Pumping up the [N I] nebular lines
  The optical [N I] doublet near 5200 {\AA} 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]/H{\beta} 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
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 to 3000 K. In an appendix, we
describe how collisional and radiative processes are treated in the detailed
model N I atom now included in the Cloudy plasma code.
