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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 S$^{2+} rightarrow$ 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 S$^{2+} rightarrow$ S$^+$ DR rate coefficient at $sim 10^{4}$~K. We present new calculations of the DR rate coefficient for S$^{2+} rightarrow$ 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.
The existence of multiple layers in the inner Orion Nebula has been revealed using data from an Atlas of spectra at 2 and 12 km/s resolution. These data were sometimes grouped over Samples of 10x10to produce high Signal to Noise spectra and sometimes
Recent Hubble Space Telescope images have allowed the determination with unprecedented accuracy of motions and changes of shocks within the inner Orion Nebula. These originate from collimated outflows from very young stars, some within the ionized po
The common assumption that Theta-1-Ori C is the dominant ionizing source for the Orion Nebula is critically examined. This assumption underlies much of the existing analysis of the nebula. In this paper we establish through comparison of the relative
Dielectronic recombination (DR) of singly charged ions is a reaction pathway that is commonly neglected in chemical models of molecular clouds. In this study we include state-of-the-art DR data for He$^+$, C$^+$, N$^+$, O$^+$, Na$^+$, and Mg$^+$ in c
We present a theoretical investigation of dielectronic recombination (DR) of Ar-like ions that sheds new light on the behavior of the rate coefficient at low-temperatures where these ions form in photoionized plasmas. We provide results for the total