VLA observations of a highly symmetric OH maser in a bipolar nebula

The Very Large Array was used to map 1667 MHz OH maser emission from the bipolar nebula OH 231.8+4.2 at 23 distinct velocities within the unusual, 100 km s/sup -1/ wide profile. The source is large (approx.10''equivalent3 x 10/sup 17/ cm) and well resolved, and displays ordered large-scale velocity gradients. At most velocities, the maser maps display an unmistakable symmetry about the bipolar axis defined by the optical and infrared reflection nebulae. Most of the data can be accounted for by an axisymmetric model in which the measuring OH is concentrated toward the system's equatorial plane and is expanding radially away from the central star. The observation of complete rings of maser emission at some velocities, however, shows that the maser is also present at high latitudes above the equatorial plane. A model which incorporates these features plus other known aspects of bipolar nebulae is presented and discussed.