Methanol masers reveal the magnetic field of the high-mass protostar
  IRAS 18089-1732
  Context. The importance of the magnetic field in high-mass-star formation is
not yet fully clear and there are still many open questions concerning its role
in the accretion processes and generation of jets and outflows. In the past few
years, masers have been successfully used to probe the magnetic field
morphology and strength at scales of a few au around massive protostars, by
measuring linear polarisation angles and Zeeman splitting. The massive
protostar IRAS 18089-1732 is a well studied high-mass-star forming region,
showing a hot core chemistry and a disc-outflow system. Previous SMA
observations of polarised dust revealed an ordered magnetic field oriented
around the disc of IRAS 18089-1732. Aims. We want to determine the magnetic
field in the dense region probed by 6.7 GHz methanol maser observations and
compare it with observations in dust continuum polarisation, to investigate how
the magnetic field in the compact maser region relates to the large-scale field
around massive protostars. Methods. We reduced MERLIN observations at 6.7 GHz
of IRAS 18089-1732 and we analysed the polarised emission by methanol masers.
Results. Our MERLIN observations show that the magnetic field in the 6.7 GHz
methanol maser region is consistent with the magnetic field constrained by the
SMA dust polarisation observations. A tentative detection of circularly
polarised line emission is also presented. Conclusions. We found that the
magnetic field in the maser region has the same orientation as in the disk.
Thus the large-scale field component, even at the au scale of the masers,
dominates over any small-scale field fluctuations. We obtained, from the
circular polarisation tentative detection, a field strength along the line of
sight of 5.5 mG which appeared to be consistent with the previous estimates.
