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We present the first astrometry catalog from the Japanese VLBI (very long baseline interferometer) project VERA (VLBI Exploration of Radio Astrometry). We have compiled all the astrometry results from VERA, providing accurate trigonometric annual parallax and proper motion measurements. In total, 99 maser sources are listed in the VERA catalog. Among them, 21 maser sources are newly reported while the rest of 78 sources are referred to previously published results or those in preparation for forthcoming papers. The accuracy in the VERA astrometry are revisited and compared with those from the other VLBI astrometry projects such as BeSSeL (The Bar and Spiral Structure Legacy) Survey and GOBELINS (the Gould's Belt Distances Survey) with the VLBA (Very Long Baseline Array). We have confirmed that most of the astrometry results are consistent with each other, and the largest error sources are due to source structure of the maser features and their rapid variation, along with the systematic calibration errors and different analysis methods. Combined with the BeSSeL results, we estimate the up-to-date fundamental Galactic parameter of $R_{0}=7.92\pm0.16_{\rm{stat.}}\pm0.3_{\rm{sys.}}$~kpc and $\Omega_{\odot}=30.17\pm0.27_{\rm{stat.}}\pm0.3_{\rm{sys.}}$~km~s$^{-1}$~kpc$^{-1}$, where $R_{0}$ and $\Omega_{\odot}$ are the distance from the Sun to the Galactic center and the Sun's angular velocity of the Galactic circular rotation, respectively., Comment: 35 pages, 7 figures, accepted for publication in the Publications of the Astronomical Society of Japan

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.

Magnetic field is ubiquitous in the Universe and it plays essential roles in various astrophysical phenomena, yet its real origin and evolution are poorly known. This article reviews current understanding of magnetic fields in the interstellar medium, the Milky Way Galaxy, external galaxies, active galactic nuclei, clusters of galaxies, and the cosmic web. Particularly, the review concentrates on the achievements that have been provided by centimeter and meter wavelength radio observations. The article also introduces various methods to analyze linear polarization data, including synchrotron radiation, Faraday rotation, depolarization, and Faraday tomography., 40 pages, 16 figures, accepted for publication in PASJ

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 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 1.23$^{\circ}$. In observations spanning nearly two years, we have detected the maser spot at the LSR velocity of 34.7 km s$^{-1}$, for which we measured the annual parallax of 2.33$\pm$0.13 mas corresponding to a distance of 430$^{+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 northeast-southwest direction. Using the distance and photospheric temperature, we estimate the stellar radius of S Crt and compare it with those of Mira variables., 9 pages, 4 figures, accepted for publication in PASJ (Vol.60, No.5, October 25, VERA special issue)

We report astrometric observations of H2O masers around the red supergiant VY Canis Majoris (VY CMa) carried out with VLBI Exploration of Radio Astrometry (VERA). Based on astrometric monitoring for 13 months, we successfully measured a trigonometric parallax of 0.88 +/- 0.08 mas, corresponding to a distance of 1.14 +0.11/-0.09 kpc. This is the most accurate distance to VY CMa and the first one based on an annual parallax measurement. The luminosity of VY CMa has been overestimated due to a previously accepted distance. With our result, we re-estimate the luminosity of VY CMa to be (3 +/- 0.5) x 10^5 L_sun using the bolometric flux integrated over optical and IR wavelengths. This improved luminosity value makes location of VY CMa on the Hertzsprung-Russel (HR) diagram much closer to the theoretically allowable zone (i.e. the left side of the Hayashi track) than previous ones, though uncertainty in the effective temperature of the stellar surface still does not permit us to make a final conclusion., Comment: 7 pages, 4 figures, accepted for publication in PASJ, VERA special issue

