Abundances of Metal‐rich HiiRegions in M51

We have obtained multi-object spectroscopy of H II regions in the spiralgalaxy M51 with the Keck I telescope and the Low Resolution ImagingSpectrometer. For 10 objects we have detected the auroral line [N II] 5755,while [S III] 6312 has been measured in 7 of these. This has allowed us tomeasure the electron temperature of the gas and to derive O, S and N abundancesfor the 10 H II regions. Contrary to the expectations from previousphotoionization models of a few H II regions in M51 and from strong-lineabundance indicators, the O/H abundance is below the solar value for mostobjects, with the most metal-rich H II regions, P 203 and CCM 72, havinglog(O/H) = -3.16 and log(O/H) = -3.29, respectively. The reduction of O/H byfactors up to two or three with respect to previous indirect determinations hasimportant consequences for the calibration of empirical abundance indicators,such as R23, in the abundance and excitation range found in the central regionsof spiral galaxies. Based on our new abundances we revise the effective yieldfor M51, now found to be almost four times lower than previous estimates, andwe discuss this result within the context of chemical evolution in galacticdisks. Features from Wolf-Rayet stars are detected in a large number of H IIregions in M51, with the C III 5696 line found preferentially in the central,most metal-rich objects. (abridged)Comment: Accepted for publication in the Astrophysical Journal, 12 pages (emulateapj