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[75.15] Modeling the Radial Velocity Curve of the Water Vapor Maser in VX UMa

D. M. Salter, P. J. Benson (Wellesley College), V. S. Strelnitski (Maria Mitchell Observatory)

VX UMa is a unique Mira-type star that demonstrates a triple-peaked spectrum of its 1.35-cm H₂O maser emission. We used the high-precision curves of radial velocities of the spectral peaks, obtained by Benson & Little-Marenin from 1988 to 1992, as probes of the kinematics of the masing region. The pronounced periodicity of the radial velocity of the central component, with a period equal to the pulsational period of the optical variations, suggests the involvement of pulsations in the observed excursions of radial velocity. However, the radial velocity of the central spectral component produced by a symmetrical, pulsating spherical layer should be constantly zero. Rotation seems to be the most obvious mechanism to impart a small non-zero component to the central feature. We assume that the bulk of maser radiation originates in the equatorial "belt" around the star and approximate this region as a two-dimensional, rotating and pulsating ring. We found that any combination of rotation and pulsation produces a quadruple peaked, not a triple peaked spectrum. Therefore, some asymmetry in the disk or unequal absorption of the two central peaks by ionized gas (e.g. in the shock responsible for the maser emission) is needed. We demonstrate that one of the central peaks can then undergo periodic changes of its radial velocity with the period of pulsation, as observed. A VLBA experiment that may verify our model is under way. This project was supported by the NSF/REU grant AST-9820555.