Abstract
In order to investigate the genesis of a powerful radio jet, we have mapped the central 10 pc region of the nearby radio galaxy, NGC 6251, with a 0.2 pc resolution using Very Long Baseline ...Interferometer (VLBI) at two radio frequencies, 5 GHz and 15 GHz. We have found the sub-parsec-scale counterjet for the first time in this radio galaxy. This discovery allows us to investigate the jet acceleration based on the relativistic beaming model.
Water masers around an AGB star, IRC
$ +$
60169, were observed at four epochs using the Japanese VLBI networks. The distribution of the maser features is limited in a thick-shell region, which has ...inner and outer expansion velocities of
$ 7 \,\mathrm{km} \,\mathrm{s}^{-1}$
and
$ 14 \,\mathrm{km} \,\mathrm{s}^{-1}$
at radii of 25 mas and 120 mas, respectively. The distribution of the red-shifted features exhibits a ring-like structure, the diameter of which is 30 mas, and corresponds to the inner radius of the maser shell. This implies that dense gas around the star obscures red-shifted emission. Although a position–radial velocity diagram for the maser features is consistent with a spherical shell model, the relative proper motions do not indicate an expansion motion of the shell. A remarkable property has been found that is a possible periodic change of the alignment pattern of water maser spots.
$H_2O$ maser emission at 22 GHz in the circumstellar envelope is one of the good tracers of detailed physics and kinematics in the mass loss process of asymptotic giant branch stars. Long-term ...monitoring of an $H_2O$ maser spectrum with high time resolution enables us to clarify acceleration processes of the expanding shell in the stellar atmosphere. We monitored the $H_2O$ maser emission of the semi-regular variable R Crt with the Kagoshima 6-m telescope, and obtained a large data set of over 180 maser spectra over a period of 1.3 years with an observational span of a few days. Using an automatic peak detection method based on least-squares fitting, we exhaustively detected peaks as significant velocity components with the radial velocity on a $0.1kms^{-1}$ scale. This analysis result shows that the radial velocity of red-shifted and blue-shifted components exhibits a change between acceleration and deceleration on the time scale of a few hundred days. These velocity variations are likely to correlate with intensity variations, in particular during flaring state of $H_2O$ masers. It seems reasonable to consider that the velocity variation of the maser source is caused by shock propagation in the envelope due to stellar pulsation. However, it is difficult to explain the relationship between the velocity variation and the intensity variation only from shock propagation effects. We found that a time delay of the integrated maser intensity with respect to the optical light curve is about 150 days.
The next-generation space VLBI mission, VSOP-2, is expected to provide unprecedented spatial resolution at 8.4, 22, and 43 GHz. 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 set to keep the phase errors below one radian. Simulations with ARIS reveal that phase referencing achieves good performance at 8.4 GHz, even under poor tropospheric conditions. At 22 and 43 GHz, 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
$\sim$
10 cm 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 8.4 GHz is promising. However, the chance of finding suitable calibrators at 22 and 43 GHz is significantly reduced; it is important to conduct specific investigations for each target at those frequencies.
We present astrometric very long baseline interferometry (VLBI) studies of AGB stars. To understand the properties and evolution of AGB stars, distances are an important parameter. The distribution ...and kinematics of their circumstellar matter are also revealed with the VLBI method. We used the VERA array to observe 22\,GHz H\(_2\)O masers in various subclasses of AGB stars. Parallaxes of the three OH/IR stars NSV17351, OH39.7\(+\)1.5, IRC\(-\)30363, and the Mira-type variable star AW~Tau were newly obtained. We present the circumstellar distribution and kinematics of H\(_2\)O masers around NSV17351. The absolute magnitudes in mid-infrared bands of OH/IR stars with very long pulsation periods were investigated and a period-magnitude relation in the WISE W3 band, \(M_{\mathrm{W3}} = (-7.21\pm1.18)\log P + (9.25\pm3.09)\), was found for the Galactic AGB stars. The VLBI is still a powerful tool for parallax measurements of the Galactic AGB stars surrounded by thick dust shells.
