Methanol masers reveal the magnetic field of a high mass protostar

Abstract: The role played by magnetic fields in high-mass star formation is not yet fully clear. Theoretical simulations have shown that magnetic fields appear to suppress fragmenta- tion in the star forming cloud, to enhance accretion via disc and to provide feedback in the form of outflows and jets. However, models require specific magnetic configurations and need more observational constraints to properly test the impact of magnetic fields. The identification of massive protostars is complicated due to their quick evolution, and their location inside distant, dense, and dark clusters. In the past few years, masers have been successfully used to probe the magnetic field strength and morphology at the small scales of about 10 astronomical units (au), around massive protostars. Thanks to the narrow and strong spectral lines of masers, we can measure linear polarization angles and Zeeman splitting and obtain information about the magnetic field intensity and geometry. Radio-interferometers, such as the Multi- Element Radio Linked Interferometer Network (MERLIN), can provide the sensitivity and the spatial and spectral resolution needed to detect the signatures of protostellar processes at the required scale of few au. In this work we make use of MERLIN data to investigate the magnetic field structure of the massive protostar IRAS18089-1722, analyzing 6.7 GHz methanol maser obser- vations. IRAS18089-1732 is a well studied high mass protostar, showing a hot core chemistry, an accretion disc and a bipolar outflow. An ordered magnetic field oriented around its disc has been detected from previous observations of polarized dust. This gives us the chance to investigate how the magnetic field at the small scale probed by masers relates to the large scale field probed by the dust. Our analysis of the 6.7 GHz polarized methanol maser observations, indicates that the magnetic field in the maser region is consistent with the magnetic field constrained by the previous dust polarized observations. We find that the magnetic field in the maser region presents the same orientation as in the disc. Thus the large scale field component, even at the few au scale of the masers, dominates over any small scale field fluctuations. We present a tentative detection of circularly polarized line emission, from which we obtain a field strength along the line of sight of 5.5 mG, consistent with previous estimates.