| Abstract: | Planck observations at 353GHz provide the first fully-sampled maps of the
polarized dust emission of interstellar filaments, offering unprecedented
information on the structure of the magnetic field. We present the polarization
properties of three nearby filaments, Musca, B211, and L1506. These three
filaments have similar total intensities (Stokes I), while the variations of
the Stokes Q and U are all different: the Musca filament is visible in the Q
and U maps, B211 and L1506 are seen in the Q map but are not distinguishable in
the U map, and the Q increase for L1506 is not spatially coincident with that
of I. They all offer 3pc segments, along which both the filament and the
background Stokes parameters are almost uniform. In all three cases, the
polarization fraction (p) towards the filaments is smaller than that of their
background. The polarized emission results from the combination of the magnetic
field (B) structure and the dust polarization properties. We model the
variations of the Stokes parameters across the filaments using variations
solely of the orientation of B, assuming constant dust polarization fraction
(p_0). Our modelling shows that the magnetic fields in the filaments and their
background have an ordered component. We find that for L1506, the
depolarization arises only from the rotation by 65^circ of the plane of the
sky (POS) projection of the field in the filament with respect to that of its
background. For Musca and B211, the drop in p is due mostly to different
orientations of B with respect to the POS inside and outside the filaments. The
magnetic fields inside Musca and B211 are nearly orthogonal to their long axes,
but almost parallel in the case of L1506. In spite of the degeneracy between
p_0 and the angle of B with respect to the POS, we find that for Musca and B211
the quality of the fit is better for p_0 values larger than 13% and 7%,
respectively. |
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