Astrometry of Galactic Star-Forming Region ON 2 N with VERA: Estimation of the Galactic Constants

We performed the astrometry of H₂O masers in the Galactic star-forming region Onsala 2 North (ON 2 N) with the VLBI Exploration of Radio Astrometry (VERA). We obtained a trigonometric parallax of 0.261\ ±\0.009 mas, corresponding to a heliocentric distance of 3.83\ ±\0.13 kpc. ON 2 N is expected to be on the solar circle, because its radial velocity with respect to the local standard of rest (LSR) is nearly zero. By using the 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 R₀=$ 7.80$\ ±\ $0.26 kpc and $Θ_0$$=$ 213\ ±\5 km s⁻¹, respectively. The ratio of Galactic constants, namely the angular rotation velocity of the LSR, can be determined more precisely, and is found to be Ω₀=Θ₀/R₀=$ 27.3$\ ±\ $0.8 km s$⁻¹$kpc$⁻¹$, which is consistent with recent estimations, but different from 25.9 km s$⁻¹$kpc$⁻¹$ derived from the recommended values of $Θ_0$ and $R_0$ by the International Astronomical Union (1985).