The abundance of carbon and magnesium in the Orion Nebula

Abstract

The Orion nebula has been observed in two positions with IUE (International Ultraviolet Explorer) in the low-resolution mode (about 7 A) and in the spectral range 1150-3200 A. Emission lines of C II, C III, O II, and He I have been measured and used to determine what is probably the first reliable abundance of carbon in H II regions. The logarithmic total abundance of carbon is found to be 8.4 close to the solar value. In contrast with the situation in the planetary nebula of similar excitation, IC 418, where the resonance Mg II wavelength 2800 line is observed to be relatively strong, in the Orion nebula the wavelength 2800 line is not detectable. An upper limit for the magnesium abundance of the order of 10 times smaller than in the sun is suggested.