Tomoharu Kurayama

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— —

Astrometry and Spatio-kinematics of H2O Masers in the Massive Star-forming Region NGC?6334I(North) with VERA

We measured the trigonometric parallax of an H2O maser source associated with the massive star-forming region NGC 6334I(North), hereafter as NGC 6334I(N), with the VLBI Exploration of Radio Astrometry. The derived annual parallax is 0.789 ± 0.161 mas, corresponding to a distance of 1.26 kpc. Using the new distance, we recalculated the physical parameters (masses and luminosities) of the dust continuum cores in the region, and the revised parameters are only ~50% of their originally reported values. We also traced 23 relative proper motions of the H2O masers associated with SMA1 (central millimeter source in the region) between epochs, which exhibit an average amplitude of maser proper motion of ~2.03 mas yr–1 (~12.22 km s–1), tracing a bipolar outflow. The bipolar outflow structure extends through ~600 mas (~720 AU), with a dynamical timescale of ~295 yr. Using an expanding flow model, we derived the most plausible dynamical center of the outflow, pointing to SMA1b (1.3 cm and 7 mm continuum source) as the outflow driving source. Based on our results and other existing parallax results, we derive the pitch angles of the Sagittarius arm as 62 ± 54 along the Galactic longitude range of ~07 – ~401 assuming a perfect logarithmic spiral. We derived the peculiar motion of NGC 6334I(N) to be –4 ± 1 km s–1 toward the Galactic center, 8 ± 2 km s–1 in the direction of the Galactic rotation, and 25 ± 2 km s–1 toward the Galactic north pole.

First Fringe Detection with VERA's Dual-Beam System and Its Phase-Referencing Capability

We present the results of the first dual-beam observations with VERA (VLBI Exploration of Radio Astrometry). The observations of a pair of H2O maser sources, W 49N and OH 43.8−0.1, were carried out on 2002 May 29 and July 23, and fringes of the H2O maser lines at 22GHz were successfully detected. While the residual fringe phases of both sources showed rapid variations over 360 ◦ due to the atmospheric fluctuation, the differential phase between the two sources remained constant for 1 hour with an r.m.s. of 8 ◦ , demonstrating that the atmospheric phase fluctuation was effectively removed by dual-beam phase referencing. An analysis based on the 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 an arbitrarily long time. These results demonstrate VERA’s high capability of phase referencing, indicating that it is a promising tool for phase-referencing VLBI astrometry at 10 µas-level accuracy.

Astrometric Microlensing of Distant Sources Caused by Stars in the Galaxy

We investigate properties of astrometric microlensing of distant sources (such as quasi-stellar objects [QSOs] and radio galaxies) caused by stars in the Galaxy, mainly focusing on application of the VERA (VLBI Exploration of Radio Astrometry) project. Assuming typical parameters for the Galactic disk and bulge, we show that the maximum optical depth for astrometric shift of the 10 μas level is 8.9 × 10-2 for the QSO-disk lensing case and 3.8 × 10-2 for the QSO-bulge lensing case. We also find that the maximum optical depth for QSO-disk lensing is larger by an order of magnitude than that for disk-disk or bulge-disk lensing (assuming a typical source distance of 8-10 kpc). In addition to optical depth, we also calculate the event rate and find that the maximum event rate for the QSO-disk lensing case is 1.2 × 10-2 events yr-1, which is about 30 times greater than that for disk-disk lensing. This high event rate implies that if one monitors 10 QSOs behind the Galactic center region for 10 yr, at least one astrometric microlensing event should be detected. Therefore, monitoring distant radio sources with VERA can be a new tool to study astrometric microlensing caused by stars in the Galaxy. We also study the event duration of astrometric microlensing and find that the mean event duration for QSO-disk lensing is 7.5 yr for QSOs located near the Galactic center. This event duration for QSO-disk lensing is reasonably short compared to the project lifetime of VERA, which is anticipated to be ~20 yr. We also find that while the minimum event duration for bulge-bulge lensing is as short as 2.6 yr, the event duration for disk-disk lensing cannot be shorter than 15 yr. Thus, although astrometric microlensing of bulge sources/lenses can be studied by optical astrometric missions like SIM and GAIA, detections of disk events with the space astrometric missions are fairly difficult because of the limited project lifetime (typically ~5 yr) as well as the heavy dust extinction. Therefore, for studying astrometric microlensing by disk stars, VERA can be a powerful tool based on observations of distant sources like QSOs and radio galaxies. We discuss the implications of astrometric microlensing for VERA by focusing on estimating the lens mass, and we also present some possible candidates of radio sources toward which astrometric microlensing events should be searched for with VERA.

THE FIRST VERY LONG BASELINE INTERFEROMETRY IMAGE OF A 44 GHz METHANOL MASER WITH THE KVN AND VERA ARRAY (KaVA)

We have carried out the first very long baseline interferometry (VLBI) imaging of a 44 GHz classI methanol maser (70‐61A + ) 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 sizeof2.7milliarcseconds ×1.5milliarcseconds(mas).Thesefeaturesaredetectedatalimitednumber ofbaselines within the length of shorter than ≈ 650 km corresponding to 100 Mλ 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 ∼5mas×2mas, which corresponds to the linear size of ∼15 AU × 6 AU assuming a distance of 3 kpc. The brightness temperatures of these features range from ∼3.5 × 10 8 to 1.0 × 10 10 K, which are higher than the estimated lower limit from a previous Very Large Array observation with the highest spatial resolution of ∼50 mas. The 44 GHz classI 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 classII methanol maser.

Properties of intrinsic polarization angle ambiguities in Faraday tomography

Faraday tomography is a powerful method to diagnose polarizations and Faraday rotations along the line of sight. The quality of Faraday tomography is, however, limited by several conditions. Recently, it is reported that Faraday tomography indicates false signals in some specific situations. In this paper, we systematically investigate the condition of the appearance of false signals in Faraday tomography. We study this by pseudo-observing two sources within a beam, and change in the intrinsic polarization angles, rotation measures, intensities, and frequency coverage. We find that false signals arise when rotation measure between the sources is less than 1.5 times the full width at half maximum of the rotation measure spread function. False signals also depend on the intensity ratio between the sources and are reduced for large ratio. On the other hand, the appearance of false signals does not depend on frequency coverage, meaning that the uncertainty should be correctly understood and taken into consideration even with future wide-band observations such as Square Kilometer Array (SKA).

Parallax of a Mira variable R Ursae Majoris studied with astrometric VLBI

We have measured an annual parallax of the Mira variable R~Ursae~Majoris (R~UMa) with the VLBI exploration for Radio Astronomy (VERA). From the monitoring VLBI observations spanning about two years, we detected H

Outer rotation curve of the Galaxy with VERA. II. Annual parallax and proper motion of the star-forming region IRAS 21379+5106

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VERA Observation of the W49NH2O Maser Outburst in 2003 October

O maser spots in the LSR velocities ranges from 37 to 42 km\,s

Pilot VLBI Survey of SiO v = 3 J = 1 → 0 Maser Emission around Evolved Stars

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