Kazuyoshi Yamashita

Astrometry of H2O Masers in Nearby Star-Forming Regions with VERA II SVS 13 in NGC 1333

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.

Distance to Orion KL Measured with VERA

We present the initial results of multiepoch VLBI observations of 22 GHz H2O masers in the Orion KL region with VERA (VLBI Exploration of Radio Astrometry). With the VERA dual-beam receiving system, we carried out phase-referencing VLBI astrometry, and successfully detected the annual parallax of Orion KL to be 2.29 ˙ 0.10 mas, corresponding to a distance of 437 ˙ 19 pc from the Sun. The distance to Orion KL was determined for the first time with the trigonometric parallax method in these observations. Although this value is consistent with that previously reported, 480 ˙ 80 pc, which was estimated from a statistical parallax method using the proper motions and radial velocities of the H2O maser features, our new results provide a much more accurate value with an uncertainty of only 4%. In addition to the annual parallax, we 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.

Astrometry of Galactic Star-Forming Region Sharpless 269 with VERA: Parallax Measurements and Constraint on Outer Rotation Curve

We have performed high-precision astrometry of H2O maser sources in the Galactic star-forming region Sharpless 269 (S269) with VERA. We successfully detected a trigonometric parallax of 189 ˙ 8� as, corresponding to a 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 the proper motions were 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 of the existence of a large amount of dark matter in the Galaxy’s outer disk.

Distance to VY Canis Majoris with VERA

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) × 10 5 L⊙ 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.

Astrometry of H2O Masers in Nearby Star-Forming Regions with VERA III. IRAS22198+6336 in Lynds1204G

We present results of multi-epoch VLBI observations with VERA (VLBI Exploration of Radio Astrometry) of the 22 GHz H2O masers associated with a young stellar object (YSO) IRAS 22198+6336 in a dark cloud, L 1204 G. Based on phase-referencing VLBI astrometry, we derived an annual parallax of IRAS 22198+6336 to be 1.309˙ 0.047 mas, corresponding to the distance of 764˙ 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 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 IRAS 22198+6336 and found that the bolometric lumi— —

Distance to NGC 281 in a Galactic Fragmenting Superbubble: Parallax Measurements with VERA

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 a 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 that the superbubble expansion may be confined to the disk. We estimate the expansion velocity of the superbubble as being � 20 km s � 1 , both perpendicular to and parallel to the Galactic disk with a consistent timescale of � 20 Myr.

VLBI Astrometry of AGB Variables with VERA — A Semiregular Variable S Crateris—

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 . .. . .. . .. . ..

Absolute Proper Motions of H2O Masers Away from the Galactic Plane Measured with VERA in the “Superbubble” Region NGC 281

AbstractWe report on absolute proper-motion measurements of an H 2 O maser source in the NGC 281 Westmolecular cloud, which is located ∼320 pc above the Galactic plane and is associated with an H I loopextending from the Galactic plane. We have conducted multi-epoch phase-referencing observations of themaser source with VERA (VLBI Exploration of Radio Astrometry) over a monitoring period of 6 monthssince May 2006. We find that the H 2 O maser features in NGC 281 West are systematically moving towardthe southwest and further away from the Galactic plane with a vertical velocity of ∼20−30 km s −1 at itsestimated distance of 2.2−3.5 kpc. Our new results provide the most direct evidence that the gas in theNGC 281 region on the H I loop was blown out from the Galactic plane, most likely in a superbubble drivenby multiple or sequential supernova explosions in the Galactic plane.Keywords:Galaxy: kinematics and dynamics — ISM: bubbles — ISM: H II regions — ISM: individual(NGC 281) — masers (H 2

VERA Observation of the W49NH2O Maser Outburst in 2003 October

We report on a strong outburst of the W 49N H2O maser observed with VERA. Single-dish monitoring with VERA 20m telescopes detected a strong outburst of the maser feature at VLSR = −30.7kms −1 in 2003 October. The outburst had a duration of ∼ 100 days and a peak intensity of 7.9 ×10 4 Jy, being one of the strongest outbursts in W 49N 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.

A Balloon-Borne Very Long Baseline Interferometry Experiment in the Stratosphere: Systems Design and Developments

Abstract The balloon-borne very long baseline interferometry (VLBI) experiment is a technical feasibility study for performing radio interferometry in the stratosphere. The flight model has been developed. A balloon-borne VLBI station will be launched to establish interferometric fringes with ground-based VLBI stations distributed over the Japanese islands at an observing frequency of approximately 20 GHz as the first step. This paper describes the system design and development of a series of observing instruments and bus systems. In addition to the advantages of avoiding the atmospheric effects of absorption and fluctuation in high frequency radio observation, the mobility of a station can improve the sampling coverage (“uv-coverage”) by increasing the number of baselines by the number of ground-based counterparts for each observation day. This benefit cannot be obtained with conventional arrays that solely comprise ground-based stations. The balloon-borne VLBI can contribute to a future progress of research fields such as black holes by direct imaging.