Astrometry of Galactic Star-Forming Region ON2N with VERA: Estimation of
  the Galactic Constants
  We have performed the astrometry of H2O masers in the Galactic star-forming
region Onsala 2 North (ON2N) with the VLBI Exploration of Radio Astrometry.We
obtained the trigonometric parallax of 0.261+/-0.009 mas, corresponding to the
heliocentric distance of 3.83+/-0.13 kpc. ON2N is expected to be on the solar
circle, because its radial velocity with respect to the Local Standard of Rest
(LSR) is nearly zero. Using present parallax and proper motions of the masers,
the Galactocentric distance of the Sun and the Galactic rotation velocity at
the Sun are found to be R_0 = 7.80+/-0.26 kpc and Theta_0 = 213+/-5 km/s,
respectively. The ratio of Galactic constants, namely the angular rotation
velocity of the LSR can be determined more precisely, and is found to be
Omega_0=Theta_0/R_0 = 27.3+/-0.8 km/s/kpc, which is consistent with the recent
estimations but different from 25.9 km/s/kpc derived from the recommended
values of Theta_0 and R_0 by the International Astronomical Union (1985).