We present phase-referenced maps of the SiO maser transitions v = 1 J = 1–0 and v = 2 J = 1–0 from the symbiotic stellar system R Aquarii, which hosts an evolved AGB star plus a hot companion. Observations were performed at two epochs: 2004.98 and 2005.98. Accurate absolute coordinates and proper motions of the emission centroid were obtained; the errors expected for these parameters are also given. We compare the VERA data with the previous astrometry by Hipparcos. This represents a possibility to improve the orbital parameters of the system in a different way than that done before. Thanks to our accurate astrometry, we have also estimated the percentage of spot coincidences between both maser transitions, a parameter that has been proposed to be relevant to discriminate between different maser pumping schemes. Although the overall distributions of both lines are always similar, the spots are rarely coincident, in a percentage ranging between 3% and 20% of the cases. The lack of systematic coincidence favors, in principle, radiative pumping. However, we argue that no firm conclusion can be reached due to a lack of models that include an overlap of rovibrational lines, and that accurately address the coincidence of very intense spots of different maser transitions.

We observed the SiO (v = 1 and v = 2, J = 1–0) maser emissions toward the Mira variable IK Tauri (IK Tau, NML Tau, IRC +10050) over nine epochs from 2003 November to 2005 April using the VLBI Exploration of Radio Astrometry (VERA) telescope array. We found that the SiO maser distributions around IK Tau are highly variable, depending on the stellar pulsation phase. The velocity structures were complex and also time-variable, and are inconsistent with a simply rotating shell that was suggested by previous observations. Based on fittings of the SiO maser distributions to elliptical ring models, we found that the ring size seems to vary in correlation with the pulsation phase, as previously found for TX Cam. Comparisons of the v = 1 and v = 2 maser distributions reveal that 42–50% of the maser pairs have a positional offset less than 0.5 mas, suggesting that the maser pumping in these spots is likely to be dominated by line overlapping or collisional pumping, rather than pure radiative pumping. In order to model the velocity structure of the spoke-like features observed in IK Tau, we propose a ballistic-orbit model, in which a gas clump ejected from the stellar surface moves radially outward with a gravitational deceleration, and argue that the stellar mass can be deduced from that model.

We report on results of five-epoch VLBI observations of H2O maser emission in the M-type star IRC � 10414, carried out with the VLBI Exploration of Radio Astrometry (VERA). The maser distribution extends over an area of 70 mas � 260 mas and exhibits a bipolar structure. The relative proper motions of 17 maser features were measured during the monitoring observations spanning nine months. The distance to IRC � 10414 was re-estimated on the basis of the statistical treatment of parallax and the model-fitting method, yielding D = 3.1 ˙ 0.4 kpc and D = 2.0 ˙ 0.2 kpc, respectively. The estimated distance, D = 2–3 kpc, is much larger than that previously adopted. The stellar luminosity of IRC � 10414 is also re-estimated to be L� � 9 � 10 4 (D=2 kpc) 2 Lˇ, much brighter by a factor of 10–20 than previously adopted (L� � 10000 Lˇ). The maser motions exhibit not only a spherically expanding flow with a velocity of � 10 (D=2 kpc) km s � 1 , but also a faster bipolar outflow with a major axis in the north–south direction and at a small inclination angle with respect to the celestial sphere. These characteristics of the star and the

We report on the results of multi-epoch VLBI observations with VERA (VLBI Exploration of Radio Astrometry) of the 22 GHz H2O masers associated with the young stellar object SVS 13 in the NGC 1333 region. We have carried out phase-referencing VLBI astrometry and measured an annual parallax of the maser features in SVS 13 of 4.25+/-0.32 mas, corresponding to the distance of 235+/-18 pc from the Sun. Our result is consistent with a photometric distance of 220 pc previously reported. Even though the maser features were detectable only for 6 months, the present results provide the distance to NGC 1333 with much higher accuracy than photometric methods. The absolute positions and proper motions have been derived, revealing that the H2O masers with the LSR (local standard of rest) velocities of 7-8 km s-1 are most likely associated with VLA4A, which is a radio counterpart of SVS 13. The origin of the observed proper motions of the maser features are currently difficult to attribute to either the jet or the rotating circumstellar disk associated with VLA4A, which should be investigated through future high-resolution astrometric observations of VLA4A and other radio sources in NGC 1333.

