The magnetic field in the star-forming region Cepheus A. from H2O maser polarization observations

We present linear and circular polarization observations of the H2O masers in 4 distinct regions spread over 1 × 2 arcsec around the HW2 high-mass young stellar object in the Cepheus A star-forming region. We find magnetic fields between 100-600 mG in the central maser region, which has been argued to trace a circumstellar disk. The masers further from HW2 have field strengths between 30-100 mG. In all cases the magnetic field pressure is found to be similar to the dynamic pressure, indicating that the magnetic field is capable of controlling the outflow dynamics around HW2. In addition to several H2O maser complexes observed before, we also detect a new maser filament, ≃1 arcsec (≃690 AU) East of HW2, which we interpret as a shocked region between the HW2 outflow and the surrounding medium. We detect a linear polarization gradient along the filament as well as a reversal of the magnetic field direction. This is thought to mark the transition between the magnetic field associated with the outflow and that found in the surrounding molecular cloud. In addition to the magnetic field we determine several other physical properties of the maser region, including density and temperatures as well as the maser beaming angles.