We conducted a 4 epoch 3 month VLBA proper motion study of H _ { 2 } O 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 H _ { 2 } O maser spots at the center of the submillimeter core of MMS1 .
One group consists of more than \sim 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 \pm 0.11 mas over the 4 epochs along a line connecting them ( corresponding to a transverse velocity of 14.4 km s ^ { -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 are likely to be associated with an accelerating biconical jet which has large opening angle of about 70 ^ { \circ } .
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 .