Dust properties along anomalous extinction sightlines. II. Studying
  extinction curves with dust models
  The large majority of extinction sight lines in our Galaxy obey a simple
relation depending on one parameter, the total-to-selective extinction
coefficient, Rv. Different values of Rv are able to match the whole extinction
curve through different environments so characterizing normal extinction
curves. In this paper more than sixty curves with large ultraviolet deviations
from their best-fit one parameter curve are analyzed. These curves are fitted
with dust models to shed light into the properties of the grains, the processes
affecting them, and their relations with the environmental characteristics. The
extinction curve models are reckoned by following recent prescriptions on grain
size distributions able to describe one parameter curves for Rv values from 3.1
to 5.5. Such models, here extended down to Rv=2.0, allow us to compare the
resulting properties of our deviating curves with the same as normal curves in
a self-consistent framework, and thus to recover the relative trends overcoming
the modeling uncertainties. Such curves represent the larger and homogeneous
sample of anomalous curves studied so far with dust models. Results show that
the ultraviolet deviations are driven by a larger amount of small grains than
predicted for lines of sight where extinction depends on one parameter only.
Moreover, the dust-to-gas ratios of anomalous curves are lower than the same
values for no deviating lines of sight. Shocks and grain-grain collisions
should both destroy dust grains, so reducing the amount of the dust trapped
into the grains, and modify the size distribution of the dust, so increasing
the small-to-large grain size ratio. Therefore, the extinction properties
derived should arise along sight lines where shocks and high velocity flows
perturb the physical state of the interstellar medium living their signature on
the dust properties. (Abridged version)
