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

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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