Tracing the base of protostellar wind(s) towards the high-mass star forming region AFGL 5142: VLA continuum and VLBA H$_\mathsf{2}$O maser observations

Abstract

We have conducted phase-reference multi-epoch observations of the 22.2 GHz water masers using the Very Long Baseline Array (VLBA) and a multi-frequency study of the continuum emission using the Very Large Array (VLA) towards the high-mass star forming region (SFR) AFGL 5142. 29 maser features were identified and most of them were persistent over the four observing epochs, allowing absolute proper motions to be determined. The water maser emission comes from two elongated structures (indicated as Group I and Group II), with the measured proper motions aligned along the structures’ elongation axes. Each group consists of two (blue- and red-shifted) clusters of features separated by a few hundreds and thousands of AU respectively for Group I and Group II. The maser features of Group II have both positions and velocities aligned along a direction close to the axis of the outflow traced by HCO + and SiO emission on angular scales of tens of arcsec. We predict that the maser emission arises from dense, shocked molecular clumps displaced along the axis of the molecular outflow. The two maser clusters of Group I are oriented on the sky along a direction forming a large angle (60 ◦ ) with the axis of the jet/outflow traced by Group II maser features. We have detected a compact (8.4 and 22 GHz) continuum source (previously reported at 4.9 and 8.4 GHz) that falls close to the centroid of Group I masers, indicating that the source ionizing the gas is also responsible for the excitation of the water masers. The kinematic analysis indicates that the Group I masers trace outflowing rather than rotating gas, discarding the Keplerian disk scenario proposed in a previous paper for Group I. Since the axis joining the two maser clusters of Group II does not cross the position of the continuum source, Group II masers might be excited by an (undetected) massive YSO, distinct from the one (pinpointed by the VLA continuum emission) responsible for the excitation of the Group I masers. Our results give support to models of accretion and jet ejection related to the formation of high-mass stars.