FAINT EMISSION-LINES IN THE SPECTRUM OF THE ORION NEBULA AND THE ABUNDANCES OF SOME OF THE RARER ELEMENTS

CCD spectra were obtained of a region just north of the Trapezium in NGC 1976, the Orion Nebula, in overlapping exposures covering the wavelength region lambda-lambda-3180-11000. Approximately 225 emission lines were measured at a resolution of about 6 angstrom. A total of 35 H I lines and 57 He I lines were identified. Most of their measured intensities, corrected for interstellar extinction, agree relatively well with the recombination-line theory. At the faintest levels there are discrepancies, probably resulting from observational uncertainties. The measured collisionally excited lines give T = 9000 K, N(e) = 4 x 10(3) cm-3 as the one-point representative model. Relative abundances of He, N, O, Ne, S, Cl, Ar, Fe, and Ni were determined, the latter two elements from singly and doubly ionized stages of ionization. They show that Fe and Ni are depleted by about a factor of 5 with respect to the other ions, presumably on solid particles that exist even in the strong radiation field and hot ionized gas near the Trapezium. Permitted lines from other elements, mostly resulting from fluorescence, but a few from recombination, are also discussed.