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

¹ Division of Physics, Mathematics, and Astronomy, California Institute of Technology, MS 105-24, 1201 East California Boulevard, Pasadena, CA 91125, USA e-mail: rsf@astro.caltech.edu
² Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, Yoshinodai 3-1-1, Sagamihara, Kanagawa 229-8510, Japan e-mail: kitamura@pub.isas.jaxa.jp
³ National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903, USA e-mail: awootten@nrao.edu
⁴ National Radio Astronomy Observatory, 1003 Lopezville Road, Socorro, NM 87801, USA e-mail: mclausse@nrao.edu
⁵ National Astronomical Observatory, Osawa 2-21-1, Mitaka, Tokyo 181-8588, Japan e-mail: kawabe@nro.nao.ac.jp

Received: 13 August 2003
Accepted: 26 March 2005

Abstract

We conducted a 4 epoch, 3 month, VLBA proper motion study of H₂O masers toward an intermediate-mass class 0 source ı20 MMS1 ( pc). The region of ı20 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 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 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 is likely to be associated with an accelerating biconical jet that has large opening angle of about . 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.