Dust Properties in HII Regions in M33
  The conversion of the IR emission into star formation rate can be strongly
dependent on the physical properties of the dust, which are affected by the
environmental conditions where the dust is embedded. We study here the dust
properties of a set of HII regions in the Local Group Galaxy M33 presenting
different spatial configurations between the stars, gas and dust to understand
the dust evolution under different environments. We model the SED of each
region using the DustEM tool and obtain the mass relative to hydrogen for Very
Small Grains (YVSG), Polycyclic Aromatic Hydrocarbons (YPAH) and Big Grains
(YBG). The relative mass of the VSGs (YVSG/YTOT) is a factor of 1.7 higher for
HII regions classified as filled and mixed than for regions presenting a shell
structure. The enhancement of VSGs within NGC 604 and NGC 595 is correlated to
expansive gas structures with velocities greater than 50 km/s. The gas-to-dust
ratio derived for the HII regions in our sample exhibits two regimes related to
the HI-H2 transition of the ISM. Regions corresponding to the HI diffuse regime
present a gas-to-dust ratio compatible with the expected value if we assume
that the gas-to-dust ratio scales linearly with metallicity, while regions
corresponding to a H2 molecular phase present a flatter dust-gas surface
density distribution. The fraction of VSGs can be affected by the conditions of
the interstellar environment: strong shocks of 50-90 km/s existing in the
interior of the most luminous HII regions can lead to fragmentation of BGs into
smaller ones, while the more evolved shell and clear shell objects provide a
more quiescent environment where reformation of dust BG grains might occur. The
gas-to-dust variations found in this analysis might imply that grain
coagulation and/or gas-phase metals incorporation to the dust mass is occurring
in the interior of the HII regions in M33.
