We present analyses to determine the fundamental parameters of the Galaxy based on VLBI astrometry of 52 Galactic maser sources obtained with VERA, VLBA and EVN. We model the Galaxy's structure with a set of parameters including the Galaxy center distance R 0 , the angular rotation velocity at the LSR Ω 0 , mean peculiar motion of the sources with respect to Galactic rotation (U src , V src , W src ), rotation-curve shape index, and the V component of the Solar peculiar motions V ⊙ . Based on a Markov chain Monte Carlo method, we find that the Galaxy center distance is constrained at a 5% level to be R 0 = 8.05 ± 0.45 kpc, where the error bar includes both statistical and systematic errors. We also find that the two components of the source peculiar motion U src and W src are fairly small compared to the Galactic rotation velocity, being U src = 1.0 ± 1.5 km s −1 and W src = −1.4 ± 1.2 km s −1 . Also, the rotation curve shape is found to be basically flat between Galacto-centric radii of 4 and 13 kpc. On the other hand, we find a linear relation between V src and V ⊙ as V src = V ⊙ − 19 (±2) km s −1 , suggesting that the value of V src is fully dependent on the adopted value of V ⊙ . Regarding the rotation speed in the vicinity of the Sun, we also find a strong correlation between Ω 0 and V ⊙ . We find that the angular velocity of the Sun, Ω ⊙ , which is defined as Ω ⊙ ≡ Ω 0 + V ⊙ /R 0 , can be well constrained with the best estimate of Ω ⊙ = 31.09 ± 0.78 km s −1 kpc −1 . This corresponds to Θ 0 = 238 ± 14 km s −1 if one adopts the above value of R 0 and recent determination of V ⊙ ∼12 km s −1 .

We performed the astrometry of H$_2$O masers in the Galactic star-forming region Onsala 2 North (ON 2 N) with the VLBI Exploration of Radio Astrometry (VERA). We obtained a trigonometric parallax of 0.261$\ \pm\ $0.009 mas, corresponding to a heliocentric distance of 3.83$\ \pm\ $0.13 kpc. ON 2 N is expected to be on the solar circle, because its radial velocity with respect to the local standard of rest (LSR) is nearly zero. By using the present parallax and proper motions of the masers, the galactocentric distance of the Sun and the Galactic rotation velocity at the Sun are found to be $R_0$$=$ 7.80$\ \pm\ $0.26 kpc and $\Theta_0$$=$ 213$\ \pm\ $5 km s$^{-1}$, respectively. The ratio of Galactic constants, namely the angular rotation velocity of the LSR, can be determined more precisely, and is found to be $\Omega_0$$=$$\Theta_0/R_0$$=$ 27.3$\ \pm\ $0.8 km s$^{-1}$kpc$^{-1}$, which is consistent with recent estimations, but different from 25.9 km s$^{-1}$kpc$^{-1}$ derived from the recommended values of $\Theta_0$ and $R_0$ by the International Astronomical Union (1985).

We present results of multi-epoch VLBI observations with VERA (VLBI Exploration of Radio Astrometry) of the 22GHz H$_{2}$O masers associated with a young stellar object (YSO) IRAS22198$+$6336 in a dark cloud, L1204G. Based on phase-referencing VLBI astrometry, we derived an annual parallax of IRAS22198$+$6336 to be 1.309$\pm$0.047mas, corresponding to the distance of 764$\pm$27pc from the Sun. Although the most principal error source of our astrometry is attributed to the internal structure of the maser spots, we successfully reduced the errors in the derived annual parallax by employing position measurements for all of the 26 detected maser spots. Based on this result, we reanalyzed the spectral energy distribution of IRAS22198$+$6336 and found that the bolometric luminosity and total mass of IRAS22198$+$6336 are 450$L_{\odot}$ and 7$M_{\odot}$, respectively. These values are consistent with an intermediate-mass YSO deeply embedded in the dense dust core, which has been proposed to be an intermediate-mass counterpart of a low-mass Class 0 source. In addition, we obtained absolute proper motions of the H$_{2}$O masers for the most blue-shifted components. We propose that the collimated jets aligned along the east-west direction are the most plausible explanation for the origin of the detected maser features.

We present a distance measurement for the semiregular variable S Crateris (S Crt) based on its annual parallax. With the unique dual beam system of the VLBI Exploration for Radio Astrometry (VERA) telescopes, we measured the absolute proper motion of a water maser spot associated with S Crt, referred to the quasar J1147$-$0724 located at an angular separation of 1.23D. In observations spanning nearly two years, we detected the maser spot at an LSR velocity of 34.7kms$^{-1}$, for which we measured an annual parallax of 2.33$\pm$0.13mas, corresponding to a distance of 430$^{+25}_{-23}$pc. This measurement has an accuracy one order of magnitude better than the parallax measurements of HIPPARCOS. The angular distribution and three-dimensional velocity field of maser spots indicate a bipolar outflow with the flow axis along the northeast-southwest direction. Using the distance and photospheric temperature, we estimated the stellar radius of S Crt and compared it with those of Mira variables.

We report on the results of monitoring observations of 242 stellar H$_2$O masers, which have been made with the Iriki 20m telescope of the VLBI Exploration of Radio Astrometry (VERA) from 2003 July to 2006 November. The present paper mainly focuses on 85 stellar H$_2$O masers that have been tightly observed with a time spacing of typically 1−2 months. In particular, 46 masers out of them have been recognized concerning their periodic flux variation and have light−curve data of stellar visual light. Thus, the present paper shows some statistical views of the observed time variability properties of stellar H$_2$O masers. We found a good correlation between a time delay of the variation in the H$_2$O maser flux with respect to that in the stellar visual light and the stellar pulsation period. The corresponding phase lags are mildly scattered, but are mainly concentrated in the range, $0.7\leq\Delta \phi\leq\;$1.5. We also measured line−of−sight velocity drifts of the individual spectral peaks of H$_2$O maser emission, which indicate radial acceleration of mass−loss outflows from the evolved stars. We discuss possible pulsation−driven shock waves that are enhanced near the stellar surface, and are propagating outwards in the circumstellar envelope.