The optical [N I] doublet near 5200 Å is anomalously strong in a variety of emission-line objects. We compute a detailed photoionization model and use it to show that pumping by far-ultraviolet (FUV) stellar radiation previously posited as a general explanation applies to the Orion Nebula (M42) and its companion M43; but, it is unlikely to explain planetary nebulae and supernova remnants. Our models establish that the observed nearly constant equivalent width of [N I] with respect to the dust-scattered stellar continuum depends primarily on three factors: the FUV to visual-band flux ratio of the stellar population, the optical properties of the dust, and the line broadening where the pumping occurs. In contrast, the intensity ratio [N I]/Hβ depends primarily on the FUV to extreme-ultraviolet ratio, which varies strongly with the spectral type of the exciting star. This is consistent with the observed difference of a factor of five between M42 and M43, which are excited by an O7 and B0.5 star, respectively. We derive a non-thermal broadening of order 5 km s–1 for the [N I] pumping zone and show that the broadening mechanism must be different from the large-scale turbulent motions that have been suggested to explain the line widths in this H II region. A mechanism is required that operates at scales of a few astronomical units, which may be driven by thermal instabilities of neutral gas in the range 1000-3000 K. In an Appendix A, we describe how collisional and radiative processes are treated in the detailed model N I atom now included in the CLOUDY plasma code.

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Published in The Astrophysical Journal, v. 757, no. 1, 79, p. 1-18.

© 2012. The American Astronomical Society. All rights reserved. Printed in the U.S.A.

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G.J.F. acknowledges support by NSF (0908877; 1108928; and 1109061), NASA (07-ATFP07-0124, 10-ATP10-0053, and 10-ADAP10-0073), JPL (RSA No 1430426), and STScI (HST-AR-12125.01, GO-12560, and HST-GO-12309). W.J.H. acknowledges financial support from DGAPA-UNAM through grant PAPIIT IN102012. C.R.O. was supported in part by STScI grant GO-11232. P.v.H. acknowledges support from the Belgian Science Policy Office through the ESA Prodex program.

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This research used data from the Atomic Line List (http://www.pa.uky.edu/~peter/atomic).