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

TL;DR

This study tackles the scarcity of reliable atomic data for multiply ionized lead in hot-star atmospheres by computing level-resolved photoionization cross sections for Pb III, IV, V, and VI. Using GRASP0 to build four Pb target structures and the Dirac R-matrix (DARC) to calculate cross sections, the authors provide data in TOPBASE format suitable for non-LTE spectral modeling. Benchmarking against NIST and prior work shows good agreement for energy levels and radiative rates, while the cross sections exhibit sensible trends along the isonuclear sequence. A test case for the Pb-rich subdwarf EC 22536-5304 demonstrates meaningful non-LTE effects on Pb line profiles, underscoring the importance of incorporating these data into atmospheric models to derive accurate Pb abundances and understand diffusion processes in hot stars.

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.

Figures (13)

• Figure 1: A comparison between the A-values arising from our Pbiv model and those computed in Alonso-Medina_2011.
• Figure 2: A comparison of the oscillator strengths computed from our Pbiv model with those experimentally determined in Andersen_1972 and Ansbacher_1988, and theoretically calculated in Migdalek_2000, Safronova_2004, Glowacki_2009 and Alonso-Medina_2011.
• Figure 3: A comparison between the A-values from our Pbv model with those calculated in Colon_2014.
• Figure 4: A comparison of the A-values corresponding to electric dipole transitions arising from our Pbvii model, considering both length and velocity gauge treatments.
• Figure 5: Comparison between the energy levels derived from our grasp$^0$ models of a) Pbiv, b) Pbv, c) Pbvi & d) Pbvii, and their equivalent values in NIST and other literature sources. The experimental sources for each species were: Pbiv Moore_1971Gutmann_1973Raassen_1991, Pbv Gutmann_1969Joshi_1990Wyart_1992, Pbvi Raassen_1990_PbVI and Pbvii Raassen_1990_PbVIIRaassen_1994. The shaded gray regions indicate a $\leq$5% (light), $\leq$10% (medium) and $\leq$15% (dark) deviation from the black dashed line of equality.