We present Very Large Telescope (VLT) UVES echelle spectrophotometry of the Orion nebula in the 3100-10 400 Å range. We have measured the intensity of 555 emission lines, many of them corresponding to permitted lines of different heavy-element ions. This is the largest set of spectral emission lines ever obtained for a Galactic or extragalactic H II region. We have derived He + , C 2+ , O + , O 2+ and Ne 2+ abundances from pure recombination lines. This is the first time that O + and Ne 2+ abundances have been obtained from these kinds of lines in the nebula. We have also derived abundances from collisionally excited lines for a large number of ions of different elements. In all cases, ionic abundances obtained from recombination lines are larger than those derived from collisionally excited lines. We have obtained remarkably consistent independent estimations of the temperature fluctuation parameter, t 2 , from different methods, which are also similar to other estimates from the literature. This result strongly suggests that moderate temperature fluctuations (t 2 between 0.02 and 0.03) are present in the Orion nebula. We have compared the chemical composition of the nebula with those of the Sun and other representative objects. The heavy-element abundances in the Orion nebula are only slightly higher than the solar ones, a difference that can be explained by the chemical evolution of the solar neighbourhood.

We present very deep spectrophotometry of 14 bright extragalactic H II regions belonging to spiral, irregular, and blue compact galaxies. The data for 13 objects were taken with the HIRES echelle spectrograph on the Keck I telescope.We have measured C II recombination lines in 10 of the objects and O II recombination lines in 8 of them. We have determined electron temperatures from line ratios of several ions, specially of low ionization potential ones. We have found a rather tight linear empirical relation between T e ([N II]) and T e ([O III]). We have found that O II lines give always larger abundances than [O III] lines. Moreover, the difference of both O ++ abundance determinations -the so-called abundance discrepancy factor-is very similar in all the objects, with a mean value of 0.26 ± 0.09 dex, independently of the properties of the H II region and of the parent galaxy. Using the observed recombination lines, we have determined the O, C, and C/O radial abundance gradients for 3 spiral galaxies: M 33, M 101, and NGC 2403, finding that C abundance gradients are always steeper than those of O, producing negative C/O gradients accross the galactic disks. This result is similar to that found in the Milky Way and has important implications for chemical evolution models and the nucleosynthesis of C.where the auroral lines become very faint, or to measure recombination lines (hereafter RLs) useful for abundance determinations of heavy-element ions (Peimbert 2003;López-Sánchez et al. 2007;Bresolin 2007).The detection of C II and O II lines produced by pure recombination in EHRs was firstly reported by Esteban et al. (2002) from deep spectra taken with the 4.2 m William Herschel Telescope. In principle, these lines have the advantage that their intensity is much less dependent on the value of T e than the collisionally excited lines (hereafter CELs), which are the lines commonly used for abundance determinations in nebulae. The brightest C II RL is C II λ4267, with typical fluxes of the order of 10 −3 × I(Hβ). This line permits to derive the C ++ abundance, which is the dominant ionization stage of C for the typical conditions of EHRs. There are only a few C abundance determinations available for EHRs, most of them derived from UV CELs that can only be observed from space (Garnett et al. 1995(Garnett et al. , 1999Kobulnicky & Skillman 1998), and more recently from RLs (Esteban et al. 2002;Peimbert 2003;Tsamis et al. 2003;Peimbert et al. 2005;López-Sánchez et al. 2007;Bresolin 2007). The C abundance determinations based on UV CELs are severely affected by uncertainties in the reddening correction. To further complicate the situation, the STIS spectrograph aboard the Hubble Space Telescope, the only instrument capable to detect the UV CELs of C in bright EHRs, stopped science operations in 2004, so that nowadays the observation of the optical CII RLs provides the only possibility for determining C abundances in EHRs. The study of the behavior of C/H and C/O ratios and their galactocentric gradients in galaxies of di...

No abstract

We present results of deep echelle spectrophotometry of eight Galactic H II regions located at galactocentric distances between 6.3 and 10.4 kpc. The data have been taken with the Very Large Telescope (VLT) Ultraviolet Echelle Spectrograph (UVES) in the 3100 to 10360Å range. We have derived C ++ and O ++ abundances from recombination lines for all the objects, as well as O + abundances from this kind of lines for three of the nebulae. The intensity of recombination lines is almost independent on the assumed electron temperature as well as on the possible presence of spatial temperature variations or fluctuations inside the nebulae. These data allow the determination of the gas-phase C and O abundance gradients of the Galactic disk, of paramount importance for chemical evolution models. This is the first time the C gradient is derived from a so large number of H II regions and for a so wide range of galactocentric distances. Abundance gradients are found of the form ∆log(O/H) = −0.044±0.010 dex kpc −1 , ∆log(C/H) = −0.103±0.018 dex kpc −1 , and ∆log(C/O) = −0.058±0.018 dex kpc −1 .

Eleven models of Galactic chemical evolution, differing in the carbon, nitrogen, and oxygen yields adopted, have been computed to reproduce the Galactic O/H values obtained from H II regions. All the models fit the oxygen gradient, but only two models fit also the carbon gradient, those based on carbon yields that increase with metallicity due to stellar winds in massive stars (MS) and decrease with metallicity due to stellar winds in low and intermediate mass stars (LIMS). The successful models also fit the C/O versus O/H evolution history of the solar vicinity obtained from stellar observations. We also compare the present day N/H gradient and the N/O versus O/H and the C/Fe, N/Fe, O/Fe versus Fe/H evolution histories of the solar vicinity predicted by our two best models with those derived from H II regions and from stellar observations. While our two best models fit the C/H and O/H gradients as well as the C/O versus O/H history, only Model 1 fits well the N/H gradient and the N/O values for metal poor stars but fails to fit the N/H values for metal rich stars. Therefore we conclude that our two best models solve the C enrichment problem, but that further work needs to be done on the N enrichment problem. By adding the C and O production since the Sun was formed predicted by Models 1 and 2 to the observed solar values we find an excellent agreement with the O/H and C/H values of the solar vicinity derived from H II regions O and C recombination lines. Our results are based on an IMF steeper than Salpeter's, a Salpeter like IMF predicts C/H, N/H, and O/H ratios higher than observed. One of the most important results of this paper is that the fraction of carbon due to MS and LIMS in the interstellar medium is strongly dependent on time and on the galactocentric distance; at present about half of the carbon in the interstellar medium of the solar vicinity has been produced by MS and half by LIMS.