Abundance gradients in M31: Comparison of results from supernova remnants and H II regions

We have obtained spectra of 11 H II regions and additional spectra of six previously reported supernova remnants (SNRs) in M31. The SNR spectra have been used in conjunction with shock model calculations to give abundances of oxygen, nitrogen, and sulfur in the interstellar gas comprising each remnant. We have also determined abundances for the H II regions using the empirical method described by Pagel et al. Both nitrogen and oxygen abundances decrease by about a factor of 4 from the innermost regions studied (approx.4 kpc) to the outer regions (approx.23 kpc). These gradients are similar to those found in other intermediate and late type spiral galaxies, including our own. The mean nitrogen and sulfur abundances are similar to those of the Orion Nebula, but the mean oxygen abundance is about a factor of 2 higher, accounting for the low excitation of the M31 h II regions. A comparison of the SNR and H II region abundance gradients shows substantial agreement for nitrogen, but discordant results for oxygen; this may be due to problems with the shock models since they do not reproduce the observed relative line intensities of O0, O+, and O++. Finally we present observations of SNR candidates in NGC 2403 and IC 342 and discuss the limitations and accuracy of the methods of detecting extragalactic SNRs