Yoshiharu Asaki

THE 2014 ALMA LONG BASELINE CAMPAIGN: FIRST RESULTS FROM HIGH ANGULAR RESOLUTION OBSERVATIONS TOWARD THE HL TAU REGION

We present Atacama Large Millimeter/submillimeter Array (ALMA) observations from the 2014 Long Baseline Campaign in dust continuum and spectral line emission from the HL Tau region. The continuum images at wavelengths of 2.9, 1.3, and 0.87 mm have unprecedented angular resolutions of 0. ′′ 075 (10 AU) to 0. ′′ 025 (3.5 AU), revealing an astonishing level of detail in the cir cumstellar disk surrounding the young solar analogue HL Tau, with a pattern of bright and dark rings observed at all wavelengths. By fitting ellipses to the most distinct rings, we measure precise values for the disk inclination (46.72 ◦ ± 0.05 ◦ ) and position angle (+138.02 ◦ ± 0.07 ◦ ). We obtain a high-fidelity image of the 1.0 mm spectral index (�), which ranges from � � 2.0 in the optically-thick central peak and two brightest ring s, increasing to 2.3-3.0 in the dark rings. The dark rings are not devoid of emission, and we estimate a grain emissivity index of 0.8 for the innermost dark ring and lower for subsequent dark rings, consistent with some degree of grain growth and evolution. Additional clues that the rings arise from planet formation incl ude an increase in their central offsets with radius and the presence of numerous orbital resonances. At a resolution of 35 AU, we resolve the molecular component of the disk in HCO + (1-0) which exhibits a pattern over LSR velocities from 2-12 km s -1 consistent with Keplerian motion around a �1.3M⊙ star, although complicated by absorption at low blue-shifted velocities. We also serendipitously detect and resolve the nearby protost ars XZ Tau (A/B) and LkH�358 at 2.9 mm. Subject headings: stars: individual (HL Tau, XZ Tau, LkH�358) — protoplanetary disks — stars: formation — submillimeter: planetary systems — techniques: interferometric

Kinematics and Distance of Water Masers in W3 IRS 5

We have made multiepoch VLBA observations of water masers in the massive-star forming region W3 IRS 5. We measured radial velocities and proper motions of 108 water maser features during three observing sessions. The masers are clustered in two groups, which are associated with at least two different outflows. Positions of the outflow origins are close to the hypercompact radio continuum sources, which are clustered within a scale of 6000 AU. The continuum sources are probably driving sources of the outflows. We performed a three-dimensional kinematic model analysis of the maser motions in one of the two outflows, assuming a spherically symmetric expanding flow. We obtained a distance to the W3 IRS 5 region as 1.83 ? 0.14 kpc. The directions of the two outflows are roughly in the north-south direction, which is not significantly different from the direction of the global outflow seen in CO (J = 2-1) emission. This suggests that the massive-star cluster in the W3 IRS 5 region was created during formation of hierarchical structure in a parent molecular-cloud core. Such a structure is expected to preserve the angular momentum vector during star formation process, which may be along the directions of the present outflows.

THE ORIGIN OF LARGE PECULIAR MOTIONS OF STAR-FORMING REGIONS AND SPIRAL STRUCTURES OF OUR GALAXY

Recent Very Long Baseline Interferometer (VLBI) observations determined the distances and proper motions of star-forming regions in spiral arms directly. They showed that star-forming regions and young stars have large peculiar motions as large as 30 km s–1 with complex structures. Such a large peculiar motion is incompatible with the prediction of the standard theory of quasi-stationary spiral arms. We use a high-resolution, self-consistent N-body+hydrodynamical simulation to explore how the spiral arms are formed and maintained, and how star-forming regions move. We found that arms are not quasi-stationary but transient and recurrent, as suggested in alternative theories of spiral structures. Because of this transient nature of the spiral arms, star-forming regions exhibit a trend of large and complex non-circular motions, which is qualitatively consistent with the VLBI observations. Owing to this large non-circular motion, a kinematically estimated gas map of our Galaxy has large systematic errors of ~2-3 kpc in the distance from the Sun.

Verification of the effectiveness of VSOP-2 phase referencing with a newly developed simulation tool, ARIS

The next-generation space VLBI mission, VSOP-2, is expected to provide unprecedented spatial resolutions at 8.4, 22, and 43GHz. In this report, phase referencing with VSOP-2 is examined in detail based on a simulation tool called ARIS. The criterion for successful phase referencing was to keep the phase errors below one radian. Simulations with ARIS reveal that phase referencing achieves good performance at 8.4GHz, even under poor tropospheric conditions. At 22 and 43GHz, it is recommended to conduct phase referencing observations under good or typical tropospheric conditions. The satellite is required to have an attitude-switching capability with a one-minute or shorter cycle, and an orbit determination accuracy higher than about 10cm at apogee; the phase referencing calibrators are required to have a signal-to-noise ratio larger than four for a single scan. The probability to find a suitable phase referencing calibrator was estimated by using VLBI surveys. From the viewpoint of calibrator availability, VSOP-2 phase referencing at 8.4GHz is promising. However, the change of finding suitable calibrators at 22 and 43GHz is significantly reduced; it is important to conduct specific investigations for each target at those frequencies.

