Astrometry of Water Maser sources in the Outer Galaxy with VERA

While the rotation curve of the inner Galactic disk is well determined, study of the outer rotation curve requires observational measurements of distances and proper motions of individual sources in the Outer Galaxy. We report astrometric observation for water maser sources in the Outer Galactic disk conducted with VERA, aiming to measure the Outer Rotation Curve. We have measured annual parallaxes and proper motions for these objects. Our result was consistent with recent other works based on astrometry and classical Cepheid observations. Epicyclic frequency seems to suggest that 2 and 4 spiral mode are dominant in the inner and outer Galaxy, respectively.