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...

We present deep echelle spectrophotometry of the brightest emission-line knots of the star-forming galaxies He 2−10, Mkn 1271, NGC 3125, NGC 5408, POX 4, SDSS J1253−0312, Tol 1457−262, Tol 1924−416 and the H ii region Hubble V in the Local Group dwarf irregular galaxy NGC 6822. The data have been taken with the Very Large Telescope Ultraviolet-Visual Echelle Spectrograph in the 3100-10420Å range. We determine electron densities and temperatures of the ionized gas from several emission-line intensity ratios for all the objects. We derive the ionic abundances of C 2+ and/or O 2+ from faint pure recombination lines (RLs) in several of the objects, permitting to derive their C/H and C/O ratios. We have explored the chemical evolution at low metallicities analysing the C/O vs. O/H, C/O vs. N/O and C/N vs. O/H relations for Galactic and extragalactic H ii regions and comparing with results for halo stars and DLAs. We find that H ii regions in star-forming dwarf galaxies occupy a different locus in the C/O vs. O/H diagram than those belonging to the inner discs of spiral galaxies, indicating their different chemical evolution histories, and that the bulk of C in the most metal-poor extragalactic H ii regions should have the same origin than in halo stars. The comparison between the C/O ratios in H ii regions and in stars of the Galactic thick and thin discs seems to give arguments to support the merging scenario for the origin of the Galactic thick disc. Finally, we find an apparent coupling between C and N enrichment at the usual metallicities determined for H ii regions and that this coupling breaks in very low-metallicity objects.

We present results of deep echelle spectrophotometry of the brightest knot of the Herbig–Haro object HH 202 in the Orion Nebula – HH 202‐S – using the Ultraviolet Visual Echelle Spectrograph in the spectral range from 3100 to 10 400 Å. The high spectral resolution of the observations has permitted to separate the component associated with the ambient gas from that associated with the gas flow. We derive electron densities and temperatures from different diagnostics for both components, as well as the chemical abundances of several ions and elements from collisionally excited lines, including the first determinations of Ca+ and Cr+ abundances in the Orion Nebula. We also calculate the He+, C2+, O+ and O2+ abundances from recombination lines. The difference between the O2+ abundances determined from collisionally excited and recombination lines – the so‐called abundance discrepancy factor – is 0.35 and 0.11 dex for the shock and nebular components, respectively. Assuming that the abundance discrepancy is produced by spatial variations in the electron temperature, we derive values of the temperature fluctuation parameter, t2, of 0.050 and 0.016 for the shock and nebular components, respectively. Interestingly, we obtain almost coincident t2 values for both components from the analysis of the intensity ratios of He i lines. We find significant departures from case B predictions in the Balmer and Paschen flux ratios of lines of high principal quantum number n. We analyse the ionization structure of HH 202‐S, finding enough evidence to conclude that the flow of HH 202‐S has compressed the ambient gas inside the nebula trapping the ionization front. We measure a strong increase of the total abundances of nickel and iron in the shock component, the abundance pattern and the results of photoionization models for both components are consistent with the partial destruction of dust after the passage of the shock wave in HH 202‐S.

We present the results of long-slit spectroscopy, in several positions, of the Orion Nebula. Our goal is to study the spatial distributions of a large number of nebular quantities, including line fluxes, physical conditions, and ionic abundances, at a spatial resolution of about 1 00 . In particular, we have compared the O ++ abundance determined from collisionally excited and recombination lines in 671 individual one-dimensional spectra covering different morphological zones of the nebula. We find that protoplanetary disks ( proplyds) show prominent spikes of T e ([N ii]), which is probably produced by collisional deexcitation due to the high electron densities found in these objects. Herbig-Haro objects show also relatively high values of T e ([N ii]), but these are probably produced by local heating due to shocks. We also find that the spatial distribution of the pure recombination O ii and [O iii] lines is fairly similar. The abundance discrepancy factor (ADF) of O ++ remains rather constant along the slit positions, except in some particular small areas of the nebula, such as at the locations of the most conspicuous Herbig-Haro objects. There is also an apparent slight increase of the ADF in the inner 40 00 around 1 Ori C. We find a negative radial gradient of T e ([O iii]) and T e ([N ii]) in the nebula, based on the projected distance from 1 Ori C. In addition, the ADF of O ++ seems to increase very slightly with the electron temperature. Finally, we estimate the value of the mean-square electron temperature fluctuation, the so-called t 2 parameter. Our results indicate that the hypothetical thermal inhomogeneities, if they exist, should be smaller than our spatial resolution element.

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