We report on results of multi-epoch VLBI observations of H2O masers associated with a low-mass young stellar object, IRAS 16293 2422 in Oph East, and a fringe-phase and position reference source, ICRF J162546.8 252738, using the VLBI Exploration of Radio Astrometry (VERA) for high-precision astrometry. We obtained an annual parallax of a maser feature to be = 5.6 0:5 mas, corresponding to a distance of D= 178 +18 37 pc. We also found 10 relative proper motions of maser features with respect to the maser feature mentioned above. The motion of the accompanying young stellar object (YSO) has already been found in thermal continuum emission previously observed with the Very Large Array. The intrinsic motions of masers have been estimated from the relative proper motions after the YSO’s motion is subtracted from, and a systemic secular motion of the position reference feature is added to the proper motions originally measured. The intrinsic maser kinematical structure may trace a bipolar outflow.

VERA aims at astrometric observations using phase referencing VLBI techniques, whose goal is a 10 micro arc-second accuracy for annual parallax measurements. VERA has four 20-m diameter VLBI radio telescopes in Japanese archipelago with the maximum baseline length of 2,300 km. They have the two-beam observing system, which makes simultaneous observations of two objects possible. This leads to very accurate phase referencing VLBI observations. An important science goal is to make a 3-dimensional map of the Galaxy and reveal its dynamics. In order to achieve this, VERA has the 22GHz and 43GHz bands for H2O and SiO maser objects, respectively. Maser objects are compact and suitable for astrometry observations. VERA's construction was started in 2000 and the array became operational in 2004. We have already measured annual parallaxes and proper motions of some galactic objects. In the future, VERA will collaborate with Korean and Chinese VLBI stations.

We report on absolute proper-motion measurements of an H2O maser source in the NGC 281 West molecular cloud, which is located ~320 pc above the Galactic plane and is associated with an HI loop extending from the Galactic plane. We have conducted multi-epoch phase-referencing observations of the maser source with VERA (VLBI Exploration of Radio Astrometry) over a monitoring period of 6 months since May 2006. We find that the H2O maser features in NGC 281 West are systematically moving toward the southwest and further away from the Galactic plane with a vertical velocity of ~20-30 km/s 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 on the HI loop was blown out from the Galactic plane, most likely in a superbubble driven by multiple or sequential supernova explosions in the Galactic plane., Comment: 10 pages, 5 figures, PASJ in press (Vol. 59, No. 4; August 25, 2007 issue)

Phase characteristics, which are systematic phase offsets across the main beam of an actual telescope, may be a problem for achieving the same-beam differential very long baseline interferometer (VLBI) technique. This technique is essential for differential phase delay measurements such as those used in the Japanese Selenological and Engineering Explorer (SELENE) project, where the phase has to be determined to an accuracy of 0.075 radians rms. Accurate measurement and correction of phase characteristics are very important. The phase characteristics at 2.2375 and 2.2807 GHz of the 20-m and 10-m telescopes at Mizusawa were measured to an error of approximately 0.04 radians rms. The phase characteristics were 0.06 radians rms for the 20-m and 0.055 radians rms for the 10-m telescopes in the main beams, and the post-fit residuals decreased to 0.03 and 0.04 radians rms, respectively, after correcting by using quadratic formulas. These results confirmed the effectiveness of the same-beam differential VLBI technique for VLBI observations of SELENE

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+0.24−0.22kpc, 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 report on multi-epoch observations of H2O maser emission in the star-forming region OH 43.8−0.1, carried out with VLBI Exploration of Radio Astrometry. The large-scale maser distributions obtained by single-beam VLBI mapping reveal new maser spots scattered over the area of 0. 7 ×1. 0, in addition to a ‘shell-like’ structure with a scale of 0. 3 ×0. �� 5, which was mapped previously. Proper motions were also obtained for 43 spots based on 5-epoch monitoring with a time span of 281 days. The distributions of the proper motions show a systematic outflow in the north–south direction with an expansion velocity of ∼ 8kms −1 . The overall distributions of the maser spots as well as the proper motions are better represented by a bipolar flow plus a central maser cluster with a complex structure, rather than a shell with uniform expansion, such as those found in Cep A R5 and W75N VLA2. The distance to OH 43.8−0.1 was also estimated based on the statistical parallax, yielding D =2 .8 ±0.5kpc. This distance is consistent with a near kinematic distance, and rules out a far kinematic distance (∼ 9kpc). Also, the radial velocity of the OH 43.8−0.1 combined with the distance provides a constraint on the flatness of the galactic rotation curve, indicating that there is no systematic difference in the rotation speeds at the Sun and at the position of OH 43.8−0.1, which is located at a galacto-centric radius of ∼ 6.3kpc.

