The Central Molecular Zone (CMZ) in the Galactic Center region shows outstanding non-circular motion unlike the Galactic disk. While several models describing this non-circular motion have been proposed, a uniform kinematic model of the CMZ orbit has not yet emerged. To uncover the dynamics of the Galactic center region, we conducted VLBI astrometric observations of 22GHz water maser sources towards the Galactic center using VERA. By measuring parallaxes and proper motions, we can determine whether each source is actually located in the CMZ or not, and identify the three-dimensional positions and velocities in the non-circular orbit if the source is indeed located in the CMZ. We present the results of our astrometric study for several maser sources associated with molecular clouds towards the Galactic center. The astrometric observations toward Sgr B2(M) indicated that Sgr B2 complex is moving toward the positive Galactic longitude relative to Sgr A*.

VERA has been regularly conducting astrometry of Galactic maser sources for ∼ 20 years, producing more than 100 measurements of parallaxes and proper motions of star-forming regions as well as AGB stars. By combining the observational results obtained by VLBA BeSSeL, EVN, and LBA, maser astrometry provides a unique opportunity to explore the fundamental structure of the Galaxy. Here we present the view of the Galaxy revealed by the maser astrometry, and also discuss the importance of maser astrometry in the era of GAIA by comparing the results obtained by VLBI and GAIA. We also present our view of “proper motions toward the future” of the relevant field, expected in the next decade based on global collaborations.

We present observational results of the submillimeter H2O and SiO lines toward a candidate high-mass young stellar object Orion Source I using ALMA. The spatial structures of the high excitation lines at lower-state energies of >2500 K show compact structures consistent with the circumstellar disk and/or base of the northeast-southwest bipolar outflow with a 100 au scale. The highest excitation transition, the SiO (v=4) line at band 8, has the most compact structure. In contrast, lower-excitation transitions are more extended than 200 au tracing the outflow. Almost all the line show velocity gradients perpendicular to the outflow axis suggesting rotation motions of the circumstellar disk and outflow. While some of the detected lines show broad line profiles and spatially extended emission components indicative of thermal excitation, the strong H2O lines at 321 GHz, 474 GHz, and 658 GHz with brightness temperatures of >1000 K show clear signatures of maser action.

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.

We will report the activities of the VERA single-dish observations. We are carrying out single-dish observations with two purposes. The first purpose is the monitoring of known H2O maser sources. At present, we are carrying out monitoring observations for 312 H2O maser sources at intervals of two months. The second purpose is the search for new water maser sources. We selected 901 target sources from the AKARI FIS Bright Source Catalogue. We found 61 new H2O maser sources.

We report on interferometric observations of a face-on accretion system around the high mass young stellar object, G353.273+0.641. The innermost accretion system of 100-au radius was resolved in a 45-GHz continuum image taken with the Jansky Very Large Array. Our SED analysis indicated that the continuum could be explained by optically-thick dust emission. 6.7 GHz CH3OH masers associated with the same system were also observed with the Australia Telescope Compact Array. The masers showed a spiral-like, non-axisymmetric distribution with a systematic velocity gradient. The line-of-sight velocity field is explained by an infall motion along a parabolic streamline that falls onto the equatorial plane of the face-on system. The streamline is quasi-radial and reaches the equatorial plane at a radius of 16 au. The physical origin of such a streamline is still an open question and will be constrained by the higher-resolution thermal continuum and line observations with ALMA long baselines.

We have conducted astrometric observations toward a 22 GHz water maser source associated with the Sgr B2 complex in the Galactic center region with VERA (VLBI exploration of Radio Astrometry). We measured a trigonometric parallax and absolute proper motion of the Sgr B2 complex with respect to an extra-galactic source by observing the water maser source at 10 epochs from 2014 to 2017. The measured distance was 7.52+3.01−1.67 kpc for the Sgr B2M region.

We also succeeded to measure internal motions of maser spots in Sgr B2M, and N region. The number of spots which we could measure the internal motions is about 400. The distribution of the maser spots shows that the maser spots are associated with envelope of HII region seen in radio continuum image obtained with VLA and ALMA. We discuss relative motions between Sgr B2M, and N by using the internal motion.

The Galactic centre region shows outstanding non-circular motion unlike the Galactic disk. As scenarios describing this non-circular motion, resonance orbits formed by the Galactic bar potential or expanding motion by past activity of the central BH are proposed. However, these both scenarios are based on line-of-sight velocities of molecular clouds in this region, and such one-dimension velocity information is insufficient to separate these scenarios.

To reveal dynamics of the Galactic centre region, we conducted astromertic observations of 22 GHz water maser sources toward the Galactic centre direction. We conducted astrometric observations toward water maser source associated with Sgr D HII region. As a result, we succeeded to measure the parallax and proper motion of the maser source. The measured distance was 2.36(+0.58/-0.39) kpc. This result clearly indicates that this source is not associated with the Galactic centre, but located on the Galactic disk.

