2023MNRAS.526.1000N

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.