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 0".06 resolution of the 22 GHz water maserstoward the Class 0 source S106 FIR (d=600 pc; 15" west of S106-IRS4) on twoepochs separated by ~3 months. Two compact clusters of the maser spots werefound in the center of the submillimeter core of S106 FIR. The separation ofthe clusters was ~80 mas (48 AU) along P. A. = 70 degrees and the size of eachcluster was ~20 mas x 10 mas. The western cluster, which had three masercomponents, was 7.0 km/s redshifted with respect to the ambient cloud velocity.Each component was composed of a few spatially localized maser spots and wasaligned on a line connecting the clusters. We found relative proper motions ofthe components with ~30 mas/yr (18 AU/yr) along the line. In addition, a seriesof single-dish observations show that the maser components drifted with aradial acceleration of ~1 km/s/yr. These facts indicate that the masers could be excited by a 10 AU-scalejet-like accelerating flow ejected from an assumed protostar located betweenthe two clusters. The outflow size traced by the masers was 50 AU x 5 AU aftercorrection for an inclination angle of 10 degrees which was derived from therelative proper motions and radial velocities of the maser components. Thethree-dimensional outflow velocity ranged from 40 to 70 km/s assuming symmetricmotions for the blue and red components. Since no distinct CO molecularoutflows have been detected so far, we suggest that S106 FIR is an extremelyyoung protostar observed just after the onset of outflowing activity.Comment: 24 pages, 6 figures, No. 5 color. Accepted, Astrophysical Journa