T_e-fluctuations and N⁺/O⁺ in the Orion Nebula

Using the Faint Object Spectrograph (FOS) on the Hubble Space Telescope, we measured the flux of the N II] (2s2p^{3 5}S₂ → 2s²2p^{2 3}P_2,1) lines at λ_vac = 2143.45, 2139.68 Å in the Orion Nebula. In order to assess the N⁺/O⁺ ratio, we also measured the flux of the [O II] (2p^{3 2}P_1/2,3/2⁰ → ²P^{3 4}S_3/2⁰) line 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 (t ²) 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 [O II] 2471 and N II] 2142 lines) and (N⁺/O⁺)_opt (obtained from the [O II] 3728 and [N II] 6585 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, t ² is 0.032, and N⁺/O⁺ is 0.14. In order to obtain N/O, the empirically-derived N⁺/O⁺ value requires a correction for the possibility that the N⁺ and O⁺ regions are not identical.