THE CHEMICAL COMPOSITION OF THE 30 DORADUS NEBULA DERIVED FROM VERY LARGE TELESCOPE ECHELLE SPECTROPHOTOMETRY

摘要

Echelle spectrophotometry of the 30 Doradus nebula in the LMC is presented. The data consist of VLT UVES observations in the 3100-10350 A range. The intensities of 366 emission lines have been measured, including 269 identified permitted lines of H~0, He~0, C~0, C~+, N~+, N~(++), O~0, O~+, Ne~0, Ne~+, S~+, S~(++), Si~0, Si~+, Si~(++), Ar~+, and Mg~+; many of them are produced by recombination only while others mainly by fluorescence. Electron temperatures and densities have been determined using different line intensity ratios. The He~+, C~(++), O~+, and O~(++) ionic abundances have been derived from recombination lines; these abundances are almost independent of the temperature structure of the nebula. Alternatively, abundances from collisionally excited lines have been derived for a large number of ions of different elements; these abundances depend strongly on the temperature structure. Accurate t~2 values have been derived from the Balmer continuum and by comparing the C~(++), O~+, and O~(++) ionic abundances obtained from collisionally excited and recombination lines. The chemical composition of 30 Doradus is compared with those of Galactic and extragalactic H Ⅱ regions. The values of ΔY/ΔO, ΔY/ΔZ, and Y_p are also discussed.