An analysis of intense OH maser emission in star-forming regions.

The interferometric observations of OH masers in star-forming regions are reviewed, with emphasis on VLBI data. A model of the OH maser zone described in Gray et al. (1991) and a theory of maser propagation in Field and Gray (1988) are employed to establish the physical conditions under which intense maser beams may form in the 18-, 5-, and 6-cm lines. It is shown that bright 1665-MHz spots indicate the presence of velocity shifts caused by accelerating flows of gold (not greater than 75 K) gas. Similar results are presented for the 1667-, 1720-, 1612-, 6035-, and 4765-MHz lines, and the association of 1720 and 4765 MHZ is demonstrated. A specific region of W3(OH) in which 1720-, 4765-, and 1665-MHz masers are spatially coincident at different V(lsr) is analyzed.