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We conducted a 4 epoch 3 month VLBA proper motion study of H$_2$O maserstoward an intermediate-mass class 0 source IRAS 20050+2720 MMS1 (d=700 pc).  From milli-arcsecond (mas) resolution VLBA images, we found two groups of H2Omaser spots at the center of the submillimeter core of MMS1. One group consistsof more than $\sim 50$ intense maser spots; the other group consisting ofseveral 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-shapedstructure 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 notsignificantly change through the 4 epochs. Furthermore, we found a relativeproper motion between the two groups. Their projected separation increased by1.13+/-0.11 mas over the 4 epochs along a line connecting them. The spatial andvelocity structures of the intense group and the relative proper motionsstrongly suggest that the maser emission is associated with a protostellar jet.Comparing the observed LSR velocities with calculated radial velocities from asimple biconical jet model, we conclude that the most of the maser emission arelikely to be associated with an accelerating biconical jet which has largeopening angle. The large opening angle of the jet traced by the masers wouldsupport the hypothesis that poor jet collimation is an inherent property ofluminous (proto)stars.Comment: 14 pages, 10 figures, Fig.3 was downsized significantly. accepted for  publication in A&

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

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