RADIO ASTROMETRIC OBSERVATIONS AND THE GALACTIC CONSTANT AS THE BASIS OF A GALACTIC KINEMATICS STUDY

We made phase-referencing Very Long Baseline Interferometry (VLBI) observations of Galactic 22 GHz $H_2O$ maser sources with VLBI Exploration of Radio Astrometry (VERA). We measured the parallax distances of G48.61+0.02, G48.99-0.30, G49.19-0.34, ON1, IRAS 20056+3350, IRAS 20143+3634, ON2N, and IRAS 20126+4104, which are located near the tangent point and the Solar circle. The angular velocity of the Galactic rotation at the LSR (i.e. the ratio of the Galactic constants) is derived using the measured parallax distances and proper motions of these sources. The derived value of ${\Omega}_0=28.8{\pm}1.7km\;s^{-1}kpc^{-1}$ is consistent with recent values obtained using VLBI astrometry but 10% larger than the International Astronomical Union (IAU) recommended value of $25.9km\;s^{-1}kpc^{-1}=(220km\;s^{-1})$ / (8.5 kpc).