MEASUREMENT OF SHIFTS IN LINE-OF-SIGHT VELOCITIES OF STELLAR WATER MASERS USING VLBI

We performed VLBI monitoring observations of water masers around a sem iregular variable RT Vir and detected for the first time systematic sh ifts of line-of-sight velocities in time for several spatially disting uished water maser spots. In some strong water maser spots, the line-o f-sight velocities linearly increased or decreased through successive observing sessions during four months. The rates of the velocity shift s ranged from -3.55 to 1.76 km s(-1) yr(-1). We also noted that the bl ue-shifted and red-shifted maser components with respect to the stella r velocity are clearly separated in the celestial plane. If mass-loss process of the evolved star is described with simple kinematic models like bipolar flow or rotation disk, we should find one-to-one correspo ndence between acceleration/deceleration and blue-shift/red-shift of t he maser spots. However, we did not find such a clear correspondence. The fact is likely to imply the more complicated nature of the real ma ss-loss process of the evolved star.