We present some of preliminary results obtained by our dense monitoring project of the M87 jet with VERA 22 and 43 GHz starting from October 2010. The aims of this monitor are to clarify the detailed physical properties of the M87 jet base near the black hole, including the connection to γ-ray productions, jet kinematics and nuclear opacity. We detected a remarkable increase of the radio flux from the jet base of M87 during an elevated very-high-energy γ-ray activity occurred in the early 2012, suggesting that the γ-ray is produced in the immediate vicinity of the central black hole.

To search for kinematic evidence of the existence of the Galactic bar, we observed 10 methanol maser sources at the near end of the bar with VERA (VLBI Exploration of Radio Astrometry). From these observations, we obtained absolute proper motions of eight sources based on the phase-referencing technique. We compared the motions with the predictions of three simple models in a 3D plane. This comparison showed that a non-flat circular rotation model and a dynamical model including a bar potential reproduce the observed data better than a flat rotation model. In addition, the bar model suggests that the inclination angle of the Galactic bar is around 35°, which is consistent with previous studies.

We present a measurement of the trigonometric parallax of IRAS 05168+3634 with VERA. The parallax is 0.532 ± 0.053 milli-arcsec, corresponding to a distance of 1.88+0.21−0.17 kpc. This is significantly closer than the previous distance estimate of 6 kpc based on a kinematic distance measurement. This drastic change in the source distance implies the need for revised values of not only the physical parameters of IRAS 05168+3634, but it also impies a different location in the Galaxy, placing it in the Perseus arm rather than the Outer arm. We also measured the proper motion of the source. A combination of the distance and proper motion with the systemic velocity yields a rotation velocity Θ = 227+9−11 km s−1 at the source position, assuming Θ0 = 240 km s−1. Our result, combined with previous VLBI results for six sources in the Perseus arm, indicates that the sources rotate systematically more slowly than the Galactic rotation velocity at the local standard of rest. In fact, we derive peculiar motions in the disk averaged over the seven sources in the Perseus arm of (Umean, Vmean) = (11 ± 3, −17 ± 3) km s−1, which indicates that these seven sources are moving systematically toward the Galactic Center and lag behind the overall Galactic rotation.

We present a distance measurement to the semi-regular variable star RX Bootis (RX Boo). Using the VLBI Exploration of Radio Astrometry (VERA) telescope, we conducted astrometric observations of a water maser spot associated with RX Boo, as well as of the continuum reference source J1419+2706. Based on monitoring observations covering a full year, the annual parallax of RX Boo was measured at 7.31 ± 0.50 mas, corresponding to a distance of 136+10−9 pc. This distance uncertainty is smaller by a factor of two than those previously published, allowing us to determine the object's stellar properties more accurately. Using our distance, we can determine the absolute magnitude and discuss more precisely the locus of RX Boo on the period–luminosity (PL) relation. RX Boo exhibits two simultaneous pulsation periods and is located on the fundamental and first overtone Mira sequences of the PL relation. In addition, we calculated the radius and mass of the star.

We aim to reveal the mass distribution of the Galaxy based on a precise rotation curve constructed using VERA observations. We have been observing Galactic H2O masers with VERA. We here report one of the results of VERA for IRAS 05168+3634. The parallax is 0.532 ± 0.053 mas which corresponds to a distance of 1.88+0.21−0.17 kpc, and the proper motions are (μαcosδ, μδ) = (0.23 ± 1.07, −3.14 ± 0.28) mas yr−1. The distance is significantly smaller than the previous distance estimate of 6 kpc based on a kinematic distance. This drastic change places the source in the Perseus arm rather than in the Outer arm. Combination of the distance and the proper motions with the systemic velocity provides a rotation velocity of 227+9−11 km s−1 at the source assuming Θ0 = 240 km s−1. The result is marginally slower than the rotation velocity at LSR with ~ 1−σ significance, but consistent with previous VLBI results for six sources in the Perseus arm. We also show the averaged disk peculiar motion over the seven sources in the Perseus arm as (Umean, Vmean) = (11 ± 3, −17 ± 3) km s−1. It suggests that the seven sources in the Perseus arm are systematically moving toward the Galactic center, and lag behind the Galactic rotation with more than 3-σ significance.

To establish the existence of the galactic bar, ten methanol maser sources around the starting points of the spiral arms were observed with VERA (VLBI Exploration of radio astrometry) using the phase-referencing technique at 6.7 GHz band. For six out of ten sources, absolute proper motions were obtained with better than 3σ accuracy. Using VLBI 3-D data of eight sources, including our five sources, we compared the observed data with three galactic models and found that the model including the bar effect is better to explain the 3-D data, than a flat circular rotation model. A non-flat circular rotation model is also consistent with the VLBI data. Based on a dynamical model with a bar, we estimate an inclination angle of the bar around 35°, which is consistent with previous studies.

