Miyuki Tsushima

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.

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

Galactic rotation measurements based on H 2 O maser astrometry with VERA

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 .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.

Parallax measurement of the Galactic Mira variables with VERA

Parallax measurements of the Galactic Mira variables with VERA have started since 2004 to establish their Period-Luminosity (PL) relationship in the Galaxy. Multi-epoch VLBI observations of a semiregular variable S Crt yielded an accurate parallax of 2.27±0.14mas corresponding to the distance of 441 −24 pc. In addition to the distance, we obtained physical properties of S Crt. Temperature of the photosphere was found to be ∼3000K by fitting the infrared spectrum with a blackbody radiation. The stellar radius was obtained based on the distance, apparent magnitude, and the temperature. Internal proper motions of circularly-arranged maser spots in S Crt were detected for the first time. Observations of the other Mira variables, such as R UMa, SY Scl, AP Lyn, and WX Psc are in progress.

VLBI monitoring of Mira variables with VERA

A. Nakagawa1, T. Omodaka1, K. M. Shibata2 , T. Kurayama2, H. Imai1, S. Kameno1, M. Tsushima1, M. Shintani1, N. Matsumoto1, M. Matsui1, S. Oizumi1, T. Yasuda1, Y. Arao1 and the VERA project2 Faculty of science, Kagoshima University, 1-21-35, Korimoto, Kagoshima, 890-0065, JAPAN email:nakagawa@astro.sci.kagoshima-u.ac.jp Mizusawa VERA observatory, National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo, 181-8588, JAPAN