Flaring up of the compact cloud G2 during the close encounter with Sgr A

A compact gas cloud G2 (Gillessen+2012) is predicted to reach the pericentre of its orbit around the super massive black hole (SMBH) of our galaxy, Sagittarius (Sgr A*), by summer 2013. This event will give us a rare opportunity to observe the interaction between SMBH and gas around it. We report the result of the fully three-dimensional simulation of the evolution of G2 during the first pericentre passage. The strong tidal force by the SMBH stretches the cloud along its orbit, and compress it strongly in the vertical direction, resulting in the heating up and flaring up of the cloud. The bolometric luminosity will reach the maximum of 100

The VSOP 5 GHz active galactic nucleus survey. V. Imaging results for the remaining 140 sources

L_{\odot}

Phase compensation experiments with the paired antennas method: 2. Millimeter‐wave fringe correction using centimeter‐wave reference

by July 2013. This flare should be easily observed in the near infrared.

Phase compensation experiments with the paired antennas method

In 1997 February, the Japanese radio astronomy satellite HALCA was launched to provide the space-bourne element for the VLBI Space Observatory Program (VSOP) mission. Approximately 25% of the mission time was dedicated to the VSOP survey of bright compact active galactic nuclei (AGNs) at 5 GHz. This paper, the fifth in the series, presents images and models for the remaining 140 sources not included in the third paper in the series, which contained 102 sources. For most sources, the plots of the (u, v) coverage, the visibility amplitude versus (u, v) distance, and the high-resolution image are presented. Model fit parameters to the major radio components are determined, and the brightness temperature of the core component for each source is calculated. The brightness temperature distributions for all of the sources in the VSOP AGN survey are discussed.

DISTANCE AND PROPER MOTION MEASUREMENT OF THE RED SUPERGIANT, PZ CAS, IN VERY LONG BASELINE INTERFEROMETRY H2O MASER ASTROMETRY

An experiment for compensating interferometer phase fluctuations due to the turbulent troposphere was conducted with the paired antennas method (PAM) using different sky frequencies for a target and reference source. A celestial source 3C279 was observed at 146.81 GHz with the Nobeyama millimeter array (NMA), while a geostationary satellite was observed as a reference source at 19.45 GHz with commercially available antennas. Each of the antennas was installed near one of the NMA antennas, thus giving us 10 pairs of nearly parallel baselines up to 316 m. Large fluctuations in the interferometer phase of 3C279 were mostly eliminated on almost all baselines by subtracting the reference phase multiplied by a ratio of the observing frequencies. The standard deviations of the compensated interferometer phase were under the level of 20° where the angular separation between the two sources was within 2°, while those of the original phase were typically at the level of 40°–50°. The phase compensation was much improved to the level of the differential excess path length of 0.06-mm rms (11° at 146.81GHz) in inserting a time lag proportional to the separation angle to the reference phase time series. These results have practical significance for the millimeter or submillimeter wave interferometry because there is rarely a suitable reference source in the vicinity of scientifically interesting sources at such high frequencies. The present experiment shows that it is quite effective in the PAM to use the reference phase at centimeter wave for compensating the millimeter-wave phase for future large millimeter-wave arrays.

The VSOP-2 Project: a second-generation space-VLBI mission ranging to mm-wavelengths

Several kinds of techniques are under development for effective correction of a tropospheric phase fluctuation in millimeter- and submillimeter-wave radio interferometry, including very long baseline interferometry (VLBI). We report here the results of phase correction experiments using a method called “paired antennas method.” In the method, pairs of closely located antennas are used for obtaining the fringe phases of both an observing source and an adjacent reference calibrator simultaneously, and the temporal variation of the obtained fringe phase of the target source is corrected by using the calibrator phase. In cases where the separation angle between the target source and the calibrator is sufficiently small, the correction will be almost perfectly made. The experiments were carried out with the Nobeyama millimeter array (NMA) with a special emphasis on examining the separation angle dependence of the degree of the phase compensation. We simultaneously observed the Japanese Communication Satellite (CS) and a quasar 3C279 passing near the satellite at 19 GHz using four antennas of the NMA in order to measure the fluctuation of the difference in their fringe phases in dependence on the angular separation. The standard deviation of the differential fringe phase estimated for 500-s time interval is reduced from 50°–70° to 10° as the angular separation decreases from 20° or greater to a few degrees. The results indicate that the method is very useful for the correction of the tropospheric phase fluctuation in millimeter- and submillimeter-wave interferometry.