The radial metallicity gradient and the history of elemental enrichment
  in M81 through emission-line probes
  We present a new set of weak-line abundances of HII regions in M81, based on
Gemini Multi-Object Spectrograph (GMOS) observations. The aim is to derive
plasma and abundance analysis for a sizable set of emission-line targets to
study the galactic chemical contents in the framework of galactic metallicity
gradients. We used the weak-line abundance approach by deriving electron
density and temperatures for several HII regions in M81. Gradient analysis is
based on oxygen abundances.Together with a set of HII region abundances
determined similarly by us with Multi-Mirror Telescope (MMT) spectra, the new
data yield to a radial oxygen gradient of -0.088$\pm$0.013 dex kpc$^{-1}$,
which is steeper than the metallicity gradient obtained for planetary nebulae
(-0.044$\pm$0.007 dex kpc$^{-1}$). This result could be interpreted as gradient
evolution with time: Models of galactic evolution with inside-out disk
formation associated to pre-enriched gas infall would produce such difference
of gradients, although stellar migration effects would also induce a difference
in the metallicity gradients between the old and young populations. By
comparing the M81 metallicity gradients with those of other spiral galaxies,
all consistently derived from weak-line analysis, we can infer that similar
gradient difference is common among spirals. The metallicity gradient slopes
for HII regions and PNe seem to be steeper in M81 than in other galactic disks,
which is likely due to the fact that M81 belongs to a galaxy group. We also
found that M81 has experienced an average oxygen enrichment of 0.14$\pm$0.08
dex in the spatial domain defined by the observations. Our data are compatible
with a break in the radial oxygen gradient slope around R$_{25}$ as inferred by
other authors both in M81 and in other galaxies.
