Bootstrapping Dielectronic Recombination from Second-Row Elements and the Orion Nebula

Authors

N. R. Badnell, University of Strathclyde, UK
Gary J. Ferland, University of KentuckyFollow
T. W. Gorczyca, Western Michigan University
D. Nikolić, California Institute of Technology
Gururaj Anil Wagle, University of KentuckyFollow

Abstract

Dielectronic recombination (DR) is the dominant recombination process for most heavy elements in photoionized clouds. Accurate DR rates for a species can be predicted when the positions of autoionizing states are known. Unfortunately such data are not available for most third- and higher-row elements. This introduces an uncertainty that is especially acute for photoionized clouds, where the low temperatures mean that DR occurs energetically through very low-lying autoionizing states. This paper discusses S²⁺ → S⁺ DR, the process that is largely responsible for establishing the [S iii]/[S ii] ratio in nebulae. We derive an empirical rate coefficient using a novel method for second-row ions, which do have accurate data. Photoionization models are used to reproduce the [O iii]/[O ii]/[O i]/[Ne iii] intensity ratios in central regions of the Orion Nebula. O and Ne have accurate atomic data and can be used to derive an empirical S²⁺ → S⁺ DR rate coefficient at ~10⁴ K. We present new calculations of the DR rate coefficient for S²⁺ → S⁺ and quantify how uncertainties in the autoionizing level positions affect it. The empirical and theoretical results are combined and we derive a simple fit to the resulting rate coefficient at all temperatures for incorporation into spectral synthesis codes. This method can be used to derive empirical DR rates for other ions, provided that good observations of several stages of ionization of O and Ne are available.

Document Type

Article

Publication Date

5-6-2015

Notes/Citation Information

Published in The Astrophysical Journal, v. 804, no. 2, article 100, p. 1-10.

© 2015. The American Astronomical Society. All rights reserved.

Reproduced by permission of the AAS.

Digital Object Identifier (DOI)

http://dx.doi.org/10.1088/0004-637X/804/2/100

Funding Information

G.J.F. acknowledges support by NSF (1108928, 1109061, and 1412155), NASA (10-ATP10-0053, 10-ADAP10-0073, NNX12AH73G, and ATP13-0153), STScI (HST-AR- 13245, GO-12560, HST-GO-12309, GO-13310.002 A, and HST-AR-13914) and is grateful to the Leverhulme Trust for support via the award of a Visiting Professorship at The Queen's University of Belfast (VP1-2012-025). T.W.G. was supported in part by the NASA APRA grant NNX11AF32G. N.R.B. was supported in part the STFC UK APAP Network grant ST/J000892/1.

Repository Citation

Badnell, N. R.; Ferland, Gary J.; Gorczyca, T. W.; Nikolić, D.; and Wagle, Gururaj Anil, "Bootstrapping Dielectronic Recombination from Second-Row Elements and the Orion Nebula" (2015). Physics and Astronomy Faculty Publications. 252.
https://uknowledge.uky.edu/physastron_facpub/252