Linear dust polarization during the embedded phase of protostar
  formation
  Measuring polarization from thermal dust emission can provide constraints on
the magnetic field structure around embedded protostars. However, interpreting
the observations is challenging without models that consistently account for
both the complexity of the protostellar birth environment and polarization
mechanisms. We aim to provide a better understanding with a focus on
bridge-like structures such as that observed towards the protostellar multiple
IRAS 16293--2422 by comparing synthetic polarization maps of thermal reemission
with observations. We analyze the magnetic field properties associated with the
formation of a protostellar multiple based on ideal MHD 3D zoom-in simulations
carried out with the RAMSES code. To compare with observations, we post-process
a snapshot of a bridge-like structure that is associated with a forming triple
star system with the radiative transfer code POLARIS and produce
multi-wavelength dust polarization maps. In the most prominent bridge of our
sample, the typical density is about 10^(-16) g cm^(-3), and the magnetic field
strength is about 1 to 2 mG. The magnetic field structure has an elongated
toroidal morphology and the dust polarization maps trace the complex
morphology. In contrast, the magnetic field strength associated with the
launching of asymmetric bipolar outflows is significantly more magnetized (~100
mG). At {\lambda}=1.3 mm, the orientation of grains in the bridge is similar
for the case accounting for radiative alignment torques (RATs) compared to
perfect alignment with magnetic field lines. However, the polarization fraction
in the bridge is three times smaller for the RAT scenario compared to assuming
perfect alignment. At shorter wavelengths ({\lambda} < 200 {\mu}m), dust
polarization does not trace the magnetic field because other effects such as
self-scattering and dichroic extinction dominate the orientation of the
polarization.
