Chemical composition of the Orion Nebula.

Results are reported for photoelectric spectrophotometry of 12 regions in the Orion Nebula performed over the wavelength range from 3700 to 7400 A. Electron temperatures and densities corresponding to various forbidden-line intensity ratios are computed, and abundances of H, He, C, N, O, Ne, S, Cl, and Ar are derived on the basis of a spatial temperature fluctuation scheme. The results are used to study four problems: (1) whether a large fraction of the heavy elements is embedded in dust grains, (2) the accuracy of ionization correction factors for those elements observed only in one ionization stage, (3) the effect of He self-absorption on the derived He abundance, and (4) the effect of improvements in atomic collision strengths and of recent radio observations on the chemical abundances. The abundances are compared with previous determinations, and those of C, N, O, and Ne are shown to be very similar to the solar values. It is concluded that no more than 27% of the heavy elements are embedded in dust, the He/H number-density ratio is approximately 0.10, and the derived N and Ne abundances are independent of the oxygen ionization degree.