In this proceeding paper, we introduce the recent results of Galactic maser astrometry by mainly focusing on those obtained with Japanese VLBI array VERA. So far we have obtained parallaxes for 86 sources including preliminary results, and combination with the data obtained with VLBA/BeSSeL provides astrometric results for 159 sources. With these most updated results we conduct preliminary determinations of Galactic fundamental parameters, obtaining R0 = 8.16 ± 0.26 kpc and Θ0 = 237 ± 8 km/s. We also derive the rotation curve of the Milky Way Galaxy and confirm the previous results that the rotation curve is fairly flat between 5 kpc and 16 kpc, while a remarkable deviation is seen toward the Galactic center region. In addition to the results on the Galactic structure, we also present brief overviews on other science topics related to masers conducted with VERA, and also discuss the future prospect of the project.

Differential VLBI observations of pulsars in our Galaxy can derive trigonometric parallax of them. Distance to pulsars derived by the parallax are very important to estimate some mean density of ionized gas between pulsars and the earth using rotation measures of them. Some preliminary results of distribution of the ionized gas density in our galaxy by using previous VLBI results are shown. Possibility of VLBI observations of pulsars using VERA and the other VLBI antennas will be described.

We report the results of multi-epoch very long baseline interferometry (VLBI) water (H2O) maser observations carried out with the VLBI Exploration of Radio Astrometry (VERA) toward the HW3d object within the Cepheus A star-forming region. We measured proper motions of 30 water maser features, tracing a compact bipolar outflow. This outflow is highly collimated, extending through ~400 mas (290 AU), and having a typical proper motion velocity of ~6 mas yr−1 (~21 km s−1). The dynamical timescale of the outflow was estimated to be ~100 years, showing that the outflow is tracing a very early star-formation phase. Our results provide strong support that the HW3d object harbors an internal massive protostar, as previous observations suggested. In addition, we have analyzed Very Large Array (VLA) archive 1.3 cm continuum data of the 1995 and 2006 epochs obtained towards Cepheus A. These results indicate possible different protostars around HW3d and/or strong variability in its radio continuum emission.

We present results of astrometric observations of S269 H2O maser performed with VERA (VLBI Exploration of Radio Astrometry). We have monitored the positions of S269 H2O masers for 1 year and successfully detected its parallax to be 189±8 micro-arcsecond. This corresponds to a source distance of 5.28+0.24−0.22 kpc, and is the smallest parallax (and thus the largest distance) that has ever been measured by means of annual parallax. Proper motions of S269 H2O maser were also measured and used to determine the Galactic rotation velocity at the position of S269. Our measurements show that the Galactic rotation velocity at S269 is the same to that at the Sun within 3%, indicating that the Galactic rotation curve is flat out to R~13 kpc.

We have carried out multi-epoch VLBI observations of the H2O maser sources associated with young stellar objects (YSOs) in nearby molecular clouds with VERA (VLBI Exploration of Radio Astrometry), which is a newly constructed VLBI network in Japan (Kobayashi et al. 2003). The main goal of our study is to measure the absolute proper motions and distances to nearby molecular clouds within 1 kpc from the Sun, to reveal their 3-dimensional structures and dynamical properties. Using the VERA dual-beam receiving system (Honma et al. 2003), we have carried out phase-referencing VLBI observations and measured annual parallaxes and absolute proper motions of the H2O maser features with respect to the extragalactic radio sources. We have successfully detected the annual parallax of one of the H2O maser features in Orion KL to be 2.29±0.10 mas, corresponding to the distance of 437±19 pc from the Sun (Hirota et al. 2007). In addition, the annual parallax of SVS13 in NGC 1333 is also determined to be4.10±0.17 mas, corresponding to the distance of 244±10 pc from the Sun, although the life time of the maser features are only 6 months. The absolute proper motions of the H2O maser features associated with Orion KL and NGC 1333 are derived, possibly indicating the outflow motions from the YSOs as well as the systemic motions of the powering sources.

We describe the proper motion measurements of water masers in the massive-star forming region W 51A and the analysis of the 3-D kinematics in three maser clusters of W 51A. We found a systematic expanding motion in one of the clusters named W 51 North, while no systematic motion was detected in other two clusters named W 51 Main and South. The 3-D motions of the clusters relative to the position reference feature in W 51 North indicate a separating motion between the W 51 North and the W 51 Main/South clusters. We estimated the distance to W 51 North as to be 6.7±2.1 kpc on basis of the statistical parallax and the model fitting methods.

It has been reported that the 22 GHz water maser in the star forming region Orion-KL has started an outburst in rate 1997 (Omodaka et al. 1998, IAUC 6893). Using Mizusawa radio telescope we started a monitoring observation of the bursting maser. We measured the linear polarization of the maser after the burst, during a phase of rapid flux density decrease. We find that the total flux density of 2.4 × 106 Jy (December 1998) exhibits about 46% linear polarization. Over the next six months we find that the total intensity decrease about two orders of magnitude while the fractional linear polarization gradually fell to 30%. These results suggest that the present bursting phenomenon has an origin similar to the super maser event starting in 1979, and the phenomenon of the extremely bright masers in this region is geometric in nature and related to the strong magnetic field.

A high-spatial-resolution observation of the NGC 7538 molecular cloud core has been performed with the Nobeyama Millimeter Array. We report on the detailed structure of the region including IRS1-3 complex and IRS11 based on the CS J = 1−0 line observational results.

The observation was done in December, 1988. The field center was at R.A. (1950)=23h11m36.8s, Dec (1950)=61° 11′ 10″ which is between IRS1-3 complex and IRS11. The primary beam, 2.5’(FWHM), was large enough to cover both IRS1-3 complex and IRS11. We used 18 baselines, and the synthesized beam became 10.6″×10.4″ (natural weight).

Millimeter-wave continuum sources in NGC 7538 region were observed with the NRO 45-m telescope and Nobeyama Millimeter Array. NRO 45-m telescope observations showed that the compact region which includes IRS1, IRS2, and IRS3 has a strong millimeter-wave intensity excess, cf. figures 1, and 2.

Observation of the high-velocity flows in the core region of the NGC7538 molecular cloud are carried out with CO (J=1-0) molecular line using NRO 45-m radio telescope. The beam size is 14″ and the mapping area covers about 4'x3′. Four high-velocity flows are found in the core region. The high-velocity flows are prominent at and around IRS1, 9, and 11.

B335 is now recognized as the smallest isolated star forming region. The detection of a Far-IR source and a bipolar flow were succesful, on the other hand, the distribution of the quiet gas is poorly understood. We are trying to determine the density distribution in B335. As the first step, we have carried out HC3N (J = 5-4 and 4-3) observations of B335. The observations of the J = 5-4 line have revealed a high density core with a 30″-60″ size. The Far-IR source is located just at the center of the core, and the core lies at the center of the bipolar flow. A mean hydrogen molecular density in the core of about 5x104 cm−3 is derived from the line ratio J – 5-4/4-3.

CS(J = 2-1) observations of B335 are carried out using the NRO 45-m telescope with a 16″ beam. We get many self-reversed profiles with good S/N ratio. Assuming that this region consists of a core and a halo, we get the molecular hydrogen density and the CS relative abundance in the halo. The CS wings are located within the cavity of the CO wing (Goldsmith et al. 1984).