Proper motions and distances of H2O maser sources. III - W51NORTH

Abstract

Accurate relative positions of the W51NORTH masers, determined by VLBI observations of the water vapor maser emission at 22 GHz, show changes of a few milli-arcsec over a two-year period. These displacements are interpreted as the result of transverse motions of about 50 km/sec, comparable with the radial velocity dispersion for this source, which are not random and show that clusters of maser features travel as a unit. The members of the maser cluster seem to be embedded in cloudlets of molecular gas having a scale size of 5 x 10 to the 14th cm. B-alpha observations made to establish the ambient velocity of the W51 region show that the ambient LSR velocity of the H II region is consistent with the molecular cloud velocity of about 56 km/sec. The distance to W51NORTH is estimated to be 8.3 + or - 2.5 kpc.