We present the initial results of multi-epoch VLBI observations of the 22 GHz H 2 O masers in the Orion KL region with VERA (VLBI Exploration of Radio Astrometry). With the VERA dual-beam receiving system, we have carried out phase-referencing VLBI astrometry and successfully detected an annual parallax of Orion KL to be 2.29±0.10 mas, corresponding to the distance of 437±19 pc from the Sun. The distance to Orion KL is determined for the first time with the annual parallax method in these observations. Although this value is consistent with that of the previously reported, 480±80 pc, which is estimated from the statistical parallax method using proper motions and radial velocities of the H 2 O maser features, our new results provide the much more accurate value with an uncertainty of only 4%. In addition to the annual parallax, we have detected an absolute proper motion of the maser feature, suggesting an outflow motion powered by the radio source I along with the systematic motion of source I itself.
We report on results of multi-epoch VLBI observations of H$_2$O masers associated with a low-mass young stellar object, IRAS 16293$-$2422 in $\rho$ Oph East, and a fringe-phase and position reference source, ICRF J162546.8$-$252738, using the VLBI Exploration of Radio Astrometry (VERA) for high-precision astrometry. We obtained an annual parallax of a maser feature to be $\pi=$ 5.6$^{+1.5}_{-0.5}$ mas, corresponding to a distance of $D=178^{+18}_{-37}$ pc. We also found 10 relative proper motions of maser features with respect to the maser feature mentioned above. The motion of the accompanying young stellar object (YSO) has already been found in thermal continuum emission previously observed with the Very Large Array. The intrinsic motions of masers have been estimated from the relative proper motions after the YSO’s motion is subtracted from, and a systemic secular motion of the position reference feature is added to the proper motions originally measured. The intrinsic maser kinematical structure may trace a bipolar outflow.
We have performed high-precision astrometry of H 2 O maser sources in Galactic star forming region Sharpless 269 (S269) with VERA. We have successfully detected a trigonometric parallax of 189 ± 8 µas, corresponding to the source distance of 5.28 +0.24 −0.22 kpc. This is the smallest parallax ever measured, and the first one detected beyond 5 kpc. The source distance as well as proper motions are used to constrain the outer rotation curve of the Galaxy, demonstrating that the difference of rotation velocities at the Sun and at S269 (which is 13.1 kpc away from the Galaxy's center) is less than 3 %. This gives the strongest constraint on the flatness of the outer rotation curve and provides a direct confirmation on the existence of large amount of dark matter in the Galaxy's outer disk.
Aims. We aim to study the spatial distribution of the v = 1 and v = 2 J = 1−0 SiO maser emission from R Aqr, a well known stellar symbiotic system. In particular, we intend to determine the annual parallax and proper motion of the source by means of measurements of the absolute coordinates of the maser spots. Methods. We performed VLBI observations of the v = 1 and v = 2 J = 1−0 maser emission, at 7 mm wavelength, using VERA. We present observations made in 11 epochs, between December 2004 and October 2006, and observations by other authors are also discussed. VERA provides very high spatial resolution and accurate astrometric measurements, thanks to its double-beam observing system. From fitting ring-like structures to the maser spot distributions, we determine absolute J2000 coordinates of the central star.(SiO maser emission is known to be distributed around the star forming spot rings at a few stellar radii.) Results. Maps with accurate absolute coordinates were obtained in 8 epochs. From the coordinates determined for the central star, we measured parallax and linear proper motion. We obtain π = 4.7 ± 0.8 mas, compatible with, but much more accurate than, the Hipparcos value (5.07 ± 3.15 mas), and deduce a distance of R Aqr D = 214 +45 −32 pc. Our accurate astrometry also yields a reliable comparison between the spot distributions of both v = 1 and v = 2 J = 1−0 lines. We find that both masers come from similar regions, i.e. at similar distances from the star and defining common spot clusters, confirming previous results. But the coincidences between spots of both lines are very rare, within the spatial and spectral resolution of our experiments. This result is found for eight epochs, spanning more than one pulsation cycle (of the cool Mira-type component). We argue that explaining the finer details of the v = 1, 2 J = 1−0 distributions found here would require new theoretical efforts. Finally, our observations also allow the study of the structure and dynamics of the close circumstellar shells in R Aqr. We do not confirm the previous suggestion that these shells are rotating; instead, we conclude that the observed kinematics is very probably caused by pulsations and random movements. The spatial distribution of the maser spots is found to be variable, but to show a stable axial symmetry.
We present results of phase-referencing VLBI observations of SiO masers in the Orion-KL region made with VERA. Using a strong maser spot in the 43GHz $v =$ 2 $J=$ 1-0 emission, we derived the trigonometric parallax of Orion-KL to be 2.39$\pm$0.03mas, corresponding to a distance of 418$\pm$6pc, with the highest accuracy among existing parallax measurements of the source. We made a superimposed image of $v =$ 1 $J=$ 1-0 and $v =$ 2 $J=$ 1-0 maser features in Orion-KL based on absolute positions obtained from the phase-referencing astrometry with a common reference source. The maser features of both transitions show similar X-shaped distributions centered at Source I. However, in each of the four arms of the X-shape, the SiO $v =$ 2 features tend to lie closer to Source I than the SiO $v =$ 1 features. The radial velocities of the maser emission decrease with the distance from Source I. The spatial and radial velocity distributions of the SiO masers suggest that the SiO masers lie in the rotating materials associated with a disk around Source I, rather than a decelerating outflow.
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