cross sections, are needed to allow a greater understanding of the origin of these observed Pb abundances, and hence discern the evolutionary pathway of these stars. We have calculated level-resolved photoionization cross sections for Pb III, IV, V and VI. Four new target structures have been developed with the General Relativistic Atomic Structure Package (GRASP0), whose corresponding energy levels, Einstein A-coefficients and oscillator strengths have been found to be in good agreement with previous experimental and theoretical sources. The photoionization cross sections calculated using the Dirac Atomic R-matrix Codes (DARC) are available in TOPBASE format, and follow the trends expected for an isonuclear series. These new Pb data sets will now allow for the modelling of Pb abundances and line opacities under Non-Local Thermodynamic Equilibrium (non-LTE) conditions. Using the helium-rich hot subdwarf EC 22536-5304 as a test case, we show that there are noticeable differences in the Pb line profiles across the ultraviolet and optical wavelength regions under LTE and non-LTE conditions. There is both depletion and enrichment of individual Pb species. This highlights the importance of applying non-LTE conditions when modelling EC 22536-5304, as well as other O/B-type stars.