We have been monitoring the flux density of Sagittarius A* (Sgr A*) at 22 GHz since DOY=42 (11 Feb. 2013) with a sub-array of the Japanese VLBI Network in order to search the increase of 22-GHz emission from Sgr A* induced by the interaction of the G2 cloud with the accretion disk. The flux densities observed until DOY=322 (18 Nov. 2013) are consistent with the previously observed values before the approaching of the cloud. We have detected no large flare during this period.

We report on a strong outburst of the W49N H2O maser observed with VERA. Single-dish monitoring with VERA 20 m telescopes detected a strong outburst of the maser feature at V_LSR = -30.7 km/s in 2003 October. The outburst had a duration of ~100 days and a peak intensity of 7.9 x 10^4 Jy, being one of the strongest outbursts in W49N observed so far. VLBI observations with the VERA array were also carried out near to the maximum phase of the outburst, and the outburst spot was identified in the VLBI map. While the map was in good agreement with previous studies, showing three major concentrations of maser spots, we found a newly formed arc-like structure in the central maser concentration, which may be a shock front powered by a forming star or a star cluster. The outburst spot was found to be located on the arc-like structure, indicating a possible connection of the present outburst to a shock phenomenon., Comment: 4 pages, 2 figures included, to appear in PASJ letter (Vol.56, #3, 2004 June)

We present a magnetic field mapping of water maser clouds in the star-forming region W3 IRS5, which has been made on the basis of the linear polarization VLBI observation. Using the Very Long Baseline Array (VLBA) at 22.2 GHz, 16 of 61 detected water masers were found to be linearly polarized with polarization degrees up to 13%. Although 10 polarized features were widely distributed in the whole W3 IRS5 water maser region, they had similar position angles of the magnetic field vectors (~75 deg from the north). The magnetic field vectors are roughly perpendicular to the spatial alignments of the maser features. They are consistent with the hourglass model of the magnetic field, which was previously proposed to explain the magnetic field in the whole W3 Main region (r~0.1 pc). They are, on the other hand, not aligned to the directions of maser feature proper motions observed previously. This implies that the W3 IRS5 magnetic field was controlled by a collapse of the W3 Main molecular cloud rather than the outflow originated from W3 IRS5., 19 pages, 5 files, 1 table, accepted for publication in the Astrophysical Journal vol. 595, No.1 (2003 September 20 issue)

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.

著者人数: 14名, Accepted: 2014-10-26, 資料番号: SA1004968000

A SiO maser survey in the J =1 0, v = 1 and 2 transitions has been made for IRAS sources in the North Galactic Cap (b > 30) with the Nobeyama 45 m radio telescope. The sources were selected on the basis of their IRAS 12/25-m and 25/60-m flux ratios as likely oxygen-rich AGB candidates. SiO masers were detected from 24 out of the 97 selected sources, where 17 were new detections. The distances and heights above the Galactic plane are calculated. The Galactic distribution of detected and undetected stars indicates that metallicity is likely to govern the detection rate. The Galactocentric angular velocities of the subsampled stars are derived and their variation with the Galactic height is discussed.

