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 occur, 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 (λ≈ 1.7) and the relation between density and magnetic field is consistent with the collapse along the magnetic field lines. Thus, the observations indicate that the magnetic field likely regulates accretion on to the disc. The magnetic field dominates the turbulent energies by approximately a factor of 3 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.