1999ApJ...527..474B

We investigate the effects of photoexcitation of [N I] and [O I] lines by stellar continuum radiation under nebular conditions. It is found that the [N I] optical lines at 5198 and 5200 Å are affected by fluorescence in many objects, including the Orion Nebula. [O I] lines are much less sensitive to fluorescence, except under conditions of low temperatures (≤5000 K) and high-radiation fields. Furthermore, it is shown that recent diagnostics by Esteban et al. of the physical conditions of partially ionized zones in Orion using combinations of [N I] and [O I] are unreliable because the lines from these species are likely to come from different regions. The hypothesis about different conditions in [N I]- and [O I]/[Fe II]-emitting regions is supported by the lack of correlation between these lines in samples of planetary nebulae, supernova remnants, and Herbig-Haro objects.