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 II] (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] (2p32Po1/2,3/2-->2p34So3/2) 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. Based on observations made with the NASA/ESA Hubble Space Telescope, obtained at STScI, which is operated by AURA, Inc., under NASA contract NAS 5-26555.
Digital Object Identifier (DOI)
Rubin, R. H.; Martin, P. G.; Dufour, R. J.; Ferland, Gary J.; Baldwin, J. A.; Hester, J. J.; and Walter, D. K., "Temperature Variations and N/O in the Orion Nebula from HST Observations" (1998). Physics and Astronomy Faculty Publications. 118.