Estimating dust distances to Type Ia supernovae from colour excess
  time-evolution
  We present a new technique to infer dust locations towards reddened Type Ia
supernovae and to help discriminate between an interstellar and a circumstellar
origin for the observed extinction. Using Monte Carlo simulations, we show that
the time-evolution of the light-curve shape and especially of the colour excess
\ebv~places strong constraints on the distance between dust and the supernova.
We apply our approach to two highly-reddened Type Ia supernovae for which dust
distance estimates are available in the literature: SN 2006X and SN 2014J. For
the former, we obtain a time-variable $E(B-V)$ and from this derive a distance
of $27.5^{+9.0}_{-4.9}$ or $22.1^{+6.0}_{-3.8}$ pc depending on whether dust
properties typical of the Large Magellanic Cloud (LMC) or the Milky Way (MW)
are used. For the latter, instead, we obtain a constant $E(B-V)$ consistent
with dust at distances larger than 50 and 38 pc for LMC$-$ and MW$-$type dust,
respectively. Values thus extracted are in excellent agreement with previous
estimates for the two supernovae. Our findings suggest that dust responsible
for the extinction towards these supernovae is likely to be located within
interstellar clouds. We also discuss how other properties of reddened Type Ia
supernovae $-$ such as their peculiar extinction and polarization behaviour and
the detection of variable, blue-shifted sodium features in some of these events
$-$ might be compatible with dust and gas at interstellar-scale distances.
