Temperature Structure and Metallicity in H II Regions
  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.
