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We conducted a long-slit spectrophotometry analysis to obtain the chemical abundances of seven metal-poor HII regions in three galaxies: UM 160, UM 420, and TOL 0513-393. The data have been taken with the Focal Reducer Low Dispersion Spectrograph 1 (FORS1) at the 8.2-m Very Large Telescope. We derived the physical conditions and the chemical abundances of N, O, Ne, S, Ar, and Cl. We also performed a detailed analysis that involves abundance determinations using the $t^2$ formalism. Based on HeI recombination line intensity ratios, together with the Helio14 code, we derived the abundance of He. In addition, for a value $\Delta Y/\Delta Z_O =3.3\pm 0.7$, we have estimated that the primordial helium abundance by mass is $Y_{\rm P}=0.2448\pm0.0033$. This value agrees with values derived from Standard Big Bang Nucleosynthesis and with other recent determinations of $Y_{\rm P}$., Comment: 11 pages, 4 figures. This paper has been accepted in MNRAS

A review of some of the papers that discuss the relevance of the temperature structure present in H II regions and planetary nebulae is presented. Particular attention is given to the determination of the chemical abundances of these objects., Comment: 8 pages. Invited review given in the II workshop on Chemical Abundances in Gaseous Nebulae, held between 11-14 March 2019 in Sao Jose dos Campos, Sao Paulo, Brazil

Based on observations of HII regions and the new computations of the recombination coefficients of the He I lines by Porter et al. (2013) we obtain a primordial helium abundance by mass of $Y_P = 0.2446\pm0.0029$. We consider thirteen sources of error for the $Y_P$ determination, some of them are mainly due to systematic effects, while the rest are mainly due to statistical effects. We compare our results with other determinations of $Y_P$ present in the literature. Combining our $Y_P$ value with computations of primordial nucleosynthesis we find a number of neutrino species $N_{eff} = 2.90\pm0.22$, and a neutron mean life $\tau_{\nu} = 872\pm14(s)$., Comment: 6 pages. Accepted for publication in the Revista Mexicana de Astronom\'ia y Astrof\'isica

Based on high quality observations of multiplet V1 of OII and the NLTE atomic computations for OII we study the density and temperature of a sample of PNe. We find that, in general, the densities derived from recombination lines of OII are similar than the densities derived from forbidden lines. This implies that the signature for oxygen rich clumps of high density and low temperature is absent in most of the objects of our sample. Electron pressures derived from the hotter zones are similar or slightly larger than those derived from the colder zones, suggesting the presence of shock waves. The average temperatures and t2 values derived from H, He and O lines are similar and consistent with chemical homogeneity. These results suggest that the abundances of these objects are the ones derived from recombination lines., 14 pages, 7 figures. To apear en the October 2014 issue of the "Revista Mexicana de Astronomia y Astrofisica"

We present long slit spectrophotometry considering the presence of thermal inhomogeneities (t^2) of two HII regions in the Small Magellanic Cloud (SMC): NGC 456 and NGC 460. Physical conditions and chemical abundances were determined for three positions in NGC 456 and one position in NGC 460, first under the assumption of uniform temperature and then allowing for the possibility of thermal inhomogeneities. We determined t^2 values based on three different methods: i) by comparing the temperature derived using oxygen forbidden lines with the temperature derived using helium recombination lines, ii) by comparing the abundances derived from oxygen forbidden lines with those derived from oxygen recombination lines, and iii) by comparing the abundances derived from ultraviolet carbon forbidden lines with those derived from optical carbon recombination lines. The first two methods averaged t^2=0.067+-0.013 for NGC 456 and t^2=0.036+-0.027 for NGC 460. These values of t^2 imply that when gaseous abundances are determined with collisionally excited lines they are underestimated by a factor of nearly 2. From these objects and others in the literature, we find that in order to account for thermal inhomogeneities and dust depletion, the O/H ratio in low metallicity HII regions should be corrected by 0.25-0.45 dex depending on the thermal structure of the nebula, or by 0.35 dex if such information is not available., Accepted for publication in The Astrophysical Journal. 41 pages in pre-print format. 3 figures

Chemical evolution models are useful for understanding the formation and evolution of stars and galaxies. Model predictions will be more robust as more observational constraints are used. We present chemical evolution models for the dwarf irregular galaxy NGC 6822 using chemical abundances of old and young Planetary Nebulae (PNe) and \ion{H}{ii} regions as observational constraints. Two sets of chemical abundances, one derived from collisionally excited lines (CELs) and one, from recombination lines (RLs), are used. We try to use our models as a tool to discriminate between both procedures for abundance determinations. In our chemical evolution code, the chemical contribution of low and intermediate mass stars is time delayed, while for the massive stars the chemical contribution follows the instantaneous recycling approximation. Our models have two main free parameters: the mass-loss rate of a well-mixed outflow and the upper mass limit, $M_{up}$, of the initial mass function (IMF). To reproduce the gaseous mass and the present-day O/H value we need to vary the outflow rate and the $M_{up}$ value., 18, pages, 8 figures, Accepted for publication in A&A

