Temperature Variations and N/O in the Orion Nebula from HST Observations^*

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] (2s2p³⁵S₂ → 2s²2p²³P_2,1) lines at λ_vac = 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] (2p^{3 2}P^o_1/2,3/2→2p^{3 4}S^o_3/2) lines at λ_vac = 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.