2005A&A...438..571F

We conducted a 4 epoch, 3 month, VLBA proper motion study of H₂O masers toward an intermediate-mass class 0 source i20 MMS1 (d=700pc). The region of i20 contains at least 3 bright young stellar objects at millimeter to submillimeter wavelengths and shows three pairs of CO outflow lobes: the brightest source MMS1, which shows an extremely high velocity (EHV) wing emission, is believed to drive the outflow(s). From milli-arcsecond (mas) resolution VLBA images, we found two groups of H₂O maser spots at the center of the submillimeter core of MMS1. One group consists of more than ∼50 intense maser spots; the other group consisting of several weaker maser spots is located at 18AU south-west of the intense group. Distribution of the maser spots in the intense group shows an arc-shaped structure which includes the maser spots that showed a clear velocity gradient. The spatial and velocity structures of the maser spots in the arc-shape did not significantly change through the 4 epochs. Furthermore, we found a relative proper motion between the two groups. Their projected separation increased by 1.13±0.11mas over the 4 epochs along a line connecting them (corresponding to a transverse velocity of 14.4km/s). The spatial and velocity structures of the intense group and the relative proper motions strongly suggest that the maser emission is associated with a protostellar jet. Comparing the observed LSR velocities with calculated radial velocities from a simple biconical jet model, we conclude that the most of the maser emission is likely to be associated with an accelerating biconical jet that has large opening angle of about 70°. The large opening angle of the jet traced by the masers would support the hypothesis that poor jet collimation is an inherent property of luminous (proto)stars.