We present a new CO observation toward the Type Ia supernova remnant (SNR) 3C 397 using the Nobeyama 45-m radio telescope at an unprecedent angular resolution of \(\sim\)18''. We newly found that the ...CO cloud at \(V_{\mathrm{LSR}}\) = 55.7-62.2 km s\(^{-1}\) (60 km s\(^{-1}\) cloud) shows a good spatial correspondence with the radio continuum shell. We also found an expanding gas motion of the 60 km s\(^{-1}\) cloud with an expansion velocity of \(\sim\)3 km s\(^{-1}\), which is thought to be formed by the pre-and/or post-supernova feedback. By considering the positions of Galactic spiral arms and the X-ray/HI absorption studies, we concluded that 3C 397 is physically associated with the 60 km s\(^{-1}\) cloud rather than the previously known CO cloud at \(V_{\mathrm{LSR}}\) \(\sim\)30 km s\(^{-1}\). Given that the previously measured pre-shock density is \(\sim\)2-5 cm\(^{-3}\), the expanding motion of the 60 km s\(^{-1}\) cloud was likely formed by the pre-supernova feedback known as optically thick wind. The scenario is consistent with that 3C 397 exploded inside a wind-blown bubble as a single degenerate system.
We present observational results of water vapor maser emission with our high-sensitivity 22 GHz VLBI imaging of the Seyfert galaxy NGC 1068. In this galaxy, there are the following four nuclear radio ...sources; NE, C, S1, and S2. Among them, the S1 component has been identified as the nucleus while the C component has been considered as attributed to the radio jet. In our VLBI observation, we find the following two types of the water maser emission at the S1 component. One is the linearly aligned component that is considered as an edge-on disk with the inner radius of 0.62 pc. The dynamical mass enclosed within the inner radius was estimated to be \(1.5\times10^7 M_{\odot}\) by assuming the circular Keplerian motion. Note, however, that the best fit rotation curve shows a sub-Keplerian rotation (\(v\propto r^{-0.24\pm0.10}\)). The other is the water maser emission distributed around the rotating disk component up to 1.5 pc from the S1 component, suggesting the bipolar outflow from the S1 component. Further, we detected the water maser emission in the C component for the first time with VLBI, and discovered a ring-like distribution of the water maser emission. It is known that a molecular cloud is associated with the C component (both HCN and HCO\(^+\) emission lines are detected by ALMA). Therefore, the ring-like maser emission can be explained by the jet collision to the molecular cloud. However, if these ring-like water masing clouds constitute a rotating ring around the C component, it is likely that the C component also has a supermassive black hole with the mass of \(\sim 10^6 M_{\odot}\) that could be supplied from a past minor merger of a nucleated satellite galaxy.
Multi-epoch VLBA observations of 3C 66A Cai, H.-B.; Shen, Z.-Q.; Sudou, H. ...
Astronomy and astrophysics (Berlin),
06/2007, Letnik:
468, Številka:
3
Journal Article
Recenzirano
Odprti dostop
We present the results of six-epoch Very Long Baseline Array (VLBA) observations of 3C 66A. The high-resolution Very Long Baseline Interferometer (VLBI) maps obtained at multi-frequency (2.3, 8.4, ...and 22.2 GHz) simultaneously enabled us to identify the brightest compact component with the core. We find that the spectrum of the core can be reasonably fitted by the synchrotron self-absorption model. Our VLBA maps show that the jet of 3C 66A has two bendings at about 1.2 and 4 mas from the core. We also give possible identifications of our jet components with the components in previous VLBA observations by analysing their proper motions. We find consistent differences of the position from the core in one component between different frequencies at six epochs.
We present the proper motions of H
$_2$
O masers in NML Cygni, observed with the Japanese VLBI Network at three epochs spanning 455d. We detected about 15 maser features at each epoch. Overall, 13 ...features that were detected at least twice were tracked by their radial velocities and proper motions. The three-dimensional kinematics of the maser features indicate the presence of an expanding outflow. The major axis of the outflow is estimated to be at a position angle of
$\sim\;$
108°, and an inclination angle of
$\sim\;$
8° with respect to the line of sight. The H
$_2$
O masers are located between an apparent minimum radius of
$\sim\;9.6 \times 10^{12}$
m (64AU) and a maximum radius of
$\sim\;3.0 \times 10^{13}$
m (202AU), where the expansion velocity increases from 12 to 27kms
$^{-1}$
. A comparison with the distributions of SiO, H
$_2$
O, and OH masers suggests that the outflow of NML Cygni is expanding outside a radius of
$\sim\;$
1.5
$\times$
10
$^{13}$
m (100AU). This radius corresponds to 6 stellar radii, and is consistent with the radius of the inner boundary for the dust shell.
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