Akiharu Nakagawa

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.

Fundamental Parameters of the Milky Way Galaxy Based on VLBI Astrometry

We present analyses to determine the fundamental parameters of the Galaxy based on VLBI astrometry of 52 Galactic maser sources obtained with VERA, VLBA, and EVN. We model the Galaxy’s structure with a set of parameters, including the Galaxy center distance R0, the angular rotation velocity at the LSR Ω0, the mean peculiar motion of the sources with respect to Galactic rotation (Usrc, Vsrc, Wsrc), the rotation-curve shape index, and the V component of the Solar peculiar motions, Vˇ. Based on a Markov chain Monte-Carlo method, we find that the Galaxy center distance is constrained at a 5% level to be R0 = 8.05 ˙ 0.45 kpc, where the error bar includes both statistical and systematic errors. We also find that the two components of the source peculiar motion Usrc and Wsrc are fairly small compared to the Galactic rotation velocity, being Usrc = 1.0 ˙ 1.5 km s � 1 and Wsrc = � 1.4 ˙ 1.2 km s � 1 . Also, the rotation curve shape is found to be basically flat between Galacto-centric radii of 4 and 13 kpc. On the other hand,

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— —

Annual Parallax Measurements of an Infrared Dark Cloud, MSXDC G034.43+00.24 with VERA

We have measured the annual parallax of the H2O maser source associated with an infrared dark cloud MSXDC G034.43+00.24 from the observations with VERA (VLBI Exploration of Radio Astrometry). The parallax is 0.643 +/- 0.049 mas, corresponding to the distance of 1.56 +0.12/-0.11 kpc. This value is less than the half of the previous kinematic distance of 3.7 kpc. We revise the core mass estimates of MSXDC G034.43+00.24, based on virial masses, LTE masses and dust masses and show that the core masses decrease from the previous estimations of ~1000 Mo to hundreds of Mo. The spectral type derived from the luminosity also changes from O9.5 to B1 in the case of MM1. This spectral type is still consistent with that of the massive star. The radial velocity derived from the flat rotation model is smaller than the observed velocity, which corresponds to the peculiar motion of ~40 km/s in the line-of-sight direction.

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.

Astrometry of Galactic Star-Forming Region ON2N with VERA: Estimation of the Galactic Constants

We performed the astrometry of H2O masers in the Galactic star-forming region Onsala 2 North (ON 2 N) with the VLBI Exploration of Radio Astrometry (VERA). We obtained a trigonometric parallax of 0.261 ˙ 0.009 mas, corresponding to a heliocentric distance of 3.83 ˙ 0.13 kpc. ON 2 N is expected to be on the solar circle, because its radial velocity with respect to the local standard of rest (LSR) is nearly zero. By using the present parallax and proper motions of the masers, the galactocentric distance of the Sun and the Galactic rotation velocity at the Sun are found to be R0 = 7.80 ˙ 0.26 kpc and Θ0 = 213 ˙ 5k m s � 1 , respectively. The ratio of Galactic constants, namely the angular rotation velocity of the LSR, can be determined more precisely, and is found to be Ω0 =Θ 0=R0 = 27.3 ˙ 0.8 km s � 1 kpc � 1 , which is consistent with recent estimations, but different from 25.9 km s � 1 kpc � 1 derived from the recommended values of Θ0 and R0 by the International Astronomical Union (1985).

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