Synthetic observations of dust emission and polarisation of Galactic
  cold clumps
  The Planck Catalogue of Galactic Cold Clumps (PGCC) contains over 13000
sources detected based on their cold dust signature. They are believed to
consist of a mixture of quiescent, pre-stellar, and already star-forming
objects. We extracted PGCC-type objects from cloud simulations and examined
their physical and polarisation properties. The comparison with the PGCC
catalogue helps to characterise the PGCC sample and provides valuable tests for
numerical simulations of interstellar medium.
  We used several MHD snapshots to define the density field of our models.
Sub-millimetre images of the surface brightness and polarisation were obtained
with radiative transfer calculations. We examined the statistics of synthetic
cold clump catalogues and examined the variations of the clump polarisation
fraction p. The clump sizes, aspect ratios, and temperatures in the synthetic
catalogue are similar to the PGCC. The fluxes and column densities are smaller
by a factor of a few. Rather than with an increased dust opacity, this could be
explained by increasing the average column density of the models by a factor of
two to three, close to N(H2)= 10^22 cm-2. When the line of sight is parallel to
the mean magnetic field, the polarisation fraction tends to increase towards
the clump centres, contrary to observations. When the field is perpendicular,
the polarisation fraction tends to decrease towards the clumps, but the drop in
$p$ is small (e.g. from p~8% to p~7%).
  Magnetic field geometry reduces the polarisation fraction in the simulated
clumps by only \Delta p~1% on average. The larger drop seen towards the actual
PGCC clumps suggests some loss of grain alignment in the dense medium, such as
predicted by the radiative torque mechanism. The statistical study is not able
to quantify dust opacity changes at the scale of the PGCC clumps.
