Absorption-line spectroscopy is a powerful tool used to estimate element abundances in both the nearby and distant universe. The accuracy of the abundances thus derived is naturally limited by the accuracy of the atomic data assumed for the spectral lines. We have recently started a project to perform new extensive atomic data calculations used for optical/UV spectral lines in the plasma modeling code Cloudy using state of the art quantal calculations. Here, we demonstrate our approach by focussing on S II, an ion used to estimate metallicities for Milky Way interstellar clouds as well as distant damped Lyman-alpha (DLA) and sub-DLA absorber galaxies detected in the spectra of quasars and gamma-ray bursts. We report new extensive calculations of a large number of energy levels of S II, and the line strengths of the resulting radiative transitions. Our calculations are based on the configuration interaction approach within a numerical Hartree-Fock framework, and utilize both non-relativistic and quasirelativistic one-electron radial orbitals. The results of these new atomic calculations are then incorporated into Cloudy and applied to a lab plasma, and a typical DLA, for illustrative purposes. The new results imply relatively modest changes (≈ 0.04 dex) to the metallicities estimated from S II in past studies. These results will be readily applicable to other studies of S II in the Milky Way and other galaxies.

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Published in The Astrophysical Journal, v. 780, no. 1, 76, p. 1-11.

© 2014. The American Astronomical Society. All rights reserved. Printed in the U.S.A.

The copyright holder has granted permission for posting the article here.

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This work is supported by the National Science Foundation grant AST/1109061 to the University of Kentucky and AST/1108830 to the University of South Carolina. V.P.K. also acknowledges partial support from the NSF (AST/0908890) and NASA (HST-GO-12536). G.J.F. acknowledges additional support by the NSF (AST/1108928), NASA (10-ATP10-0053, 10-ADAP10-0073, and NNX12AH73G), and STScI (HST-AR-12125.01, HST-AR- 13245, GO-12560, and HST-GO-12309). R.K.'s and P.B.'s research is partially funded the European Social Fund under the Global Grant measure, project VP1-3.1-ŠMM-07-K-02-013.

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