| Abstract: | Planck has mapped the polarized dust emission over the whole sky, making it
possible to trace the Galactic magnetic field structure that pervades the
interstellar medium (ISM). We combine polarization data from Planck with
rotation measure (RM) observations towards a massive star-forming region, the
Rosette Nebula in the Monoceros molecular cloud, to study its magnetic field
structure and the impact of an expanding HII region on the morphology of the
field. We derive an analytical solution for the magnetic field, assumed to
evolve from an initially uniform configuration following the expansion of
ionized gas and the formation of a shell of swept-up ISM. From the RM data we
estimate a mean value of the line-of-sight component of the magnetic field of
about +3 microG in the Rosette nebula, for a uniform electron density of about
11cm-3. The dust shell that surrounds the Rosette HII region is clearly
observed in the Planck intensity map at 353 GHz. The Planck observations
constrain the plane-of-the-sky orientation of the magnetic field in the region
to be mostly aligned with the large-scale field along the Galactic plane. The
data are compared with the analytical model, which predicts the mean
polarization properties of a spherical and uniform dust shell for a given
orientation of the field. This comparison leads to an upper limit of about
45deg on the angle between the line of sight and the magnetic field in the
Rosette complex, for an assumed intrinsic dust polarization fraction of 4%.
This field direction can reproduce the RM values detected in the ionized region
if the magnetic field strength in the Monoceros molecular cloud is in the range
9-12.5 microG. The present analytical model is able to reproduce the RM
distribution across the ionized nebula, as well as the mean dust polarization
properties of the swept-up shell, and can be directly applied to other similar
objects. |
REGISTER
Did you like this article? 
