We report measurements of parallaxes and proper motions of ten high-mass star-forming regions in the Sagittarius spiral arm of the Milky Way as part of the BeSSeL Survey with the VLBA. Combining these results with eight others from the literature, we investigated the structure and kinematics of the arm between Galactocentric azimuths beta approximate to -2 degrees. and 65 degrees. We found that the spiral pitch angle is 7 degrees.3 +/- 1 degrees.5; the arm's half-width, defined as the rms deviation from the fitted spiral, is approximate to 0.2 kpc; and the nearest portion of the Sagittarius arm is 1.4 +/- 0.2 kpc from the Sun. Unlike for adjacent spiral arms, we found no evidence for significant peculiar motions of sources in the Sagittarius arm opposite to Galactic rotation.

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Trigonometric parallaxes of star-forming regions in the Sagittarius spiral arm

Astrometry provides the foundation for astrophysics. Accurate positions are required for the association of sources detected at different times or wavelengths, and distances are essential to estimate the size, luminosity, mass, and ages of most objects. Very long baseline interferometry at radio wavelengths, with diffraction-limited imaging at submilliarcsecond resolution, has long held the promise of microarcsecond astrometry. However, only in the past decade has this been routinely achieved. Currently, parallaxes for sources across the Milky Way are being measured with ∼10 μas accuracy, and proper motions of galaxies are being determined with accuracies of ∼1 μas year−1. The astrophysical applications of these measurements cover many fields, including star formation, evolved stars, stellar and supermassive black holes, Galactic structure, the history and fate of the Local Group, the Hubble constant, and tests of general relativity. This review summarizes the methods used and the astrophysical applicatio...

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Microarcsecond Radio Astrometry

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.

The First VERA Astrometry Catalog

The very long baseline interferometry (VLBI) technique offers angular resolutions superior to any other instruments at other wavelengths, enabling unique science applications of high-resolution imaging of radio sources and high-precision astrometry. The East Asia VLBI Network (EAVN) is a collaborative effort in the East Asian region. The EAVN currently consists of 21 telescopes with diverse equipment configurations and frequency setups, allowing flexible subarrays for specific science projects. The EAVN provides the highest resolution of 0.5 mas at 22 GHz, allowing the fine imaging of jets in active galactic nuclei, high-accuracy astrometry of masers and pulsars, and precise spacecraft positioning. The soon-to-be-operational Five-hundred-meter Aperture Spherical radio Telescope (FAST) will open a new era for the EAVN. This state-of-the-art VLBI array also provides easy access to and crucial training for the burgeoning Asian astronomical community. This Perspective summarizes the status, capabilities and prospects of the EAVN. The East Asia Very Long Baseline Interferometry Network (EAVN) consists of 21 radio telescopes whose equipment and frequency setup allow fine imaging of jets in active galaxies, high-accuracy astrometry of masers and pulsars, and spacecraft positioning.

Capabilities and prospects of the East Asia Very Long Baseline Interferometry Network

Context. Proper motion observations of masers can provide information on dynamic motions on scales of a few milliarcseconds per year (mas yr −1 ) at radii of 100–1000 au scales from central young stellar objects (YSOs). Aims. The 6.7 GHz methanol masers are one of the best probes for investigations of the dynamics of high-mass YSOs, and in particular for tracing the rotating disk. We have measured the internal proper motions of the 6.7 GHz methanol masers associated with Cepheus A (Cep A) HW2 using Very Long Baseline Interferometery (VLBI) observations. Methods. We conducted three epochs of VLBI monitoring observations of the 6.7 GHz methanol masers in Cep A-HW2 with the Japanese VLBI Network (JVN) over the period 2006–2008. In 2006, we were able to use phase-referencing to measure the absolute coordinates of the maser emission with an accuracy of a few milliarcseconds. We compared the maser distribution with other molecular line observations that trace the rotating disk. Results. We measured the internal proper motions for 29 methanol maser spots, of which 19 were identified at all three epochs and the remaining ten at only two epochs. The magnitude of proper motions ranged from 0.2 to 7.4 km s −1 , with an average of 3.1 km s −1 . Although there are large uncertainties in the observed internal proper motions of the methanol maser spots in Cep A, they are well fitted by a disk that includes both rotation and infall velocity components. The derived rotation and infall velocities at the disk radius of 680 au are 0.5 ± 0. 7a nd 1.8 ± 0. 7k m s −1 , respectively. Conclusions. Assuming that the modeled disk motion accurately represents the accretion disk around the Cep A-HW2 high-mass

/pdf/rotating-and-infalling-motion-around-the-high-mass-young-2cbu4acn5y.pdf

Rotating and Infalling Motion around the High-Mass Young Stellar Object Cepheus A-HW2 Observed with the Methanol Maser at 6.7 GHz

We present the results of multiepoch VLBI observations for a 6.7 GHz methanol maser toward the UC H II region of W 3(OH) with Japanese VLBI Network (JVN). Based on phase-referencing VLBI astrometry, we derived the trigonometric annual parallax to be 0.598 ˙ 0.067 mas, corresponding to a distance of 1.67 +0:21 � 0:17 kpc. This is the first detection of parallax for a 6.7 GHz methanol maser with JVN, and demonstrates that JVN/VERA is capable of conducting VLBI astrometry for 6.7 GHz methanol maser sources within few kpc from the Sun. Based on in-beam mapping of the W 3(OH) methanol maser, we also measured the internal proper motions of its 6.7 GHz methanol maser for the first time. The internal proper motions basically show north–south expansion with a velocity of few km s � 1 , being similar to OH masers. The spatial distribution and the internal proper motions of 6.7 GHz

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Astrometry of 6.7 GHz Methanol Maser toward W 3(OH) with Japanese VLBI Network

Multimode horns were developed for 8.4GHz, 22GHz, 43GHz receiver systems of ASTRO-G/VSOP-2 satellite for Space-VLBI, and for 6.7GHz of VERA 20m radio telescopes of National Astronomical Observatory in JAPAN and 25m Shanghai Radio telescope in China. Rational bandwidth of these horns in low cross polar level are 20-30%, and their thin walls without corrugation and shortened length are the best for the satellite use. Also, wideband feeds will be presented, which are now developing for SKA and VLBI2010, which is a kind of Tapered Slot Antenna or other of traveling wave antenna.

/pdf/delopement-of-mutimode-horns-and-wideband-feed-for-radio-352qbpjemm.pdf

Kouhei Matsumoto

Papers

Astrometry of 6.7 GHz Methanol Maser toward W 3(OH) with Japanese VLBI Network

Delopement of mutimode horns and wideband feed for radio telescopes