KINEMATICAL MODELS OF MASER SHELLS FORMED BY ASPHERICAL STELLAR OUTFLOWS (U-ORIONIS, OH 231.8+4.2, ORION-IRC2)

Kinematical models are presented of maser emission uniformly distributed throughout ellipsoidal shells with various orientations to the line of sight. The velocity fields consist of different combinations of outflow, rotation, or radial acceleration which may be produced by the central star. The models are intended to be a guide for analysis of complex, aspherical outflows. They do not depend on the evolutionary nature of the star, nor are they limited to interpretation of a specific molecule. The models are applied to and reasonably describe three complex maser sources. The first case is the 1665 MHz OH emission associated with the classical Mira variable U Orionis, which can be modeled by radial outflow in an ellipsoidal shell. This model supports the possibility that nonbinary AGB stars may lose mass in aspherical outflows. The second case is the 1667 MHz OH emission associated with the bipolar nebula OH 231.8+4.2 (OH 0739-14). The velocity field of the nebula is modeled as a prolate spheroid with the outflow velocity increasing from about 10 km s-1 in the equatorial plane to about 200 km s-1 along the polar axis, and the OH is distributed at latitudes within 75-degrees of the plane. The dynamical time scale for mass loss in the equatorial plane is estimated to be about 4800 yr while that along the polar axis is about 700 yr. The results are consistent with a two-wind mechanism for the formation of the bipolar outflow. The third case consists of the low- and high-velocity outflows ostensibly associated with the luminous star IRc2 in the Orion molecular cloud. It is first shown that the vibrationally excited SiO emission, which occurs close to the star, can be modeled by an oblate spheroid with comparable components of rotation and outflow, demonstrating that a rotational component is present and that an ellipsoidal geometry provides a better approximation than a disk geometry. It is then proposed that the kinematics of the low- and high-velocity outflows, which occur much farther from the star, can be roughly described by a prolate spheroid with the outflow velocity increasing with latitude from the equatorial plane, qualitatively similar to the case of OH 231.8+4.2. Finally, a model is presented for the near region (SiO masers) and the far region (specifically, H2O masers), where the velocity fields are identical with the exception that there is additionally a rotational component in the near region. The sizes of the SiO and H2O maser regions are found to be comparable to those of a late-type supergiant with a high rate of mass loss (greater-than-or-equal-to 10(-4) M. yr-1).