Atomic data for neutron-capture elements

II. Photoionization and recombination properties of low-charge krypton ions^⋆

Michigan State UniversityDepartment of Physics and Astronomy, 3248 Biomedical Physical Sciences, East Lansing MI 48824-2320, USA
e-mail: sterling@pa.msu.edu

Received: 13 June 2011
Accepted: 11 July 2011

Abstract

We present multi-configuration Breit-Pauli distorted-wave photoionization (PI) cross sections and radiative recombination (RR) and dielectronic recombination (DR) rate coefficients for the first six krypton ions. These were calculated with the AUTOSTRUCTURE code, using semi-relativistic radial wavefunctions in intermediate coupling. Kr has been detected in several planetary nebulae (PNe) and H II regions, and is a useful tracer of neutron-capture nucleosynthesis. PI, RR, and DR data are required to accurately correct for unobserved Kr ions in photoionized nebulae, and hence to determine elemental Kr abundances. PI cross sections have been determined for ground configuration states of Kr⁰−Kr⁵⁺ up to 100 Rydbergs. Our Kr⁺ PI calculations were significantly improved through comparison with experimental measurements. RR and DR rate coefficients were respectively determined from the direct and resonant PI cross sections at temperatures (10¹−10⁷)z² K, where z is the charge. We account for Δn = 0 DR core excitations, and find that DR is the dominant recombination mechanism for all but Kr⁺ at photoionized plasma temperatures. Internal uncertainties are estimated by comparing results computed with three different configuration-interaction expansions for each ion, and by testing the sensitivity to variations in the orbital radial scaling parameters. The PI cross sections are generally uncertain by 30−50% near the ground state thresholds. Near 10⁴ K, the RR rate coefficients are typically uncertain by less than 10%, while those of DR exhibit uncertainties of factors of 2 to 3, due to the unknown energies of near-threshold autoionizing resonances. With the charge transfer rate coefficients presented in the third paper of this series, these data enable robust Kr abundance determinations in photoionized nebulae for the first time, providing a new tool for studying heavy element enrichments in PNe and for investigating the chemical evolution of trans-iron elements.