Towards Determination of Galactic Rotation Curve through Phase- Referencing VLBI Observations of Astronomical Maser Sources

VERA（VLBI Exploration of Radio Astronomy）是国家天文台新组建的VLBI阵列，主要用于对天文脉泽源进行高精度相位参考VLBI天体测量，研究银河系的结构和动力学。该项目最近获得资助，四站阵列正在建设中。在本研究中，我们进行了22 GHz的开关相位参考VLBI观测的两对银河系水脉泽源和相邻的河外连续射电源使用VLBA的NRAO作为第一步确定的旋转曲线的太阳圈外的银河系。研究的主要目的是根据真实的VLBI数据确定VERA计划的基本概念，为VERA观测做准备。这两对射电源分别于1998年和1999年以一个月的间隔在40秒的快速切换相位参考模式下进行了两次观测。主要由首席研究员的研究生Kazuya Hachisuka对观测结果进行的分析表明：（1）主要由湍流对流层中的水汽引起的VLBI条纹相位的大波动在相位参考中得到了成功的补偿，40秒积分相位差的剩余波动可观测到小至10度（对应于2000公里投影基线的50微弧秒角精度），（2）首次清楚地探测到两个脉泽源在一个月内的天体位移，假定它们的距离分别约为7千秒差距和2千秒差距，由于银河系自转、年视差和太阳运动的综合影响，观测结果与预期的自行基本一致，证实了VERA相位参考VLBI观测在10微弧秒级高精度脉泽天体测量中的适用性。

VERA (VLBI Exploration of Radio Astronomy) is a new VLBI array of National Astronomical Observatory mainly aimed at study of the structure and dynamics of our Galaxy through high-precision phase-referencing VLBI astrometry of astronomical maser sources. The project was recently funded and the four station array is under construction. In the present study, we conducted 22 GHz switching phase-referencing VLBI observations of two pairs of Galactic water maser sources and adjacent extragalactic continuum radio sources using VLBA of NRAO as the first step towards determination of the rotation curve of the Galaxy outside the Solar Circle. The primary purpose of the study was to confirm basic concepts of VERA project on the basis of the real VLBI data and to prepare to VERA observations. The two pairs of radio sources were observed twice with a month interval in 1998 and 1999, respectively, in the fast-switching phase-referencing mode with 40 second switching cycle. Analyses of the observed results which were mainly performed by Kazuya Hachisuka, a graduate student of the head investigator, revealed(1) the large fluctuation in the VLBI fringe phase due mostly to the water vapor in the turbulent troposphere was successfully compensated in the phase-referencing, and the residual fluctuation in 40-second-integrated phase difference observable was as small as 10 degrees (which corresponds to 50 microarcsecond angular precision with 2000 km projected baseline) ,(2) celestial displacements of the two maser sources with assumed distances of roughly 7 kpc and 2 kpc, respectively, during a month were clearly detected for the first time, and the were mostly consistent with the expected proper motions due to the combained effects of the Galactic rotation, annual parallax and solar motion.The results confirmed the eligibility of the phase-referencing VLBI observations with VERA in the high-precision maser astrometry with 10 microarcsecond-level accuracy.