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...

We have derived a primordial helium abundance of Yp = 0.2477 +- 0.0029, based on new atomic physics computations of the recombination coefficients of He I and of the collisional excitation of the H I Balmer lines together with observations and photoionization models of metal-poor extragalactic H II regions. The new atomic data increase our previous determination of Yp by 0.0086, a very significant amount. By combining our Yp result with the predictions made by the standard Big Bang nucleosynthesis model, we find a baryon-to-photon ratio, \eta, in excellent agreement both with the \eta value derived by the primordial deuterium abundance value observed in damped Lyman-\alpha systems and with the one obtained from the WMAP observations.Comment: 35 pages, 1 figure. AAS LaTeX. Accepted for publication in the Astrophysical Journal. The accepted version is more explicit in the description of the procedure and in the treatment of errors with respect to the first version, and the final numerical result is slightly changed. Scheduled for ApJ, Vol. 667, Sept 20, 200

We study the e †ect that the temperature structure has on the determination of the primordial helium abundance,We provide an equation linking T (O III), the temperature derived from the [O III] lines, Y p . and T (He II), the temperature of the He I lines, both for H II regions with O``only and for H II regions where a fraction of O`is present. By means of T (He II), which is always smaller than T (O III), we derive the helium abundances of Ðve objects with low and very low metallicity (NGC 346, NGC 2363, Haro 29, SBS 0335[052, and I Zw 18) ; these objects were selected from the literature because they include the three low-metallicity objects with the best line determinations and the two objects with the lowest metallicity. From these abundances we obtain that a value 0.0088 lower than that Y p (nHc) \ 0.2356^0.0020, derived by using T (O III). We call this determination because the collisional contribution to the Y p (nHc) Balmer-line intensities has not been taken into account. All the recent determinations in the literature Y p have not taken into account the collisional contribution to the Balmer-line intensities. By considering the collisional contribution to the Balmer-line intensities of these Ðve objects, we derive that Y p (]Hc) \ 0.2384^0.0025.

Spectrophotometry in the λλ 3400-7400 range is presented for 13 areas of the brightest H ii region in the SMC: NGC 346. The observations were obtained at CTIO with the 4-m telescope. Based on these observations its chemical composition is derived. The helium and oxygen abundances by mass are given by: Y (SMC)= 0.2405 ± 0.0018 and O(SMC)= 0.00171 ± 0.00025. From models and observations of irregular and blue compact galaxies it is found that ∆Y /∆O = 3.5 ± 0.9 and consequently that the primordial helium abundance by mass is given by: Y p = 0.2345 ± 0.0026(1σ). This result is compared with values derived from Big Bang nucleosynthesis, and with other determinations of Y p .