CNO ABUNDANCES AND TEMPERATURE-FLUCTUATIONS IN THE ORION NEBULA

Abundances of carbon, nitrogen, and oxygen in 22 regions of the Orion Nebula are derived using diagnostic emission lines measured from IUE and ground-based spectrophotometry and imagery. In order to minimize geometrical complications, the regions chosen lie in the relatively smooth area of the nebula west of the Trapezium and extend outward to about 5'. For 20 of the regions, both C III] lambda-1909 and C n] lambda-2326 were measured from the IUE spectra, and combined with optical line strengths of [O II], [O III], [N II], H-beta, and Ha to study spatial variations in the ionic and total elemental concentrations of C, N, and O. A detailed analysis of UV-optical extinction, temperature gradients, and density gradients using new and previously reported observations is given. Empirical CNO abundances are derived for each position using both on-the-spot values for T(e) (from [O III] and EN II]) and N(e) (from [S II]), as well as global mean values for the nebula. For both situations of T(e)-N(e) gradients and an isothermal nebula we find no evidence of the existence of radial gradients in C, N, or O westward from the Trapezium (within +/- 0.2 dex rms). However, the mean concentrations of C, N, and O relative to H found using directly observed temperatures are much lower than solar; in terms of 12 + log (X/H) we derived (C, N, O) = (7.94, 7.50, 8.45). Rather than conclude that the Orion Nebula is depleted in CNO compared to the Sun, we propose the existence of significant temperature fluctuations in the nebula. For such, the auroral transitions of the [O III] and [N II] multiplets are enhanced by a hotter medium (possibly wind-driven or shock-ionized) compared to the mass-loaded and denser regions which emit the bulk of the nebular transitions. We show that an assumed rms temperature fluctuation of 0.055 gives abundances of (C, N, 0) = (8.78, 7.73, 8.90), where the oxygen is solar and, within our error bars, the carbon is near solar. Additional support for Orion having near-solar abundances comes from the results of a grid of constant-density nebular models based on a single ionizing source with T(eff) = 37,500 K (similar to theta-1 Ori C). We find that the " mean " UV-optical spectrum of a " core " region approximately 1' W of the Trapezium can be readily matched with a model of solar O, N, Ne, and S with enhanced C and Ar. The lower temperatures found in such a model are consistent with a rms temperature fluctuation of 0.055. We compare our simple model results with more sophisticated "blister" and "dust" models of M42 published recently and discuss some of the implications that the complications of Orion have with regard to the greater problem of abundance determinations in Galactic and extragalactic H II regions.