Interplay of dust alignment, grain growth and magnetic fields in
  polarization: lessons from the emission-to-extinction ratio
  Polarized extinction and emission from dust in the interstellar medium (ISM)
are hard to interpret, as they have a complex dependence on dust optical
properties, grain alignment and magnetic field orientation. This is
particularly true in molecular clouds. The data available today are not yet
used to their full potential.
  The combination of emission and extinction, in particular, provides
information not available from either of them alone. We combine data from the
scientific literature on polarized dust extinction with Planck data on
polarized emission and we use them to constrain the possible variations in dust
and environmental conditions inside molecular clouds, and especially
translucent lines of sight, taking into account magnetic field orientation.
  We focus on the dependence between \lambda_max -- the wavelength of maximum
polarization in extinction -- and other observables such as the extinction
polarization, the emission polarization and the ratio of the two. We set out to
reproduce these correlations using Monte-Carlo simulations where the relevant
quantities in a dust model -- grain alignment, size distribution and magnetic
field orientation -- vary to mimic the diverse conditions expected inside
molecular clouds.
  None of the quantities chosen can explain the observational data on its own:
the best results are obtained when all quantities vary significantly across and
within clouds. However, some of the data -- most notably the stars with low
emission-to-extinction polarization ratio -- are not reproduced by our
simulation. Our results suggest not only that dust evolution is necessary to
explain polarization in molecular clouds, but that a simple change in size
distribution is not sufficient to explain the data, and point the way for
future and more sophisticated models.
