2009A&A...498..753D

We have performed observations of water maser emission towards a sample of low-mass protostars, in order to investigate the properties of jets associated with the earliest stages of star formation and their interaction with the surrounding medium. The main aim is to measure the absolute positions and proper motions of the H₂O spots in order to investigate the kinematics of the region from where the jet is launched. We imaged the protostars in the nearby region NGC1333-IRAS4 in the water maser line at 22.2GHz by using the VLBA in phase-reference mode at the milliarcsecond scale over four epochs, spaced by one month to measure proper motions. Two protostars (A2 and B) were detected in a highly variable H₂O maser emission, with an active phase shorter than four weeks. The H₂O maps allow us to trace the fast jet driven by the B protostar: we observed both the red- and blue-shifted lobes very close to the protostar, ≤35AU, moving away with projected velocities of ∼10-50km/s. The comparison with the molecular outflow observed at larger scale suggests a jet precession with a 18'/yr rate. By measuring the positional spread of the H₂O spots we estimate a jet width of ∼2AU at a distance of ∼12AU from the driving protostar.