SIO MASERS IN ASYMMETRIC MIRAS. I. R LEONIS

This is the first paper in a series of multi-epoch observations of the SiO masers at 7 mm wavelength in several asymptotic giant branch (AGB) stars. This is a sample of Mira variable stars showing evidence of asymmetric structure in the infrared which were observed interferometrically in the infrared by Infrared Optical Telescope Array and with Very Long Baseline Array measurements of the SiO masers. In this paper, we present the observations of R Leonis (R Leo). During the period of observations, this star shows extended emission with large-scale coherent patterns in the radial velocity, possibly the result of ejecting a substantial amount of material, largely to the west. This is interpreted as an event in which material is expelled in a collimated flow, possibly following an energetic event. If common, these events may help explain the asymmetric nature of the planetary nebulae that develop from AGB stars. The systemic velocity of R Leo is estimated to be +1.0 +/- 0.3 km s(-1). All observed radial velocities are well below the escape velocity.