| We present the first 22 GHz polarimetric VLBI images of low-velocity water masers in the star-forming region W51 M. The compact maser concentration found near the reference position of W51 M is identified as a protostellar cocoon with both rotational and radial motions. The inner and outer radii of this water maser cocoon are 5 and 66 AU, respectively. Adjacent to this protostellar cocoon we see a 1200 AU long linear maser structure at a position angle of 200° (the streamer), which is roughly aligned with the Galactic magnetic field projection on the sky and the polarization position angle of these masers. The streamer masers move longitudinally along this direction with a median space velocity of 25 (+/-8.4) km s-1 relative to the centroid of the cocoon. In contrast to the cocoon masers, which show a mean linear polarization of only 3% (maximum 13%), the masers in the streamer exhibit higher degrees of linear polarization (mean 12%; maximum 35%). The level of distortion in the polarization directions of the streamer masers from the magnetic field direction together with the observationally estimated nonthermal velocity dispersion of the streamer spots with respect to the mean velocity of the streamer yield a preshock magnetic field strength, perpendicular to the shock velocity, of 0.9-1.2 (+/-0.25) mG. Inside the streamer masers we estimate the typical magnetic field strength to be 38 (+/-15) mG. We present statistics of the cocoon and streamer masers and discuss the origin of the maser stream, which is difficult to explain as an outflow from W51 M. Most likely the streamer is produced by shocks caused by the nearby expanding H II region, which interacts with the dense molecular core of W51 M on its western side. The proximity to the protostar suggests that these shocks have affected, or even triggered, the star formation in W51 M. |
