Search for Astronomical Maser Sources and Reference Radio Sources for VLBI Astrometry

目前的研究结果总结如下，清楚地证实了VERA(VLBI探索射电天体测量)计划中以高精度银河系测速为目标的可行性。(1)寻找参考射电源利用射电源星表和VLBI观测进行的统计调查表明，通常至少有一个紧凑和足够明亮的候选参考源在距离任何目标源2度的范围内找到。(2)脉泽源的物理性质国内VLBI观测揭示了晚期类型恒星周围存在水脉泽点，基线分辨率为1000公里，存留时间超过半年，因此适合VLBI天体测量。首次探测到了恒星脉泽斑视线速度的系统性漂移(加速)。在ori-kl sar形成区发现了新的脉泽斑，它们几乎沿双极外流的对称轴排列，并可能与中心yun…的喷流活动有关。利用日本国内的VLBI网络(J-Net)进行的切换差分VLBI实验中，成功地探测到了22 GHz的两个相邻类星体之间的条纹相位差观测。(4)大气相位漂移的补偿。利用Nobeama毫米波阵列的实验和基于湍流对流层统计模型的数值模拟表明，在目标和参考源相差不超过几度的情况下，利用差分VLBI进行的相位补偿具有很高的性能。模拟也表明了相位补偿的有效性在高达22-43 GHz的观测频率下，开关周期为10-20秒的VLBI。(5)天体测量精度的估计基于模型模拟的VERA阵列测量年视差的估计精度对于在有利大气条件下观测到的一对点状脉泽和类星体源，估计精度优于10微角秒。较少

Results of present research as summarized below clearly confirmed feasibility of the the high-precision Galactic astorometry aimed at in VERA (VLBI Exploration of Radio Astrometry) Project.(1) Search for Reference Radio SourcesStatistical survey using radio-source catalogs and VLBI observations showed that at least one compact and bright enough reference-source candidate is normally found within 2 degrees from any target source.(2) Physical Properties of Maser SourcesDomestic VLBI observations revealed existence of water maser spots around late-type stars which are not resolved with 1000km baseline, surviving longer than a half year and, therefore, suited to VLBI astrometry. Systematic shift (acceleration) in line-of-sight velocity of the stellar maser spots was detected for the first time. New maser spots were discovered in Ori-KL sar-forming region which are almost aligned along the symmetry axis of the bipolar outflow and possibly associated with the jet-activity of the central youn … More g stellar object.(3) Test Observation for Maser AstrometryFringe-phase-difference observable was successfully detected between two adjacent quasars at 22 GHz in a switching differential VLBI experiment carried out with Japanese domestic VLBI Network (J-net).(4) Compensation of Atmospheric Phase FluctuationsExperiments using Nobeyama Millimeter Array and numerical simulations based on a statistical model of the turbulent troposphere demonstrated high performance expected in the phase compensation by means of the differential VLBI provided that the target and reference sources are separated by no more than a few degrees.The simulations also revealed effectiveness of the phase compensation in terms of the fast-switching defferential VLBI with the switching cycle of 10-20 second at observing frequency up to 22-43 GHz.(5) Estimation of Astrometric AccuracyEstimated accuracy of the annual parallax determination with VERA array based on a model simulation was better than 10 microarcsecond for a pair of point-like maser and quasar sources observed in favorable atmospheric conditions. Less