A survey has been made in the SiO J = 1-0, v = 1 and 2 transitions (~43 GHz) for the color-selected IRAS sources in the central region of the Galaxy, -10? < l < 15? (except |l| ? 3?) and |b| ? 3?, with the Nobeyama 45 m telescope. We have detected 124 out of 200 observed sources in SiO masers. Distances to the sources range approximately from 2 to 11 kpc, if estimated from the IRAS 12 and 25 ?m flux densities. Radial velocities of the detected sources spread between ?260 km s-1. The distribution of SiO sources in the longitude-velocity diagram is compared with the H I contours. A number of sources are out of the H I limiting velocities; a group of negative-velocity (~-200 to -80 km s-1) sources is found at l = 3?-12? region. The subsamples of sources exhibit negative and positive shifts of the average radial velocity depending on distance, suggesting evidence of a streaming motion of stars in the bulge bar.

We have made multiepoch VLBA observations of water masers in the massive-star forming region W3 IRS 5. We measured radial velocities and proper motions of 108 water maser features during three observing sessions. The masers are clustered in two groups, which are associated with at least two different outflows. Positions of the outflow origins are close to the hypercompact radio continuum sources, which are clustered within a scale of 6000 AU. The continuum sources are probably driving sources of the outflows. We performed a three-dimensional kinematic model analysis of the maser motions in one of the two outflows, assuming a spherically symmetric expanding flow. We obtained a distance to the W3 IRS 5 region as 1.83 ? 0.14 kpc. The directions of the two outflows are roughly in the north-south direction, which is not significantly different from the direction of the global outflow seen in CO (J = 2-1) emission. This suggests that the massive-star cluster in the W3 IRS 5 region was created during formation of hierarchical structure in a parent molecular-cloud core. Such a structure is expected to preserve the angular momentum vector during star formation process, which may be along the directions of the present outflows.

A survey has been made in the SiO J = 1-0, v = 1 and 2 transitions (~43 GHz) for color-selected IRAS sources in the Galactic-center region of |l| ≤ 3° and |b| ≤ 3° using the Nobeyama 45 m telescope. We have detected 86 of 176 observed sources in the SiO masers. Distances to the sources are deduced from IRAS 12 and 25 μm flux densities and range approximately from 5 to 12 kpc. Radial velocities of the detected sources spread between -300 and 310 km s-1. The longitude-velocity (l-v) diagram exhibits an empty region of sources at l = -04-10 and Vlsr = 20-150 km s-1, which is interpreted as an undersampling effect of sources in the IRAS catalog at the Galactic central disk. The rotation rate, velocity dispersion, tilt angle of the rotation axis, and velocity shift at l = 0° are derived by fitting the velocities of the sources with a straight line as a function of Galactic longitude. These quantities are compatible with those obtained from previous observations of bulge stars with |b| > 3°. The average radial velocity of subsamples of stars tends to increase with distance, suggesting the presence streaming motion of stars in a barlike bulge. We conclude that most of the IRAS sources in the sample belong to the bulge.

We discuss the results on 26 water masers candidates observed with the 45 m millimeter wave telescope of the NRO of Japan, and propose using the tail length and integration intensity of the spectral line profile as criteria for molecular outflow. 9 out of the 26 candidates are shown to be without outflows.

CS (2-1) data of the Orion A Cloud obtained with the FCRAO 14 m telescope are shown. The CS (2-1) image shows the clumpy and filamentary structure of the Orion A cloud, as did our previous CS (1-0) image, obtained with the NRO 45 m telescope. The peak intensity TMB (CS 2-1) decreases more steeply than TMB (CS 1-0) toward the south along the filament. It is found that the gas density of the CS cores tends to be lower in the south of the cloud, and this explains the above difference in the line-intensity decrease. From CS and C34S data, we estimated the optical depth of CS (2-1) to be moderately high (2-5) and that of CS (1-0) to be typically less than unity.

