The recent star formation ( SF ) in the early-type spiral galaxy M81 is characterized using imaging observations from the far-ultraviolet ( UV ) to the far-infrared ( IR ) .
We compare these data with models of the stellar , gas , and dust emission for sub-galactic regions .
Our results suggest the existence of a diffuse dust emission not directly linked to the recent SF .
We find a radial decrease of the dust temperature and dust mass density , and in the attenuation of the stellar light .
The IR emission in M81 can be modeled with three components : 1 ) cold dust with a temperature < T _ { c } > = 18 \pm 2 K , concentrated near the HII regions but also presenting a diffuse distribution ; 2 ) warm dust with < T _ { w } > = 53 \pm 7 K , directly linked with the HII regions ; and 3 ) aromatic molecules , with diffuse morphology peaking around the HII regions .
We derive several relationships to obtain total IR luminosities from IR monochromatic fluxes , and we compare five different star formation rate ( SFR ) estimators for HII regions in M81 and M51 : the UV , H \alpha , and three estimators based on Spitzer data .
We find that the H \alpha luminosity absorbed by dust correlates tightly with the 24 \mu m emission .
The correlation with the total IR luminosity is not as good .
Important variations from galaxy to galaxy are found when estimating the total SFR with the 24 \mu m or the total IR emission alone .
The most reliable estimations of the total SFRs are obtained by combining the H \alpha emission ( or the UV ) and an IR luminosity ( especially the 24 \mu m emission ) , which probe the unobscured and obscured SF , respectively .
For the entire M81 galaxy , about 50 % of the total SF is obscured by dust .
The percentage of obscured SF ranges from 60 % in the inner regions of the galaxy to 30 % in the outer zones .