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H$_2$O maser emission at 22 GHz in the circumstellar envelope is one of thegood tracers of detailed physics and kinematics in the mass loss process ofasymptotic giant branch stars. Long-term monitoring of an H$_2$O maser spectrumwith high time resolution enables us to clarify acceleration processes of theexpanding shell in the stellar atmosphere. We monitored the H$_2$O maseremission of the semi-regular variable R Crt with the Kagoshima 6-m telescope,and obtained a large data set of over 180 maser spectra during 1.3 years withan observational span of a few days. Based on an automatic peak detectionmethod based on a least-squares fitting with a Gaussian basis function model,we exhaustively detected peaks as significant velocity components with theradial velocity on a 0.1 km/s scale. This analysis result shows that the radialvelocity of red-shifted and blue-shifted components exhibits a change betweenacceleration and deceleration on the time scale of a few hundred days. Thesevelocity variations are likely to correlate with the intensity variations, inparticular during flaring state of H$_2$O masers. It seems reasonable toconsider that the velocity variation of the maser source is caused by the shockpropagation in the envelope due to stellar pulsation. However, it is difficultto explain the relationship between the velocity variation and the intensityvariation only from shock propagation effects. We found that a time delay ofthe integrated maser intensity with respect to the optical light curve is about150 days.

Time Variations of the Radial Velocity of H$_2$O Masers in the Semi-regular Variable R Crt