The Oxygen Abundance in the Inner H Ii Regions of M101. Implications for the Calibration of Strong-line Metallicity Indicators1

I present deep spectroscopy of four H II regions in the inner, metal-rich zone of the spiral galaxy M101 obtained with the LRIS spectrograph at the Keck telescope. From the analysis of the collisionally excited lines in two of the target H II regions, H1013 and H493, I have obtained oxygen abundances 12 + log(O/H) = 8.52 and 12 + log(O/H) = 8.74, respectively. These measurements extend the determination of the oxygen abundance gradient of M101 via the direct method to only 3 kpc from the center. The intensity of the C IIλ4267 line in H1013 leads to a carbon abundance 12 + log(C/H) = 8.66, corresponding to nearly twice the solar value. From a comparison of the continuum temperature derived from the Balmer discontinuity, T (Bac) = 5000 K, and the line temperature derived from [O III] λ4363/λ5007, T [O III] = 7700 K, an average temperature T0 = 5500 K and a mean square temperature fluctuation t2 = 0.06 have been derived. Accounting for the spatial inhomogeneity in temperature raises the oxygen abundance obtained from the oxygen auroral lines to 12 + log(O/H) = 8.93. These findings are discussed in the context of the calibration of strong-line metallicity indicators, in particular of the upper branch of R23. There is no evidence for the strong abundance biases arising from temperature gradients predicted theoretically for metal-rich H II regions. Subject headings: ISM: abundances — galaxies: ISM — galaxies: abundances — galaxies: individual (M101)