First results from a VLBA proper motion survey of H2O masers in low-mass YSOs:: the Serpens core and RNO 15-FIR

This article reports first results of a long-term observational program aimed to study the earliest evolution of jet/disk systems in low-mass YSOs by means of VLBI observations of the 22.2 GHz water masers. We report here data for the cluster of low-mass YSOs in the Serpens molecular core and for the single object RNO 15-FIR. Towards Serpens SMM1, the most luminous sub-mm source of the Serpens cluster, the water maser emission comes from two small (<= 5 AU in size) clusters of features separated by approximate to 25 AU, having line of sight velocities strongly red-shifted ( by more than 10 km s(-1)) with respect to the LSR velocity of the molecular cloud. The two maser clusters are oriented on the sky along a direction that is approximately perpendicular to the axis of the radio continuum jet observed with the VLA towards SMM1. The spatial and velocity distribution of the maser features lead us to favor the interpretation that the maser emission is excited by interaction of the receding lobe of the jet with dense gas in the accretion disk surrounding the YSO in SMM1. The line of sight velocities of several features decrease at a rate of approximate to 1 km s(-1) month(-1) and the sky-projected relative motion of two features appears to be accelerated (decelerated) at a rate of approximate to 10-15 km s(-1) month(-1). We propose that the shocks harboring the maser emission are slowed down as they proceed through the dense material surrounding the YSO. Towards RNO 15-FIR, the few detected maser features have both positions and (absolute) velocities aligned along a direction that is parallel to the axis of the molecular outflow observed on much larger angular scales. In this case the maser emission likely emerges from dense, shocked molecular clumps displaced along the axis of the jet emerging from the YSO. The protostar in Serpens SMM1 is more massive than the one in RNO 15-FIR. We discuss the case where a high mass ejection rate can generate jets sufficiently powerful to sweep away from their course the densest portions of circumstellar gas. In this case, the excitation conditions for water masers might preferably occur at the interface between the jet and the accretion disk, rather than along the jet axis.