First results from VLBA survey of water masers towards low-mass YSOs: the Serpens core and RNO 15-FIR.

Abstract

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. Up to now we collected 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 submm source of the Serpens cluster, the water maser emission comes from two small (le 5 AU in size) cluster of features separated by ≈25 AU, having line of sight velocities strongly red-shifted (by more than 10 km s<SUP>-1</SUP>) 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 jet observed with the VLA towards SMM1. The spatial and velocity distribution of the maser features let us favour the interpretation that the maser emission can be excited by interaction of the receding lobe of the jet with dense gas in the accretion disk surrounding the YSO in SMM1. Towards RNO 15-FIR, the few detected maser features have both positions and (absolute) velocities aligned along a direction which 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 an higher mass ejection rate can generate jets sufficiently powerful to evacuate along the direction of motion, close to the YSO, the densent portions of circumstellar gas. In that 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.