Abundances of metal-rich HII regions in M51

We have obtained multiobject spectroscopy of H II regions in the spiral galaxy M51 with the Keck I telescope and the Low Resolution Imaging Spectrometer. For 10 objects we have detected the auroral line [N II] lambda5755, while [S III] lambda6312 has been measured in seven of these. This has allowed us to measure the electron temperature of the gas and to derive oxygen, sulfur, and nitrogen abundances for the 10 H II regions. Contrary to expectations from previous photoionization models of a few H II regions in M51 and from strong-line abundance indicators, the O/H abundance is below the solar value for most objects, with the most metal-rich H II regions, P203 and CCM 72, having log (O/H) = -3.16 [similar to1.4(O/H)(circle dot)] and log (O/H) = -3.29 [similar to1.0 (O/H)(circle dot)], respectively. The reduction of O/H by factors of up to 2 or 3 with respect to previous indirect determinations has important consequences for the calibration of empirical abundance indicators, such as R-23, in the abundance and excitation range found in the central regions of spiral galaxies. The abundance gradients in these galaxies can therefore be considerably flatter than those determined by using such empirical calibrations. The H II regions with a measured electron temperature span the range (0.19-1.04) R-0 in galactocentric radius and indicate a shallow abundance gradient for M51: -0.02 +/- 0.01 dex kpc(-1). The S/O abundance ratio is found to be similar to previous determinations of its value in other spiral galaxies, log (S/O) approximate to -1.6. Therefore, we find no evidence for a variation in massive-star initial mass function or nucleosynthesis at high oxygen abundance. An overabundance of nitrogen is measured, with log (N/O) similar or equal to -0.6. On the basis of our new abundances, we revise the effective yield for M51, now found to be almost 4 times lower than previous estimates, and we discuss this result in the context of chemical evolution in galactic disks. Features from Wolf-Rayet stars (the blue bump at 4660 Angstrom and the C III line at 5696 Angstrom) are detected in a large number of H II regions in M51, with the C III lambda5696 line found preferentially in the central, most metal-rich objects.