We have used the Japanese VLBI array VERA to perform high-precision astrometry of an H2O maser source in the Galactic star-forming region NGC 281 West, which has been considered to be part of a 300-pc superbubble. We successfully detected a trigonometric parallax of 0.355+/-0.030 mas, corresponding to a source distance of 2.82+/-0.24 kpc. Our direct distance determination of NGC 281 has resolved the large distance discrepancy between previous photometric and kinematic studies; likely NGC 281 is in the far side of the Perseus spiral arm. The source distance as well as the absolute proper motions were used to demonstrate the 3D structure and expansion of the NGC 281 superbubble, ~650 pc in size parallel to the Galactic disk and with a shape slightly elongated along the disk or spherical, but not vertically elongated, indicating the superbubble expansion may be confined to the disk. We estimate the expansion velocity of the superbubble as ~20 km/s both perpendicular to and parallel to the Galactic disk with a consistent timescale of ~20 Myr., 16 pages, 5 figures, accepted for publication in PASJ, VERA special issue

We present results of multi-epoch VLBI observations with VERA (VLBI Exploration of Radio Astrometry) of the 22 GHz H$_{2}$O masers associated with a young stellar object (YSO) IRAS 22198+6336 in a dark cloud L1204G. Based on the phase-referencing VLBI astrometry, we derive an annual parallax of IRAS 22198+6336 to be 1.309$\pm$0.047 mas, corresponding to the distance of 764$\pm$27 pc from the Sun. Although the most principal error source of our astrometry is attributed to the internal structure of the maser spots, we successfully reduce the errors in the derived annual parallax by employing the position measurements for all of the 26 detected maser spots. Based on this result, we reanalyze the spectral energy distribution (SED) of IRAS 22198+6336 and find that the bolometric luminosity and total mass of IRAS 22198+6336 are 450$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 obtain 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., Comment: 15 pages, 7 figures, accepted for publication in PASJ (Vol.60, No.5, October 25, VERA special issue)

We present the results of the multi-frequency observations of radio outburst of the microquasar Cyg X-3 in February and March 2006 with the Nobeyama 45-m telescope, the Nobeyama Millimeter Array, and the Yamaguchi 32-m telescope. Since the prediction of a flare by RATAN-600, the source has been monitored from Jan 27 (UT) with these radio telescopes. At the eighteenth day after the quench of the activity, successive flares exceeding 1 Jy were observed successfully. The time scale of the variability in the active phase is presumably shorter in higher frequency bands. We also present the result of a follow-up VLBI observation at 8.4 GHz with the Japanese VLBI Network (JVN) 2.6 days after the first rise. The VLBI image exhibits a single core with a size of, 17 pages,6 figures; accepted by PASJ

We have performed high-precision astrometry of H2O maser sources in Galactic star forming region Sharpless 269 (S269) with VERA. We have successfully detected a trigonometric parallax of 189+/-8 micro-arcsec, corresponding to the source distance of 5.28 +0.24/-0.22 kpc. This is the smallest parallax ever measured, and the first one detected beyond 5 kpc. The source distance as well as proper motions are used to constrain the outer rotation curve of the Galaxy, demonstrating that the difference of rotation velocities at the Sun and at S269 (which is 13.1 kpc away from the Galaxy's center) is less than 3%. This gives the strongest constraint on the flatness of the outer rotation curve and provides a direct confirmation on the existence of large amount of dark matter in the Galaxy's outer disk., 7 pages and 4 figures, Accepted by PASJ (Vol. 59, No. 5, October 25, 2007 issue)

We present the initial results of multi-epoch VLBI observations of the 22 GHz H2O masers in the Orion KL region with VERA (VLBI Exploration of Radio Astrometry). With the VERA dual-beam receiving system, we have carried out phase-referencing VLBI astrometry and successfully detected an annual parallax of Orion KL to be 2.29+/-0.10 mas, corresponding to the distance of 437+/-19 pc from the Sun. The distance to Orion KL is determined for the first time with the annual parallax method in these observations. Although this value is consistent with that of the previously reported, 480+/-80 pc, which is estimated from the statistical parallax method using proper motions and radial velocities of the H2O maser features, our new results provide the much more accurate value with an uncertainty of only 4%. In addition to the annual parallax, we have detected an absolute proper motion of the maser feature, suggesting an outflow motion powered by the radio source I along with the systematic motion of source I itself., 7 pages, 3 figures. PASJ, in press (Vol. 59, No. 5, October 25, 2007 issue)

