Abstract
We present the VEra Data Analyzer (VEDA) software package for Very Long Baseline Interferometry (VLBI) phase-referencing observations and parallax measurements. The Japanese VLBI project ...VLBI Exploration of Radio Astrometry (VERA) provides high-precision astrometric results at the 10 μas level. To achieve this precision, accurate calibration of the atmospheric phase fluctuation, the instrumental phase, and the source structural effect is required. VEDA specializes in phase-referencing data analysis, including these calibrations. In order to demonstrate its performance we analyzed H2O maser observations of W 3(OH) and Orion KL with VERA. Their parallaxes were obtained to be 0.527 ± 0.016 mas and 2.459 ± 0.029 mas, respectively. We also analyzed their data using AIPS, which is widely used for VLBI data analysis, and confirmed that the parallaxes obtained using VEDA and AIPS are coincident within 10 μas. VEDA is available for high-precision parallax and proper motion measurements of Galactic maser sources.
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
$ R_0$
, the angular rotation velocity at the LSR
$ \Omega_0$
, the mean peculiar motion of the sources with respect to Galactic rotation (
$ U_{\rm src}$
,
$ V_{\rm src}$
,
$ W_{\rm src}$
), the rotation-curve shape index, and the
$ V$
component of the Solar peculiar motions,
$ V_\odot $
. Based on a Markov chain Monte-Carlo method, we find that the Galaxy center distance is constrained at a 5% level to be
$ R_0$
$ =$
8.05
$ \pm$
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
$ U_{\rm src}$
and
$ W_{\rm src}$
are fairly small compared to the Galactic rotation velocity, being
$ U_{\rm src}$
$ =$
1.0
$ \pm$
1.5 km s
$ ^{-1}$
and
$ W_{\rm src}$
$ =$
$-$
1.4
$ \pm$
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, we find a linear relation between
$ V_{\rm src}$
and
$ V_\odot $
as
$ V_{\rm src}$
$ =$
$ V_\odot $
$-$
19 (
$ \pm$
2) km s
$ ^{-1}$
, suggesting that the value of
$ V_{\rm src}$
is fully dependent on the adopted value of
$ V_\odot $
. Regarding the rotation speed in the vicinity of the Sun, we also find a strong correlation between
$ \Omega_0$
and
$ V_\odot $
. We find that the angular velocity of the Sun,
$ \Omega_{\odot}$
, which is defined as
$ \Omega_\odot$
$ \equiv$
$ \Omega_0$
$ +$
$ V_\odot/R_0$
, can be well constrained with the best estimate of
$ \Omega_\odot$
$ =$
31.09
$ \pm$
0.78 km s
$ ^{-1}$
kpc
$ ^{-1}$
. This corresponds to
$ \Theta_0$
$ =$
238
$ \pm$
14 km s
$ ^{-1}$
if one adopts the above value of
$ R_0$
and recent determination of
$ V_\odot $
$ \sim$
12 km s
$ ^{-1}$
.
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 the 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 ratios. 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 the Square Kilometer Array (SKA).
Abstract
We report on measurements of parallax and proper motion for four 22 GHz water maser sources as part of the VERA Outer Rotation Curve project. All the sources show Galactic latitudes of >2° ...and Galactocentric distances of >11 kpc at the Galactic longitude range of 95° < l < 126°. The sources trace the Galactic warp reaching to 200–400 pc, and also the signature of the warp to 600 pc toward the north Galactic pole. The new results, along with previous results in the literature, show that the maximum height of the Galactic warp increases with Galactocentric distance. Also, we examined velocities perpendicular to the disk for the sample, and found oscillatory behavior between the vertical velocities and Galactic heights. This behavior suggests the existence of bending (vertical density) waves, possibly induced by a perturbing satellite (e.g., the passage of the Sagittarius dwarf galaxy).
We report on results of astrometric observations of H
$ _{2}$
O masers in the ``water fountain'' source IRAS 18286
$-$
0959 (I18286) with the VLBI Exploration of Radio Astrometry (VERA). These ...observations yielded an annual parallax of IRAS 18286
$-$
0959,
$ \pi$
$ =$
0.277
$ \pm$
0.041 mas, corresponding to a heliocentric distance of
$ D$
$ =$
3.61
$ ^{+0.63}_{-0.47}$
kpc. The maser feature, whose annual parallax was measured, showed an absolute proper motion of (
$ \mu_{\alpha}$
,
$ \mu_{\delta}$
)
$ =$
(
$-$
3.2
$ \pm$
0.3,
$-$
7.2
$ \pm$
0.2) mas yr
$ ^{-1}$
. The intrinsic motion of the maser feature in the internal motions of the cluster of features in I18286 does not seem to trace the motion of the bipolar jet of I18286. Taking into account this intrinsic motion, the derived motion of the maser feature is roughly equal to that of the maser source I18286 itself. The proximity of I18286 to the Galactic midplane (
$ z$
$ \approx$
10 pc) suggests that the parental star of the water fountain source in I18286 should be an intermediate-mass AGB/post-AGB star, but the origin of a large deviation of the systemic source motion from that expected from the Galactic rotation curve is still unclear.
