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.

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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)


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.

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