Temperature Structure and Metallicity in H II Regions

Abstract

The metallicities implied by collisionally excited lines (CELs) of heavy elements in H II regions are systematically lower than those implied by recombination lines (RLs) by factors ~2, introducing uncertainties of the same order in the metallicities inferred for the interstellar medium of any star-forming galaxy. Most explanations of this discrepancy are based on the different sensitivities of CELs and RLs to electron temperature, and invoke either some extra heating mechanism producing temperature fluctuations in the ionized region or the addition of cold gas in metal-rich inclusions or ionized by cosmic rays or X rays. These explanations will change the temperature structure of the ionized gas from the one predicted by simple photoionization models and, depending on which one is correct, will imply different metallicities for the emitting gas. We select nine H II regions with observed spectra of high quality and show that simple models with metallicities close to the ones implied by oxygen CELs reproduce easily their temperature structure, measured with Te([N II])/Te([O III]), and their oxygen CELs emission. We discuss the strong constraints that this agreement places on the possible explanations of the discrepancy and suggest that the simplest explanation, namely errors in the line recombination coefficients by factors ~2, might be the correct one. In such case, CELs will provide the best estimates of metallicity.

10 pages, 4 figures, uses emulateapj, to be published in ApJ

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• Ionized gas diagnostics from protoplanetary discs in the Orion Nebula and the abundance discrepancy problem
• The Discrepant Kinematics of ORLs and CELs in NGC 7009 as a Function of Ionization Structure
• The Kinematics of the Permitted C II $\lambda$ 6578 Line in a Large Sample of Planetary Nebulae
• The Oxygen Abundance in the Solar Neighborhood
• The massive multiple system HD 64315
• Small-spatial scale variations of nebular properties and the abundance discrepancy in three Galactic HII regions
• Wide field-of-view study of the Eagle Nebula with the Fourier transform imaging spectrograph SITELLE at CFHT
• SDSS-V LVM: A spatially resolved study of the physical conditions and the chemical abundance discrepancy in the Lagoon Nebula (M 8)
• The ADF and the t${}^2$ formalism in H II regions based on the upper mass limit of the IMF for the MW