We present echelle spectroscopy in the 3500‐ to 7060‐?… range for two positions of the Orion nebula. The data were obtained using the 2.1‐m telescope at Observatorio Astron??mico Nacional in San Pedro M??rtir, Baja California. We have measured the intensities of about 220 emission lines, in particular 81 permitted lines of C+, N+, N++, O0, O+, Ne0, 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 t2 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.

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We present measurements of the gas-phase abundance ratio C/O in six H II regions in the spiral galaxies M101 and NGC 2403, based on ultraviolet spectroscopy using the Faint Object Spectrograph on the Hubble Space T elescope. The ratios of C to O increase systematically with O/H in both galaxies, from log C/O B [0.8 at log O/H \ [4.0 to log C/O B [0.1 at log O/H \ [3.4. C/N shows no correlation with O/H. The rate of increase of C/O is somewhat uncertain because of uncertainty as to the appropriate UV reddening law and uncertainty in the metallicity dependence on grain depletions. However, the trend of increasing C/O with O/H is clear, conÐrming and extending the trend in C/O indicated previously from observations of irregular galaxies. Our data indicate that the radial gradients in C/H across spiral galaxies are steeper than the gradients in O/H. Comparing the data to chemicalevolution models for spiral galaxies shows that models in which the massive star yields do not vary with metallicity predict radial C/O gradients that are much Ñatter than the observed gradients. The most likely hypothesis at present is that stellar winds in massive stars have an important e †ect on the yields and thus on the evolution of carbon and oxygen abundances. C-to-O and N-to-O abundance ratios in the outer disks of spirals determined to date are very similar to those in dwarf irregular galaxies. This implies that the outer disks of spirals have average stellar-population ages much younger than those of the inner disks.

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We present high resolution observations of the giant extragalactic H II regions NGC 604, NGC 2363, NGC 5461 and NGC 5471, based on observations taken with the ISIS spectrograph on the William Herschel Telescope. We have detected -by the first time-C II and O II recombination lines in these objects. We find that recombination lines give larger C ++ and O ++ abundances than collisionallly excited lines, suggesting that temperature variations can be present in the objects. We detect [Fe IV] lines in NGC 2363 and NGC 5471, the most confident detection of optical lines of this kind in H II regions. Considering the temperature structure we derive their H, He, C, N, O, Ne, S, Ar, and Fe abundances. From the recombination lines of NGC 5461 and NGC 5471 we determine the presence of C/H and O/H gradients in M101. We calculate the ∆Y /∆O and ∆Y /∆Z values considering the presence of temperature variations and under the assumption of constant temperature. We obtain a better agreement with models of galactic chemical evolution by considering the presence of temperature variations than by assuming that the temperature is constant in these nebulae.