We present chemical evolution models of the Galactic disk with different Z-dependent yields. We find that a moderate mass loss rate for massive stars of solar metallicity produces an excellent fit to the observed C/H and C/O gradients of the Galactic disk. The best model also fits: the H, He, C, and O abundances derived from recombination lines of M17, the protosolar abundances,and the C/O-O/H, C/Fe-Fe/H, and O/Fe-Fe/H relations derived from solar vicinity stars. The agreement of the model with the protosolar abundances implies that the Sun originated at a galactocentric distance similar to the one it has. Our model for $r=3$ kpc implies that a fraction of the stars in the direction of the bulge formed in the inner disc. We obtain a good agreement between our model and the C/O versus O/H relationship derived from extragalactic H~{\sc ii} regions in spiral galaxies., Comment: 30 pages, 11 figures. Rev. Mex. Astron. Astrof. accepted

I present a brief review on the determination of the primordial helium abundance by unit mass, Yp. I discuss the importance of the primordial helium abundance in: (a) cosmology, (b) testing the standard big bang nucleosynthesis, (c) studying the physical conditions in H II regions, (d) providing the initial conditions for stellar evolution models, and (e) testing the galactic chemical evolution models., Comment: 27 pages, 8 figures. Appeared in Current Science, vol. 95, pg. 1165-1176, 2008. (http://www.ias.ac.in/currsci/nov102008/1165.pdf)

The determination of the heavy element abundances from giant extragalactic H II regions is based on collisionally excited lines. We argue that in the presence of temperature variations the abundances determined are lower limits to the real heavy element abundances. To determine the real abundances it is necessary to take into account the temperature variations present in these nebulae. We discuss the relevance of obtaining accurate line intensities of recombination lines of H, He, C, and O to determine the chemical composition of extragalactic H II regions. We suggest that Pagel's method to derive the O/H ratio should be calibrated by using recombination lines instead of photoionization models or abundances derived from collisionally excited lines., 4 pages, conference on "Science with the GTC-10m Telescope", Granada, Spain, February, 2002

Recent advances on the quest to determine the primordial or pregalactic helium abundance, Yp, are reviewed. There are seven problems affecting the He/H abundance determinations of H II regions that are briefly discussed: the underlying absorption lines in the observed spectra, the ionization, temperature, and density structures of each object, the collisional contribution to the intensity of the He I and H I lines, the optical thickness of the He I lines, and the extrapolation to derive Yp based on the Y and O/H values., Comment: 10 pages. Conference on Star Formation through Time (ASP Conference Series)

Observations of the interstellar and circumstellar absorption components obtained with the Hubble Space Telescope Space Telescope Imaging Spectrograph (STIS) along the line of sight toward the Wolf- Rayet-luminous blue variable (LBV) system HD 5980 in the Small Magellanic Cloud are analyzed., Facultad de Ciencias Astronómicas y Geofísicas

A new determination of the pregalactic helium abundance based on the Magellanic Clouds H II regions is discussed. This determination amounts to Yp = 0.2345 +- 0.0030 and is compared with those derived from giant extragalactic H II regions in systems with extremely low heavy elements content. It is suggested that the higher primordial value derived by other authors from giant H II region complexes could be due to two systematic effects: the presence of neutral hydrogen inside the helium Stromgren sphere and the presence of temperature variations inside the observed volume., 10 pages, 1 figure, IAU Symposium 198

By comparing observed abundance gradients with those predicted by a chemodynamical model of the Galaxy the following results are obtained: there is a defficiency of O/H poor PNe in the solar vicinity which implies that only a fraction of intermediate mass stars produce PNe, a similar result is obtained from the observed abundance distributions of Ne/H, S/H and Ar/H; the age distributions of the stellar progenitors of type II and type III PNe show a substantial overlap, but the average age of the progenitors of type III PNe is larger than that of the progenitors of type II PNe; the S/H and the Ar/H gradients of type I PNe and H II regions are similar, the flatter O/H gradient of type I PNe relative to that of H II regions is probably to the effect of temperature fluctuations on the abundance determinations; the similarity of the abundance gradients derived from H II regions, and PNe imply that transient phenomena are not important in shaping the present day H II gradients., 5 pages, 2 figures, 1 table

Observations of star formation sites in a number of molecular clouds, including NGC 7538, NGC 2071, Cepheus A and K3-50A, have been made using the Owens Valley millimeter wave interferometer. The measurements of ^(12)CO, ^(13)CO, CS and continuum emission are at resolutions of order 7″. Continuum emission at 2.7 mm is detected from all the cloud cores and, in general, the ^(12)CO interferometer maps reveal the presence of high velocity, bipolar outflows within 10″ of these continuum sources. Thus, the origins of the outflows are readily identifiable; the flows themselves appear to remain collimated on scales of less than 10^(17) cm. The orientations of the flows are similar to those seen at lower resolution, except in Cepheus A, where the interferometer is capable of distinguishing between two outflows which are confused in single telescope maps.

7″-resolution CO (1-0) observations of M82 with the Owens Valley millimeter-wave interferometer have resolved 2 components of molecular gas in the central 1.5 arcmin of the galaxy: (1) a high plane of M82, and (2) shell-like or filamentary structures of molecular gas, with size-scale as large as 400 pc, extending most likely out of the plane of the galaxy.