Abstract
We have measured the annual parallax of the water maser around a Mira star, U Lyncis (U Lyn) with VLBI Exploration of Radio Astrometry (VERA). The value of the parallax is 1.27 ± 0.06 mas, ...corresponding to a distance of
$786^{+40}_{-36}\:$
pc. This is the first VLBI measurement of the parallax of U Lyn. We obtained the period and the apparent magnitude of U Lyn based on near-infrared observations with the Kagoshima University 1 m telescope. The period and the mean magnitude of the K′ band are 437.9 day and +1.15 ± 0.09 mag, respectively. Using our measured parallax and period, we find that the location of U Lyn on the period–luminosity plane is consistent with its nature as a Mira variable fundamental mode pulsator. Also, we estimated the distance using the period–luminosity relationship, our period and apparent magnitude and this gave a difference of about 100 pc between our parallactic distance and the distance estimated by using the period–luminosity relationship. We estimated the luminosity, radius, and mass of U Lyn to be 10100 ± 420 L
⊙, 391 ± 34 R
⊙, and 1.9 ± 0.7 M
⊙, respectively. We found 50 water maser spots with a distribution ∼ 13 au around the star and the motion implies the existence of an outflow.
AGB variable stars are at the transient phase between low and high mass-loss rates; estimating the masses of these stars is necessary to study the evolutionary processes and mass-loss processes ...during the AGB stage. We applied the pulsation constant theoretically derived by Xiong and Deng (2007 MNRAS, 378, 1270) to 15 galactic AGB stars in order to estimate their masses. We found that using the pulsation constant is effective to estimate the mass of a star pulsating with two different pulsation modes, such as S Crt and RX Boo, which provides mass estimates comparable to theoretical results of AGB star evolution. We also extended the use of the pulsation constant to single-mode variables, and analyzed the properties of AGB stars related to their masses.
We report simultaneous multifrequency observing performance at 22 and 43 GHz of the 21 m shaped-Cassegrain radio telescopes of the Korean VLBI Network (KVN). KVN is the first millimeter-dedicated ...VLBI network in Korea having a maximum baseline length of 480 km. It currently operates at 22 and 43 GHz and is planned to operate in four frequency bands: 22, 43, 86, and 129 GHz. The unique quasi optics of KVN enable simultaneous multifrequency observations based on efficient beam filtering and accurate antenna-beam alignment at 22 and 43 GHz. We found that the offset of the beams is within less than 5′′ over all pointing directions of the antenna. The dual-polarization, cooled, high electron mobility transistor (HEMT) receivers at 22 and 43 GHz result in receiver noise temperatures less than 40 K at 21.25–23.25 GHz and 80 K at 42.11–44.11 GHz. The pointing accuracies have been measured to be 3′′ in azimuth and elevation for all antennas. The measured aperture efficiencies are65%(K)/67%(Q)
65
%
(
K
)
/
67
%
(
Q
)
,62%(K)/59%(Q)
62
%
(
K
)
/
59
%
(
Q
)
, and66%(K)/60%(Q)
66
%
(
K
)
/
60
%
(
Q
)
for the three KVN antennas, KVNYS, KVNUS, and KVNTN, respectively. The main-beam efficiencies are measured to be50%(K)/52%(Q)
50
%
(
K
)
/
52
%
(
Q
)
,48%(K)/50%(Q)
48
%
(
K
)
/
50
%
(
Q
)
, and50%(K)/47%(Q)
50
%
(
K
)
/
47
%
(
Q
)
for KVNYS, KVNUS, and KVNTN, respectively. The estimated Moon efficiencies are77%(K)/90%(Q)
77
%
(
K
)
/
90
%
(
Q
)
,74%(K)/79%(Q)
74
%
(
K
)
/
79
%
(
Q
)
, and80%(K)/86%(Q)
80
%
(
K
)
/
86
%
(
Q
)
for KVNYS, KVNUS, and KVNTN, respectively. The elevation dependence of the aperture efficiencies is quite flat for elevations greater than 20°.
We present Very Long Baseline Interferometry (VLBI) observations of 22 GHz Hsub 2O masers in the high-mass star-forming region of W75N, carried out with VLBI Exploration of Radio Astrometry (VERA) ...for three epochs in 2007 with an angular resolution of ~1 mas. We detected Hsub 2O maser emission toward the radio jet in VLA 1 and the expanding shell-like structure in VLA 2. The spatial distribution of the Hsub 2O masers detected with VERA and measured proper motions around VLA 1 and VLA 2 are similar to those found with previous VLBI observations in epochs 1999 and 2005, with the masers in VLA 1 mainly distributed along a linear structure parallel to the radio jet and, on the other hand, forming a shell-like structure around VLA 2. This suggests that we are probably observing the formation of a jet-driven Hsub 2O maser structure in VLA2, evolving from a non-collimated pulsed-outflow event during the first stages of evolution of a massive young stellar object (YSO). We discuss possible implications of our results in the study of the first stages of evolution of massive YSOs.
Abstract
We have measured the annual parallax of water masers around the semi-regular variable star RW Leporis (RW Lep) from observations with VLBI Exploration of Radio Astrometry (VERA). The ...measured parallax is 1.62 ± 0.16 mas, corresponding to a distance of $617^{+68}_{-55}\:$pc. This is the first VLBI annual parallax of RW Lep, whose previously measured parallax is a negative value. Using the estimated distance we plot RW Lep on the period–luminosity plane. RW Lep is reported to have two different periods, the longer period shows that the star is located between the sequences C and C′. The shorter period shows that it is located on the sequence B+. We estimated the luminosity, radius, and mass of RW Lep to be 15400 L⊙, 428 R⊙, and 4.7 M⊙, respectively. We found 13 maser spots around the star, with a distribution ∼ 11 times larger than the star's radius.
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