VLBI study of maser kinematics in high-mass star-forming regions

Abstract

<i>Aims. <i/>To study the high-mass star-forming process, we started a large project to unveil the gas kinematics close to young stellar objects (YSOs) through the Very Long Baseline Interferometry (VLBI) of maser associations. By comparing the high spatial resolution maser data that traces the inner kinematics of the (proto)stellar cocoon with interferometric thermal data that traces the large-scale environment of the hot molecular core (HMC) harboring the (proto)stars, we can investigate the nature and identify the sources of large-scale motions. The present paper focuses on the high-mass star-forming region G16.59–0.05.<i>Methods. <i/>Using the VLBA and the EVN arrays, we conducted phase-referenced observations of the three most powerful maser species in G16.59–0.05: H<sub>2<sub/>O at 22.2 GHz (4 epochs), CH<sub>3<sub/>OH at 6.7 GHz (3 epochs), and OH at 1.665 GHz (1 epoch). In addition, we performed high-resolution (<i>≥<i/>), high-sensitivity (< 0.1 mJy) VLA observations of the radio continuum emission from the star-forming region at 1.3 and 3.6 cm.<i>Results. <i/>This is the first work to report accurate measurements of the <i>relative<i/> proper motions of the 6.7 GHz CH<sub>3<sub/>OH masers. The different spatial and 3-D velocity distributions clearly indicate that the 22 GHz water and 6.7 GHz methanol masers trace different kinematic environments. The bipolar distribution of 6.7 GHz maser line-of-sight velocities and the regular pattern of observed proper motions suggest that these masers are tracing rotation around a central mass of about 35 . The flattened spatial distribution of the 6.7 GHz masers, oriented NW–SE, suggests that they can originate in a disk/toroid rotating around the massive YSO that drives the <sup>12<sup/>CO (2–1) outflow, oriented NE–SW, observed on an arcsec scale. The extended, radio continuum source observed close to the 6.7 GHz masers could be excited by a wide-angle wind emitted from the YSO associated with the methanol masers, and such a wind has proven to be energetic enough to drive the NE–SW <sup>12<sup/>CO (2-1) outflow. The H<sub>2<sub/>O masers are distributed across a region offset about to the NW of the CH<sub>3<sub/>OH masers, in the same area as where the emission of high-density molecular tracers, typical of HMCs, was detected. We postulate that a distinct YSO, possibly in an earlier evolutionary phase than what excites the methanol masers, is responsible for the excitation of the water masers and the HMC molecular lines.