Abundance gradient across M 81

Spectra of 18 H II regions of the early type spiral galaxy M 81 have been obtained and studied to determine the oxygen abundance gradient across the galaxy. A radial gradient in O/H with a slope of about -0.08 dex/kpc is found, with calculations from photoionization models confirming the results from the empirical calibration. N/O has a shallow gradient, with the value of N(+)/O(+) clustered around the solar value of N/O. The outermost region in M 81 has an oxygen abundance 0.7 dex smaller than that of the sun, but has essentially solar N/O. This contrasts with the results obtained for other H II regions of comparably low oxygen abundance, particularly those in dwarf emission-line galaxies, which have much lower N/O ratios. The O/H gradient and the overall abundance of oxygen in M 81 are similar to those seen in other spiral galaxies of similar mass and luminosity. 20 references