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Astron. Astrophys. 319, L1-L4 (1997)

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1. Introduction

Stellar water masers as observed with the high spatial resolution of VLBI are expected to be very useful tracers of detailed physics and kinematics in the mass-loss process of evolved stars (e.g. Elitzur 1992; Takaba et al. 1994).

Previous VLA observations revealed that the stellar water masers distribute in circumstellar shells with radii ranging from about 5-10 to 50-100 AU in Mira and semiregular variables (Lane et al. 1987; Bowers & Johnston 1994, hereafter BJ). Benson et al. (1992) and BJ found periodic variations of the water masers correlated with the optical light curves in intensity and spatial distribution, respectively. Bowers, Claussen & Johnston (1993, hereafter BCJ) as well as BJ pointed out that the angular distributions of the water masers are often significantly asymmetric relative to the estimated stellar position. Although the velocity field of the masers is generally consistent with the expanding shell model, the expansion is not strictly isotropic. In some cases, we observe clear anisotropy suggestive of weakly bipolar outflow (BJ). Yates & Cohen (1994, hereafter YC) and BJ suggest that the material in the masing region is accelerated to the terminal velocity of about 10 km s-1 at radii ranging from several to a few tens of AU.

In the light of the above rather complicated appearance of the circumstellar environment including the water masers, it is quite interesting to directly detect the acceleration of the maser spots. Having information on positions, velocities and accelerations of the maser spots, we will be able to greatly constrain the underlying dynamics of the mass-loss process. The simplest way to detect the acceleration is to measure shifts of the line-of-sight velocities in time for the maser spots, which are known to be very compact with apparent sizes of the order of 0.1 to 10 milliarcseconds (mas) (Spencer et al. 1979; Imai et al. 1997). However, tracing the velocity shifts is sometimes difficult since several maser spots are often blended within a single peak in the velocity profile obtained from single-dish observations (Sullivan III 1971, 1973; BCJ; BJ; YC). Therefore, higher spatial resolution is indispensable to trace the velocity shift of each spatially-well-distinguished spot.

Here we report first measurement of the linear shifts of line-of-sight velocities for water maser spots associated with a semiregular variable RT Vir obtained in a VLBI monitoring using Japanese domestic network called J-Net.

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© European Southern Observatory (ESO) 1997

Online publication: July 3, 1998