Water Maser Kinematics in the Jet of OH 12.8-0.9

We present Very Long Baseline Array observations of the kinematics of the water masers associated with OH 12.8-0.9, the fourth member of the so-called water-fountain class of sources. We find that the masers occupy two distinct regions at the ends of a bipolar jetlike structure oriented north-south, with the blueshifted masers located to the north and the redshifted masers to the south. The masers are distributed along arclike structures 12-20 mas across that are oriented perpendicular to the separation axis with an angular separation of ~110 mas on the sky. Our multiepoch observations show the two maser arcs to be expanding away from each other along the axis of separation. The relative proper motions of the two maser regions is 2.7 mas yr^-1 (~105 km s^-1 at the assumed distance of 8 kpc). The measured radial velocity difference between the northern blueshifted masers and the southern redshifted masers is 48.4 km s^-1. The radial velocity, when combined with the proper motion, yields a three-dimensional expansion velocity of 58 km s^-1 and an inclination angle of 24° for the jet. By combining our radial velocities with historical values, we estimate the three-dimensional acceleration of the masers to be ~0.63 km s^-1 yr^-1 and a dynamical age for the collimated outflow of ~90 yr.