Properties of extragalactic dust inferred from linear polarimetry of
  Type Ia Supernovae
  Aims: The aim of this paper is twofold: 1) to investigate the properties of
extragalactic dust and compare them to what is seen in the Galaxy; 2) to
address in an independent way the problem of the anomalous extinction curves
reported for reddened Type Ia Supernovae (SN) in connection to the environments
in which they explode. Methods: The properties of the dust are derived from the
wavelength dependence of the continuum polarization observed in four reddened
Type Ia SN: 1986G, 2006X, 2008fp, and 2014J. [...] Results: All four objects
are characterized by exceptionally low total-to-selective absorption ratios
(R_V) and display an anomalous interstellar polarization law, characterized by
very blue polarization peaks. In all cases the polarization position angle is
well aligned with the local spiral structure. While SN~1986G is compatible with
the most extreme cases of interstellar polarization known in the Galaxy,
SN2006X, 2008fp, and 2014J show unprecedented behaviours. The observed
deviations do not appear to be connected to selection effects related to the
relatively large amounts of reddening characterizing the objects in the sample.
Conclusions: The dust responsible for the polarization of these four SN is most
likely of interstellar nature. The polarization properties can be interpreted
in terms of a significantly enhanced abundance of small grains. The anomalous
behaviour is apparently associated with the properties of the galactic
environment in which the SN explode, rather than with the progenitor system
from which they originate. For the extreme case of SN2014J, we cannot exclude
the contribution of light scattered by local material; however, the observed
polarization properties require an ad hoc geometrical dust distribution.
