The origin of dust polarization in molecular outflows
  Aims: In this paper we present a case study to investigate conditions
necessary to detect a characteristic magnetic field substructure embedded in a
large-scale field. A helical magnetic field with a surrounding hourglass shaped
field is expected from theoretical predictions and self-consistent
magnetohydrodynamical (MHD) simulations to be present in the specific case of
protostellar outflows. Hence, such an outflow environment is the perfect for
our study. Methodes: We present synthetic polarisation maps in the infrared and
millimeter regime of protostellar outflows performed with the newly developed
RT and polarisation code POLARIS. The code, as the first, includes a
self-consistent description of various alignement mechanism like the imperfect
Davis-Greenstein (IDG) and the radiative torque (RAT) alignment. We investigate
for which effects the grain size distribution, and applied alignement mechanism
have. Results: We find that the IDG mechanism cannot produce any measurable
polarization degree (< 1 %) whereas RAT alignment produced polarization degrees
of a few 1 %. Furthermore, we developed a method to identify the origin of the
polarization. We show that the helical magnetic field in the outflow can only
be observed close to the outflow axis and at its tip, whereas in the
surrounding regions the hourglass field in the foreground dominates the
polarization. Furthermore, the polarization degree in the outflow lobe is lower
than in the surroundings in agreement with observations. We also find that the
orientation of the polarization vector flips around a few 100 micron due to the
transition from dichroic extinction to thermal re-emission. Hence, in order to
avoid ambiguities when interpreting polarization data, we suggest to observed
in the far-infrared and mm regime. Finally, we show that with ALMA it is
possible to observe the polarization emerging from protostellar outflows.
