Chemical composition of the Orion nebula derived from echelle spectrophotometry

We present echelle spectroscopy in the 3500- to 7060-Angstrom range for two positions of the Orion nebula. The data were obtained using the 2.1-m telescope at Observatorio Astronomico Nacional in San Pedro Martir, Baja California. We have measured the intensities of about 220 emission lines, in particular 81 permitted lines of C(+), N(+), N(++), O(0), O(+), Ne(0), Si(+), Si(++) and S(+), some of them produced by recombination only and others mainly by fluorescence. We have determined electron temperatures, electron densities and ionic abundances using different continuum and line intensity ratios. We derived the He, C and O abundances from recombination lines and find that the C/H and O/H values are very similar to those derived from B stars of the Orion association, and that these nebular values are independent of the temperature structure. We have also derived abundances from collisionally excited lines. These abundances depend on the temperature structure; accurate t(2) values have been derived comparing the O II recombination lines with the [O III] collisionally excited lines. The gaseous abundances of Mg, Si and Fe show significant depletions, implying that a substantial fraction of these atoms is tied up in dust grains. The derived depletions are similar to those found in warm clouds of the Galactic disc, but are not as large as those found in cold clouds. A comparison of the solar and Orion chemical abundances is made.