VLBI observations of the main line OH masers in VY Canis Majoris

It is pointed out that the OH/IR supergiant VY Canis Majoris is an excellent candidate for investigating the dynamics of a circumstellar cloud. VY CMa displays very strong maser emission from the SiO, H2O, and OH molecules. A series of VLBI observations of the VY CMa 1665 MHz and 1667 MHz main line OH masers was conducted to prove the inner regions of the VY CMa envelope and determine the circumstellar geometry and kinematics. These observations substantially extend the spatial and spectral resolution of previous observations. The spatial structure observed can be combined with the spectral polarization data to form a consistent picture of the kircumstellar magnetic field. The 1665 MHz Zeeman doublet permits a direct measurement of an average magnetic field strength of approximately 1 milligauss. The isolated -2.1 km/s 1667 MHz feature is 100% left circularly polarized and no spatially coincident RHC feature is seen in the 1667 MHz map.