Microwave OH maser emission in the circumstellar envelopes of late-type stars

The total angular extents and absolute positions (+-1 arc s) of OH maser clouds surrounding 20 red giant and supergiant stars have been measured using the Very Large Array in a spectral line mode. The results are consistent with a symmetric, expanding circumstellar shell model for many of the sources, but about half of the sources have more complex structure at the arc second level. A correlation is found between the sizes of the masing regions at 1,612 MHz, the expansion velocities of the circumstellar shells, and the estimated mass loss rates. For one star (U Orionis) initiation of extensive mass loss has occurred very recently and dramatic changes in its OH spectrum may represent the 'turn on' stage of OH masers in evolved stars. (author)