  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+-2 K, concentrated near
the HII regions but also presenting a diffuse distribution; 2) warm dust with
T_w=53+-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, Halpha, and three estimators based on Spitzer data. We
find that the Halpha luminosity absorbed by dust correlates tightly with the 24
microns 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 microns or the total IR emission alone. The most reliable
estimations of the total SFRs are obtained by combining the Halpha emission (or
the UV) and an IR luminosity (especially the 24 microns 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.
