Magnetic field regulated infall on the disc around the massive protostar
  Cepheus A HW2
  We present polarization observations of the 6.7-GHz methanol masers around
the massive protostar Cepheus A HW2 and its associated disc. The data were
taken with the Multi-Element Radio Linked Interferometer Network. The maser
polarization is used to determine the full three-dimensional magnetic field
structure around Cepheus A HW2. The observations suggest that the masers probe
the large scale magnetic field and not isolated pockets of a compressed field.
We find that the magnetic field is predominantly aligned along the protostellar
outflow and perpendicular to the molecular and dust disc. From the
three-dimensional magnetic field orientation and measurements of the magnetic
field strength along the line of sight, we are able to determine that the high
density material, in which the masers occurs, is threaded by a large scale
magnetic field of ~23 mG. This indicates that the protostellar environment at
~1000 AU from Cepheus A HW2 is slightly supercritical (lambda approximately
1.7) and the relation between density and magnetic field is consistent with
collapse along the magnetic field lines. Thus, the observations indicate that
the magnetic field likely regulates accretion onto the disc. The magnetic field
dominates the turbulent energies by approximately a factor of three and is
sufficiently strong to be the crucial component stabilizing the massive
accretion disc and sustaining the high accretion rates needed during massive
star-formation.
