Multibaseline VLBI observations of the 1612 MHz OH masers toward NML Cygni and VY Canis Majoris

High-resolution observations of the 1612-MHz OH maser emission from the OH/IR stars NML Cygni and VY Canis Majoris were conducted in April and October 1977 by using a four-station VLBI array with an angular resolution between 0.16 and 0.015 arcsec. Comparison with previous maps over a period of 6 years shows no evidence of expansion or contraction greater than about 0.1 arcsec. Sizes of individual maser features are typically less than or about 0.02 arcsec for both stars. The principal features of the maps for both objects exhibit a similar overall velocity structure, namely, separate condensations of the extreme blue- and redshifted velocity components are offset from each other, while features closer to the stellar velocity are more uniformly distributed. The maps are in rough agreement with the expected velocity distribution due to a toroidal emission region of inner radius about 3 x 10 to the 15th cm when viewed at an oblique angle.