A kinematic study of the disc-outflow system around a high-mass protostar G59.783+0.065 probed by methanol and water masers

TL;DR

This study uses VLBI observations of methanol and water masers to analyze the complex kinematic structures around a high-mass protostar, revealing multiple circumstellar components and their dynamic interactions.

Contribution

It provides the first detailed kinematic analysis of both methanol and water masers in G59.783+0.065, uncovering complex motions and structures that explain maser distribution complexities.

Findings

Methanol and water masers form bipolar distributions 2000 au apart.

Methanol masers trace a rotating disc-wind or infall inside an edge-on disc.

Complex maser motions suggest multiple circumstellar structures at outflow-inflow boundary.

Abstract

Class II CH3OH masers are used as a convenient tracer of disc-like structures in high-mass star formation. However, more than half of them show a complex distribution in Very Long Baseline Interferometry (VLBI) maps. The origin of such a complex distribution is still unknown. We conducted VLBI monitoring observations to unveil the origin of a complex class II CH3OH maser in the high-mass star-forming region G59.783+0.065. We observed the CH3OH maser at 6.7 GHz and the H2O maser at 22 GHz to probe detailed circumstellar kinematics and structures by the Japanese VLBI network and the VLBI Exploration of Radio Astrometry. We found similar bipolar distributions in both masers, specifically two clusters located 2000 au apart along the East-West direction. We detected a linear distribution of CH3OH masers in the Western cluster. A position-velocity diagram shows that the Western CH3OH masers trace a rotating disc-wind or infalling component inside an edge-on disc-like structure. In contrast to the simple bipolar expanding motions of the H2O masers, the CH3OH masers exhibited complex motions despite their spatial coincidence. Some of the Eastern CH3OH masers showed bipolar expansions similar to the H2O masers, while others displayed random or even inward motions. Such complex kinematics and their close association with the H2O maser could occur at the boundary between outflow and inflow. We suggest that the complex distribution of class II CH3OH masers, like G59.783+0.065 arises from several distinct circumstellar structures that simultaneously achieve maser excitation.