Outer Rotation Curve of the Galaxy with VERA II: Annual Parallax and Proper Motion of the Star-Forming Region IRAS21379+5106

We conducted astrometric VLBI observations of water-vapor maser emission in the massive star forming region IRAS 21379+5106 to measure the annual parallax and proper motion, using VERA. The annual parallax was measured to be $0.262 \pm 0.031$mascorrespondingtoatrigonometricdistanceof$3.82^{+0.51}{-0.41}$kpc\mathrm{.}Thepropermotionwas$(\mu\alpha\cos{\delta}, \mu_\delta)=(-2.74 \pm 0.08, -2.87 \pm 0.18)$masyr$^{-1}$. Using this result, the Galactic rotational velocity was estimated to be $V_{\theta }=218\pm 19$ km s${}^{-1}$ at the Galactocentric distance $R=9.22\pm 0.43$ kpc, when we adopted the Galactic constants $R_0=8.05\pm 0.45$ kpc and $V_0=238\pm 14$ km s${}^{-1}$. With newly determined distance, {the bolometric luminosity of the central young stellar object was re-evaluated to $(2.15\pm 0.54)\times {10}^3{L}_{\odot }$, which corresponds to spectral type of} B2–B3. Maser features were found to be distributed along a straight line from south-west to north-east. In addition, a vector map of the internal motions constructed from the residual proper motions implies that maser features trace a bipolar flow and that it cannot be explained by simple ballistic motion.