The extremely collimated bipolar H2O jet from the NGC 1333-IRAS 4B protostar

Context. 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. Aims. 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. Methods. We imaged the protostars in the nearby region NGC 1333-IRAS 4 in the water maser line at 22.2 GHz by using the VLBA in phase-reference mode at the milliarcsecond scale over four epochs, spaced by one month to measure proper motions. Results. 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, <= 35 AU, moving away with projected velocities of similar to 10-50 km s(-1). The comparison with the molecular outflow observed at larger scale suggests a jet precession with a 18' yr(-1) rate. By measuring the positional spread of the H2O spots we estimate a jet width of similar to 2 AU at a distance of similar to 12 AU from the driving protostar.