The population of planetary nebulae and HII regions in M81. A study of
  radial metallicity gradients and chemical evolution
  We analyze the chemical abundances of planetary nebulae and HII regions in
the M81 disk for insight on galactic evolution, and compare it with that of
other galaxies, including the Milky Way. We acquired Hectospec/MMT spectra of
39 PNe and 20 HII regions, with 33 spectra viable for temperature and abundance
analysis. Our PN observations represent the first PN spectra in M81 ever
published, while several HII region spectra have been published before,
although without a direct electron temperature determination. We determine
elemental abundances of helium, nitrogen, oxygen, neon, sulfur, and argon in
PNe and HII regions, and determine their averages and radial gradients. The
average O/H ratio of PNe compared to that of the HII regions indicates a
general oxygen enrichment in M81 in the last ~10 Gyr. The PN metallicity
gradient in the disk of M81 is -0.055+-0.02 dex/kpc. Neon and sulfur in PNe
have a radial distribution similar to that of oxygen, with similar gradient
slopes. If we combine our HII sample with the one in the literature we find a
possible mild evolution of the gradient slope, with results consistent with
gradient steepening with time. Additional spectroscopy is needed to confirm
this trend. There are no Type I PNe in our M81 sample, consistently with the
observation of only the brightest bins of the PNLF, the galaxy metallicity, and
the evolution of post-AGB shells. Both the young and the old populations of M81
disclose shallow but detectable negative radial metallicity gradient, which
could be slightly steeper for the young population, thus not excluding a mild
gradients steepening with the time since galaxy formation. During its evolution
M81 has been producing oxygen; its total oxygen enrichment exceeds that of
other nearby galaxies.
