Tracking the Motions of Cosmic Masers
Session 27 -- Hat Creek
Oral presentation, Tuesday, 8:30-12:30, Dwinelle 155 Room

## [27.04] Tracking the Motions of Cosmic Masers

J.M.Moran (CfA)

The Hat Creek 85-foot telescope was used in the first VLBI experiment involving more than two elements, in July 1967. The first real map of a molecular maser resulted from that experiment, which revealed that the OH maser in W3(OH) consisted of at least seven unresolved spots. From a second experiment in 1968, crude limits on the proper motions of 40~km~s$^{-1}$ were set and some of the spots were resolved. The sensitivity of VLBI experiments has greatly improved over the last two decades and the most recent images of the OH maser in W3(OH) contain 70 or more spots and proper motions for many of them have been measured. The motions of H$_2$O masers around newly formed stars have also been extensively measured by VLBI techniques. A comparison of the transverse angular motions with the line-of-sight Doppler motions provides a method for estimating their distances. The scale size of the Galaxy, R$\rm _o$, derived by comparing the distances to seven masers with a model of the Galaxy having pure circular rotation at 220~km~s$^{-1}$ is 7.8$\pm$0.6~kpc. This method of determining distances has been extended to a maser in the nearby galaxy M33 where proper motions have been detected and a preliminary distance derived.