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 Ω 0 , mean peculiar motion of the sources with respect to Galactic rotation (U src , V src , W src ), rotation-curve shape index, and the V component of the Solar peculiar motions V ⊙ . 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 ± 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 src and W src are fairly small compared to the Galactic rotation velocity, being U src = 1.0 ± 1.5 km s −1 and W src = −1.4 ± 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 src and V ⊙ as V src = V ⊙ − 19 (±2) km s −1 , suggesting that the value of V src is fully dependent on the adopted value of V ⊙ . Regarding the rotation speed in the vicinity of the Sun, we also find a strong correlation between Ω 0 and V ⊙ . We find that the angular velocity of the Sun, Ω ⊙ , which is defined as Ω ⊙ ≡ Ω 0 + V ⊙ /R 0 , can be well constrained with the best estimate of Ω ⊙ = 31.09 ± 0.78 km s −1 kpc −1 . This corresponds to Θ 0 = 238 ± 14 km s −1 if one adopts the above value of R 0 and recent determination of V ⊙ ∼12 km s −1 .

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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.

The Korean VLBI Network (KVN) is a new mm-VLBI dedicated array with capability for simultaneous observations at multiple frequencies, up to 129 GHz.The innovative multi-channel receivers present significant benefits for astrometric measurements in the frequency domain. The aim of this work is to verify the astrometric performance of the KVN using a comparative study with the VLBA, a well established instrument. For that purpose, we carried out nearly contemporaneous observations with the KVN and the VLBA, at 14/7 mm, in April 2013. The KVN observations consisted of simultaneous dual frequency observations, while the VLBA used fast frequency switching observations. We used the Source Frequency Phase Referencing technique for the observational and analysis strategy. We find that having simultaneous observations results in a superior performance for compensation of all atmospheric terms in the observables, in addition to offering other significant benefits for astrometric analysis.We have compared the KVN astrometry measurements to those from the VLBA.We find that the structure blending effects introduce dominant systematic astrometric shifts and these need to be taken into account. We have tested multiple analytical routes to characterize the impact of the low resolution effects for extended sources in the astrometric measurements. The results from the analysis of KVN and full VLBA datasets agree within 2-σ of the thermal error estimate.We interpret the discrepancy as arising from the different resolutions. We find that the KVN provides astrometric results with excellent agreement, within 1-σ, when compared to a VLBA configuration which has a similar resolution. Therefore this comparative study verifies the astrometric performance of KVN using SFPR at 14/7 mm, and validates the KVN as an astrometric instrument. Subject headings: Astrometry -techniques: interferometric -quasars: individual (OJ287, 0854+213)GHz, along with KVN observations using a similar configuration, both carried out in April 2013. The VLBA observations were carried out using fast frequency switching between both bands, while the KVN used simultaneous dual frequency observations. This study will serve to deepen the understanding on the limitations imposed by the fast frequency switching observing mode (i.e. with the VLBA), and the benefits derived from simultaneous dual frequency simultaneous observations with the multi-channel receivers in the KVN. The driver for this is to investigate whether to equip global baselines with KVN-like systems or more conventional fast frequency-switching systems.The interest and basis of the SFPR method have been described in detail in other publications ; most recently, the application to spectral line observations of H 2 O and SiO masers in evolved stars, with a non integer frequency ratio, is presented in Dodson et al. (2014). In summary, the SFPR technique provides at mm-wavelengths the benefits that conventional PR has in the cm-wavelength regime, where the moderate tropospheric phase fluctuations can be match...

We present the results of VLBI observations of three H$_2$O maser sources in massive star-forming regions: IRAS 06058$+$2138, IRAS 19213$+$1723, and AFGL 2789. We determined the annual parallaxes and proper motions for these sources by a phase-referencing method. The estimated distances to IRAS 06058$+$2138 and AFGL 2789 are 1.76$\pm$0.11 kpc and 3.07$\pm$0.30 kpc, which are located in the Perseus spiral arm. The distance to IRAS 19213$+$1723 is 3.98$\pm$0.57 kpc, placing it in the Carina–Sagittarius arm. With estimated 3-dimensional velocity components, we have shown that the galactic rotation curve is flat in the range of 6.4 kpc $\le R \le$ 13 kpc as combining with S 269, which was observed with VERA. We have also found that the overall rotation curve of the Galaxy do not depend on $\Theta_0$. The maser sources also show deviations from flat rotation curves by a few kilometers per second. We discuss possible reasons for these peculiar motions.

We report a multi-epoch, simultaneous 22 GHz H 2 O and 44 GHz Class I CH 3 OH maser line survey toward 180 intermediate-mass young stellar objects, including 14 Class 0, 19 Class I objects, and 147 Herbig Ae/Be stars. We detected H 2 O and CH 3 OH maser emission toward 16 (9 %) and 10 (6 %) sources with 1 new H 2 O and 6 new CH 3 OH maser sources. The detection rates of both masers rapidly decrease as the central (proto)stars evolve, which is contrary to the trends in high-mass star-forming regions. This suggests that the excitations of the two masers are closely related to the evolutionary stage of the central (proto)stars and the circumstellar environments. H 2 O maser velocities deviate on average 9 km s −1 from the ambient gas velocities whereas CH 3 OH maser velocities match quite well with the ambient gas velocities. For both maser emissions, large velocity differences (|v H 2 O − v sys | > 10 km s −1 and |v CH 3 OH − v sys | > 1 km s −1 ) are mostly confined to Class 0 objects. The formation and disappearance of H 2 O masers is frequent and their integrated intensities change by up to two orders of magnitude. In contrast, CH 3 OH maser lines usually show no significant change in intensity, shape, or velocity. This is consistent with the previous suggestion that H 2 O maser emission originates from the base of an outflow while 44 GHz Class I CH 3 OH maser emission arises from the interaction region of the outflow with the ambient gas. The isotropic maser luminosities are well correlated with the bolometric luminosities of the central objects. The fitted relations are L H 2 O = 1.71 × 10 −9 (L bol ) 0.97 and L CH 3 OH = 1.71 × 10 −10 (L bol ) 1.22 .