We have searched for the SiO J=1--0 v= 1 and 2 maser lines at ~ 43 GHz in 277 2MASS/MSX/IRAS sources off the Galactic plane (|b|> 3 deg), which resulted in 119 (112 new) detections. Among the new detections, are two very faint objects with MSX 12 micron flux densities below 2 Jy. These are likely to be O-rich AGB-stars associated with dwarf-galaxy tidal tails. The sample also includes medium bright MSX objects at moderately high Galactic latitudes (3 deg, Comment: PASJ No.3 (June 25, 2007 issue in press)

We surveyed 291 MSX/2MASS infrared objects in the 7 x 2 deg area of the galactic center in the 43 GHz SiO J=1--0 v=1 and 2 maser lines, obtaining accurate radial velocities of 163 detected objects. The surveyed area is the region where the IRAS catalog is incomplete due to contamination by high source density. The objects in the present MSX/2MASS sample were chosen to have similar infrared characteristics to those of the previous SiO-maser-survey samples based on the color selected IRAS sources. The sampling based on the 2MASS catalog causes a bias to the foreside objects of the bulge due to heavy obscuration by interstellar dust; the detections are considerably leaned on the V_{lsr}, Full high resolution figures available as NRO report No.638 at http://www.nro.nao.ac.jp/library/report/list.html. PASJ 58, No. 3 (June 25 issue in press)

We have surveyed 401 color selected IRAS sources in the Galactic disk in the SiO J=1--0 v= 1 and 2 maser lines at 43 GHz, resulting in 254 (239 new) detections. The observed sources lie mostly in a strip of the inner Galactic disk with boundaries -10, Comment: 38 page 9 figures, high res. eps files are available as NRO report No. 608 (http://www.nro.nao.ac.jp/library/report/list.html). PASJ 56 No. 4 in press

Mizusawa 10m antenna performs monitoring observation of light curve and polarization of the maser spots. The 10m antenna has 22GHz and 43GHz receivers, which are able to detect water vapor masers and SiO (J=l-0) masers around late-type stars. This system has good performance for linear polarization measurement and antenna temperature monitoring for several years

We present the results of first dual-beam observations with VERA (VLBI Exploration of Radio Astrometry). The first dual-beam observations of a pair of H2O maser sources W49N and OH43.8-0.1 have been carried out on 2002 May 29 and July 23, and fringes of H2O maser lines at 22 GHz have been successfully detected. While the residual fringe phases of both sources show rapid variations over 360 degree due to the atmospheric fluctuation, the differential phase between the two sources remains constant for 1 hour with r.m.s. of 8 degree, demonstrating that the atmospheric phase fluctuation is removed effectively by the dual-beam phase referencing. The analysis based on Allan standard deviation reveals that the differential phase is mostly dominated by white phase noise, and the coherence function calculated from the differential phase shows that after phase referencing the fringe visibility can be integrated for arbitrarily long time. These results demonstrate VERA's high capability of phase referencing, indicating that VERA is a promising tool for phase referencing VLBI astrometry at 10 microarcsec level accuracy., Comment: 9 pages, 3 figures included, to appear in PASJ letter (Vol.55, #4, 2003 August)

We report preliminary results of a long-term survey of SiO v=l and v=2 J=l-0 masers towards 418 large-amplitude variables within 15′ of the Galactic center with the Nobeyama 45-m telescope. We have detected 194 masers out of 338 surveyed fields during 2001-2002. The averaged detection rate of 57% is comparable with that obtained towards IRAS point sources in the Galactic bulge. The detection rate increases clearly with the stellar pulsation periods. The Galactic longitude-radial velocity diagram of the SiO sources revealed is quite similar to that of OH masers. A hole in the diagram indicates the presence of the non-circular streaming motions of stars in the nuclear disk of the Galaxy

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., 20 pages, 8 figures, 8 tables, appear in the NRO report No. 564 (ftp://ftp.nro.nao.ac.jp/nroreport/PASJ-W51.pdf) and will appear in Publ. Astron. Soc. Japan, Vol. 54, No. 5 (10/25 issue)

