Distance and kinematics of the W49N H2O maser outflow

Abstract

Study of the motions of 105 H2O maser features clustered around a newly formed star in W49N yields the kinematics of the gas flow, the distance to the source, and the spatial scale of the Milky Way. It is found that the maser outflow is bipolar, with an opening angle of about 60 deg and an inclination of about 40 deg to the line of sight. The expansion has a constant velocity of about 18 km/s out to a radius of 0.1 pc, beyond which the outflow velocity increases to greater than 200 km/s. This increase may be due to interaction with ambient material. A rotation is also present; this rotation is nearly perpendicular to the outflow axis. The rotation may be due to ram pressure from ambient material; rotation of the ring of H II regions described by Welch et al. could produce such nonradial motion. Comparison of Doppler velocities and proper motions yields a distance of 11.4 +/- 1.2 kpc for the maser cluster. Combining this with a kinematic distance for W49N from Galactic rotation, a value of R0, the distance to the Galactic center, of 8.1 +/- 1.1 kpc, is obtained.