Pumping up the [N I] nebular lines

G. J. Ferland, W. J. Henney, C. R. O'Dell, R. L. Porter, P. A.M. Van Hoof, R. J.R. Williams

Research output: Contribution to journalArticlepeer-review

27 Scopus citations

Abstract

The optical [N I] doublet near 5200Å 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 1000-3000K. 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.

Original languageEnglish
Article number79
JournalAstrophysical Journal
Volume757
Issue number1
DOIs
StatePublished - Sep 20 2012

Keywords

  • Atomic processes
  • Dust, extinction
  • Hii regions
  • Line: formation
  • Photon-dominated region (PDR)
  • Radiative transfer

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science

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