Recalibration of Pagel's method for HII regions considering the thermal structure, the ionization structure, and the depletion of O into dust grains

Using a sample of 28 HII regions from the literature with measured temperature inhomogeneity parameter, t^2, we present a statistical correction to the chemical abundances determined with the Te(4363/5007) method. We used the t^2 values to correct the oxygen gaseous abundances and consider the oxygen depletion into dust to calculate the total abundances for these objects. This correction is used to obtain a new calibration of Pagel's strong-line method, R_{23}, to determine oxygen abundances in HII regions. Our new calibration simultaneously considers the temperature structure, the ionization structure, and the fraction of oxygen depleted into dust grains. Previous calibrations in the literature have included one or two of these factors; this is the first time all three are taken into account. This recalibration conciliates the systematic differences among the temperatures found from different methods. We find that the total correction due to thermal inhomogeneities and dust depletion amounts to an increase in the O/H ratio of HII regions by factors of 1.7 to 2.2 (or 0.22 to 0.35 dex). This result has important implications in various areas of astrophysics such as the study of the higher end of the initial mass function, the star formation rate, and the mass-metallicity relation of galaxies, among others.Comment: 16 pages (preprint), 4 figures, 1 Table, accepted in ApJ

Analysis of two SMC HII Regions Considering Thermal Inhomogeneities: Implications for the Determinations of Extragalactic Chemical Abundances

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.Comment: Accepted for publication in The Astrophysical Journal. 41 pages in pre-print format. 3 figure

On the abundance discrepancy problem in HII regions

The origin of the abundance discrepancy is one of the key problems in the physics of photoionized nebula. In this work, we analize and discuss data for a sample of Galactic and extragalactic HII regions where this abundance discrepancy has been determined. We find that the abundance discrepancy factor (ADF) is fairly constant and of the order of 2 in all the available sample of HII regions. This is a rather different behaviour than that observed in planetary nebulae, where the ADF shows a much wider range of values. We do not find correlations between the ADF and the O/H, O++/H+ ratios, the ionization degree, Te(High), Te(Low)/ Te(High), FWHM, and the effective temperature of the main ionizing stars within the observational uncertainties. These results indicate that whatever mechanism is producing the abundance discrepancy in HII regions it does not substantially depend on those nebular parameters. On the contrary, the ADF seems to be slightly dependent on the excitation energy, a fact that is consistent with the predictions of the classical temperature fluctuations paradigm. Finally, we obtain that Te values obtained from OII recombination lines in HII regions are in agreement with those obtained from collisionally excited line ratios, a behaviour that is again different from that observed in planetary nebulae. These similar temperature determinations are in contradiction with the predictions of the model based on the presence of chemically inhomogeneous clumps but are consistent with the temperature fluctuations paradigm. We conclude that all the indications suggest that the physical mechanism responsible of the abundance discrepancy in HII regions and planetary nebulae are different.Comment: 14 pages, 8 figures, 9 tables. Accepted for publication in the Ap

Faint emission lines in the Galactic H II regions M16, M20 and NGC 3603

We present deep echelle spectrophotometry of the Galactic {\hii} regions M16, M20 and NGC 3603. The data have been taken with the Very Large Telescope Ultraviolet-Visual Echelle Spectrograph in the 3100 to 10400 \AA range. We have detected more than 200 emission lines in each region. Physical conditions have been derived using different continuum and line intensity ratios. We have derived He$^{+}$, C$^{++}$ and O$^{++}$ abundances from pure recombination lines as well as abundances from collisionally excited lines for a large number of ions of different elements. We have obtained consistent estimations of the temperature fluctuation parameter, {\ts}, using different methods. We also report the detection of deuterium Balmer lines up to D$\delta$ (M16) and to D$\gamma$ (M20) in the blue wings of the hydrogen lines, which excitation mechanism seems to be continuum fluorescence. The temperature fluctuations paradigm agree with the results obtained from optical CELs and the more uncertain ones from far IR fine structure CELs in NGC 3603, although, more observations covering the same volume of the nebula are necessary to obtain solid conclusions.Comment: 22 pages, 13 Tables, 7 Figures. Accepted for publication by MNRA