Temperature variations and N/O in the orion nebula from HST observations

R. H. Rubin, P. G. Martin, R. J. Dufour, G. J. Ferland, J. A. Baldwin, J. J. Hester, D. K. Walter

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24 Scopus citations


Using the Goddard High Resolution Spectrograph (GHRS) and the Faint Object Spectrograph (FOS) on the Hubble Space Telescope, we measured the flux of the N n] (2s2p3 5S2 → 2s22p2 3P2,1) lines at λvac = 2143.45, 2139.68 Å in the Orion Nebula - the first detection of these lines in an H II region. In order to assess the N+/O+ ratio, we also measured the flux of the [O II] (2p32P1/2,3/20 → 2p3 4S3/20) lines at λvac = 2471.05, 2471.12 Å. In addition, with the FOS, other emission lines were measured in the same aperture in order to assess the average electron temperature and mean-square temperature variation (t2) in the N+ region, as well as the N+/O+ ratio. When we require that the empirically determined values be equal for (N+/O+)UV (obtained from the N II] 2142 and [O II] 2471 lines) and (N+/O+)opt (obtained from the [N II] 6585 and [O II] 3728 lines), we obtain the following. For the (N+, O+) zone, the average electron density is ∼7000 cm-3, the average electron temperature is 9500 K, t2 = 0.032, and N+/O+ = 0.14. By comparing our FOS observations to predicted fluxes, utilizing our two previous photoionization models, we are able to derive the N/O ratio. There is fairly good agreement between (N/O)UV and (N/O)opt as derived from the two models with a range between 0.13 and 0.18. This range also encompasses our model-derived values for (N/O)ir (0.17-0.18), which fit the observed far-infrared line ratio [N III] 57 μm/[O III] 52 μm. The empirically derived N+/O+ value requires a correction for the possibility that the N+ and O+ regions are not identical. Our overall results place the gas-phase Orion N/O ratio in the range 0.13-0.18, which is somewhat higher than solar.

Original languageEnglish
Pages (from-to)891-904
Number of pages14
JournalAstrophysical Journal
Issue number2 PART I
StatePublished - 1998


  • ISM: abundances
  • ISM: atoms
  • ISM: H H regions
  • ISM: individual (Orion Nebula)

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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