6.7 GHz methanol masers in three high-mass young stellar objects: G37.43+1.51, G37.479–0.105, and G37.55+0.20

¹ Engineering Research Institute “Ventspils International Radio Astronomy Centre”, Ventspils University of Applied Sciences, Inženieru iela. 101, Ventspils 3601, Latvia
² RIKEN Pioneering Research Institute, 2-1 Hirosawa, Wako-shi, Saitama 351-0198, Japan

^★ Corresponding authors: This email address is being protected from spambots. You need JavaScript enabled to view it. ; This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 31 October 2025
Accepted: 1 March 2026

Abstract

Context. Class II 6.7 GHz CH₃OH (methanol) masers are powerful probes of the physical processes in high-mass young stellar objects. We selected three closely located sources from the Irbene single-dish methanol maser monitoring program for high-resolution imaging studies.

Aims. Our goal is to investigate their milliarcsecond-scale structure and kinematics to improve our understanding of the physical processes driving maser activity and variability in these sources.

Methods. We imaged three sources (G37.43+1.51, G37.479–0.105, and G37.55+0.20) with the European Very Long Baseline Interferometer Network (EVN), supplemented by archival data and single-dish light curves from the Irbene and Ibaraki radio telescopes. Maser emission parameters were derived for all identified cloudlets, and their spatial and kinematic distributions were analyzed.

Results. We present the first milliarcsecond-resolution, high-sensitivity images of G37.479–0.105 and G37.55+0.20; other team already studied G37.43+1.51 studied with the EVN. In all three sources, most of the maser cloudlets form linear or arched structures with visible velocity gradients. In G37.43+1.51, two-epoch proper motion measurements reveal a lengthening of one of the maser groups (designated group A), with an expansion velocity of approximately 1.1 km s⁻¹. This is consistent with an overall expansion of the maser region. In G37.479–0.105, 6.7 GHz methanol maser cloudlets are distributed in parabolic structures, with the opening (i.e., the symmetry axis of the parabola) oriented roughly in the same direction as an outflow emanating from the region. This may indicate that the maser cloudlets trace the walls of the outflow cavity. In G37.55+0.20, masers form an elongated structure located at the center of a source of outflows and a formaldehyde maser. The light curve of the periodic maser components and their on-sky distribution suggest substantial differences in line-of-sight distances among cloudlets (assuming the maser pumping conditions are modulated at the speed of light) that appear close in projection. Combined with the position of the continuum maximum, this supports the colliding-wind binary model as the likely driver of maser variability. In all three high-mass protostars, the 6.7 GHz methanol masers appear to trace the innermost regions of the systems they reside in.