We have surveyed known large-amplitude variables within 15' of the Galactic center in the SiO J=1-0 v=1 and 2 maser lines at 43 GHz, resulting in 79 detections and 58 non-detections. The detection rate of 58 percent is comparable to that obtained in Bulge IRAS source surveys. SiO lines were also detected from four other sources near the program objects. The SiO detection rate increases steeply with the period, particularly for stars with P>500 d, where it exceeds 80%. We found at a given period that the SiO detection rate is approximately double that for OH. These facts suggest that the large-amplitude variables in the Nuclear Disk region are AGB stars similar in their overall properties to the inner and outer bulge IRAS/SiO sources., 5 pages, 2 figures, 1 Table. PASJ 54, No 2 April 25 issue in press

We have carried out the first very long baseline interferometry (VLBI) imaging of 44 GHz class I methanol maser (7_{0}-6_{1}A^{+}) associated with a millimeter core MM2 in a massive star-forming region IRAS 18151-1208 with KaVA (KVN and VERA Array), which is a newly combined array of KVN (Korean VLBI Network) and VERA (VLBI Exploration of Radio Astrometry). We have succeeded in imaging compact maser features with a synthesized beam size of 2.7 milliarcseconds x 1.5 milliarcseconds (mas). These features are detected at a limited number of baselines within the length of shorter than approximately 650 km corresponding to 100 Mlambda in the uv-coverage. The central velocity and the velocity width of the 44 GHz methanol maser are consistent with those of the quiescent gas rather than the outflow traced by the SiO thermal line. The minimum component size among the maser features is ~ 5 mas x 2 mas, which corresponds to the linear size of ~ 15 AU x 6 AU assuming a distance of 3 kpc. The brightness temperatures of these features range from ~ 3.5 x 10^{8} to 1.0 x 10^{10} K, which are higher than estimated lower limit from a previous Very Large Array observation with the highest spatial resolution of ~ 50 mas. The 44 GHz class I methanol maser in IRAS 18151-1208 is found to be associated with the MM2 core, which is thought to be less evolved than another millimeter core MM1 associated with the 6.7 GHz class II methanol maser., Comment: 19 pages, 3 figures

We monitored water-vapor masers around the semi-regular variable star R Crateris with the Japanese VLBI Network (J-Net) at the 22 GHz band during four epochs with intervals of one month. The relative proper motions and Doppler-velocity drifts of twelve maser features were measured. Most of them existed for longer than 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 degrees. The existence of a bipolar outflow suggests that a Mira variable star had already formed a bipolar outflow. The water masers are in a region of apparent minimum radii of 1.3 x 10^12 m and maximum radii of 2.6 x 10^12 m, between which the expansion velocity ranges from 4.3 to 7.4 km/s. These values suggest that the water masers are radially accelerated, but still gravitationally bound, in the water-maser region. The most positive and negative velocity-drifting features were found relatively close to the systemic velocity of the star. We found that the blue-shifted features are apparently accelerated and the red-shifted apparently decelerated. The acceleration of only the blue-shifted features seems to be consistent with that of the expanding flow from the star., 15 pages, 5 figures, Accepted for publication in PASJ (2001), preprint can be obtained via WWW on http://www.nro.nao.ac.jp/library/report/list.html

We present an overview of recent astrometric results with VERA. Since 2004, we have been conducting astrometry of tens of Galactic maser sources with VERA, and recently obtained trigonometric parallaxes for several sources, with distances ranging from 180 pc to 5.3 kpc. In this paper, we briefly summarize the results for Galactic star-forming regions, including S269, Orion-KL, NGC 1333, ρ-oph, NGC 281 and others.

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 (Kobayashiet 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 (Honmaet 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 (Hirotaet 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.

著者人数：25名, 資料番号: SA1000935000

High-spatial-resolution observations of the NGC 7538 molecular cloud have been performed with the NRO 45-m telescope, JCMT, and Nobeyama Millimeter Array. We report on relationships among three high dense regions, three bipolar flows, and seven H2O masers.