Self-scattering in protoplanetary disks with dust settling
  Scattering of re-emitted flux is considered to be at least partially
responsible for the observed polarisation in the (sub-)millimetre wavelength
range of several protoplanetary disks. Although the degree of polarisation
produced by scattering is highly dependent on the dust model, early studies
investigating this mechanism relied on the assumption of single grain sizes and
simple density distribution of the dust. However, in the dense inner regions
where this mechanism is usually most efficient, the existence of dust grains
with sizes ranging from nanometres to millimetres has been confirmed.
Additionally, the presence of gas forces larger grains to migrate vertically
towards the disk midplane, introducing a dust segregation in the vertical
direction. Using polarisation radiative transfer simulations, we analyse the
dependence of the resulting scattered light polarisation at 350 $\mu$m, 850
$\mu$m, 1.3 mm, and 2 mm on various parameters describing protoplanetary disks,
including the effect of dust grain settling. We find that the different disk
parameters change the degree of polarisation mostly by affecting the anisotropy
of the radiation field, the optical depth, or both. It is therefore very
challenging to deduce certain disk parameter values directly from polarisation
measurements alone. However, assuming a high dust albedo, it is possible to
trace the transition from optically thick to optically thin disk regions. The
degree of polarisation in most of the considered disk configurations is lower
than what is found observationally, implying the necessity to revisit models
that describe the dust properties and disk structure.
