Radial velocity structure in OH and H2O masers.

It has recently been suggested that the spectra of many H2O masers are symmetric in nature, with one or more pairs of features equally spaced in radial velocity about a central axis. The present paper proposes that a similar symmetry exists in OH masers associated with H II regions and in main-line OH stars, such as VY CMa. The spectra of the maser sources W3 OH and OH 309.9+0.5 are analyzed as examples. The results demonstrate that features at nearly the same radial velocity are often present in several different OH as well as H2O transitions and probably originate in the same masing cloud. It is suggested that symmetric spectra could result from two basic geometries: expansion (or conceivably contraction) of gas shells around a central object or rotation of gas rings about a central axis. The same basic geometry is expected to apply to maser sources associated with H II regions, main-line OH stars, and type II OH/IR sources.