Junichi Nakajima

3-D Kinematics of Water Masers in the W 51A Region

We report proper motion measurements of water masers in the massive-star forming region W51A and the analyses of the 3-D kinematics of the masers in three maser clusters of W51A (W51 North, Main, and South). In W~51 North, we found a clear expanding flow that has an expansion velocity of ~70 km/s and indicates deceleration. The originating point of the flow coincides within 0.1 as with a silicon-monoxide maser source near the HII region W~51d. In W51 Main, no systematic motion was found in the whole velocity range (158 km/s =< V(lsr) =< -58 km/s) although a stream motion was reported previously in a limited range of the Doppler velocity (54 km/s =< V(lsr) =< 68 kms). Multiple driving sources of outflows are thought to explain the kinematics of W51 Main. In W51 South, an expansion motion like a bipolar flow was marginally visible. Analyses based on diagonalization of the variance-covariance matrix of maser velocity vectors demonstrate that the maser kinematics in W51 North and Main are significantly tri-axially asymmetric. We estimated a distance to W51 North to be 6.1 +/- 1.3 kpc on the basis of the model fitting method adopting a radially expanding flow.

J-Net Galactic-Plane Survey of VLBI Radio Sources for VLBI Exploration of Radio Astrometry (VERA)

In order to search for new VLBI sources in the Galactic plane that can be used as phase reference sources in differential VLBI, we have conducted 22 GHz observations of radio sources in the Galactic plane using the Japanese VLBI Network (J-Net). We have observed 267 VLBI source candidates selected from existing radio surveys and have detected 93 sources at the signal-to-noise ratio larger than 5. While 42 of the 93 detected sources had already been detected with VLBI at relatively lower frequency (typically 2 to 8 GHz), the remaining 51 are found to be new VLBI sources detected for the first time. These VLBI sources are located within

VLBI Monitoring Observations of Water Masers around the Semi-Regular Variable Star R Crateris

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VLBI Observations of Water Masers in the Circumstellar Envelope of IRC+60169

, and have a large number of Galactic maser sources around them. Thus, they are potential candidates for phase reference sources for VLBI Exploration of Radio Astrometry (VERA), which is the first VLBI array dedicated to the phase referencing VLBI astrometry aiming at measuring the parallax and proper motion of maser sources in the whole Galaxy.

Overview and initial results of the Very long baseline Interferometry Space Observatory Programme

AbstractWe monitored water-vapor masers around the semi-regular variable starR Crateris with the Japanese VLBI Network (J-Net) at the 22 GHz band duringfour epochs with intervals of one month. The relative proper motions and Doppler-velocity drifts of twelve maser features were measured. Most ofthem existed for longerthan 80 days. The 3-D kinematics of the features indicates a bipolar expanding flow.The major axis of the asymmetric flow was estimated to be at P.A. = 136 ◦ . Theexistence of a bipolar outflow suggests that a Mira variable star had already formeda bipolar outflow. The water masers are in a region of apparent minimum radii of1.3 × 10 12 m and maximum radii of 2.6 × 10 12 m, between which the expansion ve-locity ranges from 4.3 to 7.4 km s −1 . These values suggest that the water masers areradially accelerated, but still gravitationally bound, in the water-maser region. Themost positive and negative velocity-drifting features were found relatively close to thesystemic velocity of the star. We found that the blue-shifted features are apparentlyaccelerated and the red-shifted apparently decelerated. The acceleration of only theblue-shifted features seems to be consistent with that of the expanding flow from thestar.Key words: masers — stars: late-type — stars: individual (R Crateris)1

The VLBI Space Observatory Programme and the Radio-Astronomical Satellite HALCA

Water masers around an AGB star IRC+60169 have been observed at four epochs using the Japanese VLBI networks. The distribution of the red-shifted features exhibits a ring-like structure, the size of which is 30 mas and corresponds to the inner radius of the maser shell. This implies that dense gas around the star obscures red-shifted emission