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R. Furuya (NRO), Y. Kitamura (ISAS), M. Claussen (NRAO), A. Wootten (NRAO), M. Saito (CfA), K.B. Marvel (AAS), R. Kawabe (NRO)
We present results from four monthly epochs of VLBA observations of H2O masers at 22 GHz toward the Class 0 source S106 FIR. Prior to the observations, using the VLA, we found two clusters of maser spots with 50 AU separation, located in the center of the cloud core of S106 FIR. The eastern and western clusters are red- and blueshifted; both are expanding away from an assumed central protostar.\par The VLBA observations have revealed the presence of a U-shaped distribution of the masers in the blue cluster. It has a length of 4 AU, a width of 3 AU and is located at a distance of 25 AU from the star. The U-shape opens towards the star and its axis is parallel to a line connecting the two clusters. Proper motion measurements reveal that the distance separating the two clusters is expanding in the axial direction. An expansion velocity of the U-shape structure ranges from 25 to 45 km s-1 estimated from the transverse velocities and line-of-sight velocities of the masers.
The masers are thought to be excited in a ``micro bowshock'" between the head of micro jet and the ambient cloud medium. The U-shaped structure suggests that the micro bowshock does not comprise of interior knots but lies at the head of the micro jet. If it were an inner knot, then the temperature of the postshock layer of outer knots must be higher than that of quiescent ambient cloud medium which the head has not impacted. The higher temperature leads to a lower-Mach number(\approx10), and the lower-Mach number would form a more parabolic shape rather than the U-shape. In fact, MHD simulations of the micro jet modeled with our observed parameters clearly reproduce the U-shaped structure when the Mach number is larger than \approx30. These facts strongly suggest the presence of an extremely young star near the middle of the line connecting the two clusters. The extraordinarily small size of the micro jet, the absence of distinct CO molecular outflows and the cold SED argue that S106 FIR is an object in the earliest stages of star formation.
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