Proper motion of H2O masers in IRAS 20050+2720 MMS1:: an AU scale jet associated with an intermediate-mass class 0 source

We conducted a 4 epoch, 3 month, VLBA proper motion study of H2O masers toward an intermediate-mass class 0 source IRAS 20050+2720 MMS1 ( d = 700 pc). The region of IRAS 20050+2720 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 similar to 50 intense maser spots; the other group consisting of several weaker maser spots is located at 18 AU 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 +/- 0.11 mas over the 4 epochs along a line connecting them ( corresponding to a transverse velocity of 14.4 kms(-1)). 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 degrees. 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.