VLA Observations of H₂O Masers in the Class 0 Protostar S106 FIR: Evidence for a 10 AU Scale Accelerating Jetlike Flow

We conducted VLA observations at 006 resolution of the 22 GHz water masers toward the Class 0 source S106 FIR (d = 600 pc; 15'' west of S106 IRS 4) on two epochs separated by ~3 months. Two compact clusters of the maser spots were found in the center of the submillimeter core of S106 FIR. The separation of the clusters was ~80 mas (48 AU) along P.A. = 70°, and the size of each cluster was ~20 mas × 10 mas. The western cluster, which had three maser components, was 8.0 km s^-1 blueshifted, and the eastern cluster, which had a single component, was 7.0 km s^-1 redshifted with respect to the ambient cloud velocity. Each component was composed of a few spatially localized maser spots and was aligned on a line connecting the clusters. We found relative proper motions of the components with ≈30 mas yr^-1 (18 AU yr^-1) along the line. In addition, a series of single-dish observations shows that the maser components drifted with radial accelerations of ~1 km s^-1 yr^-1. These facts indicate that the masers could be excited by a 10 AU scale jetlike accelerating flow ejected from an assumed protostar located between the two clusters. The outflow size traced by the masers was 50 AU × 5 AU after correction for an inclination angle of 10°, which was derived from the relative proper motions and radial velocities of the maser components. The three-dimensional outflow velocity ranged from 40 to 70 km s^-1 assuming symmetric proper motions for the blue and red components. Since no distinct CO molecular outflows have been detected so far, we suggest that S106 FIR is an extremely young protostar observed just after the onset of outflowing activity.