Context :

Aims : 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 .

Methods : 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 .

Results : 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 .

Conclusions : 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 .