Advances in atomic data for neutron-capture elements

N. C. Sterling (1), M. C. Witthoeft (2), P. C. Stancil (3), D. A. Esteves (4), R. C. Bilodeau (5,6), R. L. Porter (3), A. Aguilar (5)

(1) Michigan State U., (2) NASA/GSFC, (3)U. Georgia, (4) U. Colorado, (5) LBNL/ALS, (6) Western Michigan U.

Neutron n-capture elements (atomic number Z > 30), which can be produced in planetary nebulae (PNe) progenitor stars via s-process nucleosynthesis, have been detected in nearly 100 PNe. This demonstrates that nebular spectroscopy is a potentially powerful tool for studying the production and chemical evolution of trans-iron elements. However, significant challenges must be addressed before this goal can be achieved. One of the most substantial hurdles is the lack of atomic data for n-capture elements, particularly that needed to solve for their ionization equilibrium (and hence to convert ionic abundances to elemental abundances).

To address this need, we have computed multi-configuration distorted-wave photoionization (PI) cross sections and radiative recombination (RR) and dielectronic recombination (DR) rate coefficients for the first six ions of Se and Kr (similar calculations are ongoing for Xe). The calculations were benchmarked against experimental PI cross section measurements conducted at the Advanced Light Source synchrotron radiation facility. We estimate the internal uncertainties to be 30-50% for PI cross sections,≤10% for RR, and from 20% to two orders of magnitude for DR rate coefficients.

In addition, we computed charge transfer (CT) rate coefficients for ions of six n-capture elements using multi-channel Landau Zener and Demkov codes. We are incorporating these data into the photoionization code Cloudy to derive ionization correction factors for Se and Kr, and will test the sensitivity of abundance determinations to atomic data uncertainties via Monte Carlo simulations. These efforts will enable the accurate determination of nebular Se, Kr, and Xe abundances, allowing robust investigations of s-process enrichments in PNe.

Poster presented at IAU Symposium 283, Planetary Nebulae: an Eye to the Future, 25-29 July 2011, Tenerife, Spain.

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