Since 2007 VERA (VLBI Exploration of Radio Astrometry) has been producing astrometric results (distances and/or proper motions) for Galactic maser sources. Nearly 30 parallaxes have been obtained for star-forming regions and late-type stars. By using VERA's astrometric results for star-forming regions, combined with those obtained with VLBA and EVN, fundamental Galactic parameters and Galactic structure may be derived. Our results show that R0 = 8.4 ± 0.4 kpc and Ω⊙ ≡ Ω0 + V⊙/R0=30.7±0.8 km s−1 kpc−1, and also show that the rotation curve of the Galaxy is nearly flat. The determinations of Galactic parameters and structures demonstrate that the maser astrometry can not only contribute significantly to research of individual maser sources, but also to studies of the structure of the Galaxy.

The water maser site associated with G353.273+0.641 is classified as a dominant blueshifted H2O maser, which shows an extremely wide velocity range (± 100 km s−1) with almost all flux concentrated in the highly blueshifted emission. The previous study has proposed that this peculiar H2O maser site is excited by a pole-on jet from high mass protostellar object. We report on the monitoring of 22-GHz H2O maser emission from G353.273+0.641 with the VLBI Exploration of Radio Astrometry (VERA) and the Tomakamai 11-m radio telescope. Our VLBI imaging has shown that all maser features are distributed within a very small area of 200 × 200 au2, in spite of the wide velocity range (> 100 km s−1). The light curve obtained by weekly single-dish monitoring shows notably intermittent variation. We have detected three maser flares during three years. Frequent VLBI monitoring has revealed that these flare activities have been accompanied by a significant change of the maser alignments. We have also detected synchronized linear acceleration (−5 km s−1yr−1) of two isolated velocity components, suggesting a lower-limit momentum rate of 10−3 M⊙ km s−1yr−1 for the maser acceleration. All our results support the previously proposed pole-on jet scenario, and finally, a radio jet itself has been detected in our follow-up ATCA observation. If highly intermittent maser flares directly reflect episodic jet-launchings, G353.273+0.641 and similar dominant blueshifted water maser sources can be suitable targets for a time-resolved study of high mass protostellar jet.

In 2011 February, a burst of the 22 GHz H2O maser in Orion KL was reported. In order to identify the bursting maser features, we have been carrying out observations of the 22 GHz H2O maser in Orion KL with VERA, a Japanese VLBI network dedicated for astrometry. The bursting maser turns out to consist of two spatially different features at 7.58 and 6.95 km s−1. We determine their absolute positions and find that they are coincident with the shocked molecular gas called the Orion Compact Ridge. We tentatively detect the absolute proper motions of the bursting features toward the southwest direction, perpendicular to the elongation of the maser features. It is most likely that the outflow from the radio source I or another young stellar object interacting with Compact Ridge is a possible origin of the H2O maser burst. We will also carry out observations with ALMA in the cycle 0 period to monitor the submillimeter H2O maser lines in the Orion Compact Ridge region. These follow-up observations will provide novel information on the physical and chemical properties of the mastering region.

We report on recent VLBA/VERA/IVS observational tests of General Relativity. First, we will summarize the results from the 2005 VLBA experiment that determined gamma with an accuracy of 0.0003 by measuring the deflection of four compact radio sources by the solar gravitational field. We discuss the limits of precision that can be obtained with VLBA experiments in the future. We describe recent experiments using the three global arrays to measure the aberration of gravity when Jupiter and Saturn passed within a few arcmin of bright radio sources. These reductions are still in progress, but the anticipated positional accuracy of the VLBA experiment may be about 0.01 mas.

VLBI phase-referencing monitoring of water vapor masers around the red supergiant, S Per, was conducted over four years. We successfully obtained proper motions and an annual parallax of the masers and determined the distance to S Per of 2.51±0.09 kpc. The proper motion of the star itself was inferred from the maser proper motions, and it was −0.38 and −1.54 mas/yr for right ascension and declination, respectively. Assuming the distance from the sun to the Galactic center, R0, of 8.5 kpc and the rotation velocity around the sun, Θ0, of 220 km/s, the Galactic rotation velocity around S Per is 200 km/s.

We report on absolute proper-motion measurements of H2O maser features in the NGC 281 West molecular cloud, located ~320 pc above the Galactic plane and associated with an HI loop extending from the Galactic plane. We conducted six-epoch phase-referencing observations of the maser source with VERA (VLBI Exploration of Radio Astrometry) over six months since May 2006. The H2O maser features are found to be systematically moving toward the southwest and further away from the Galactic plane with a vertical velocity of ~20–30 km s−1 at its estimated distance of 2.2–3.5 kpc. Our new results provide the most direct evidence that the gas in the NGC 281 region was blown out from the Galactic plane, most likely in a superbubble driven by multiple or sequential supernova explosions in the Galactic plane.