A search for the favored hyperfine transition of a 6.7 GHz methanol maser line

¹ Institute of Astronomy, Faculty of Physics, Astronomy and Informatics, Nicolaus Copernicus University, Grudziadzka 5, 87-100 Torun, Poland
² INAF – Osservatorio Astronomico di Cagliari, Via della Scienza 5, 09047 Selargius, Italy
³ Department of Space, Earth and Environment, Chalmers University of Technology, 412 96 Gothenburg, Sweden

^* Corresponding author.

Received: 19 May 2025
Accepted: 1 July 2025

Abstract

Context. The polarized emission of astrophysical masers, especially OH and CH₃OH lines, is an effective tool to study the magnetic field in high-mass star-forming regions. The magnetic field strength measurement via the Zeeman effect of OH maser emission is well established, but that of the CH₃OH maser emission is still under debate because of its complex hyperfine structure.

Aims. We aim to identify the dominating hyperfine transition of the Class II 6.7 GHz CH₃OH maser emission by comparing the magnetic field strength measured with the 6.0 GHz excited OH maser emission and the Zeeman splitting of the CH₃OH maser emission.

Methods. We used quasi-simultaneous European VLBI Network observations of the two maser emissions at 6.035 GHz (excited OH maser) and 6.668 GHz (CH₃OH. maser) toward two well-known high-mass young stellar objects: G69.540-0.976 (ON 1) and G81.871+0.781 (W75N). The observations were performed in full polarimetric mode and in phase-referencing mode to couple the maser features of the two maser emissions in each source.

Results. We detected linearly and circularly polarized emission in both maser transitions and high-mass young stellar objects. Specifically, we measured the magnetic field strength in twelve and five excited OH maser features toward ON 1 and W 75 N, respectively, and the Zeeman splitting of the CH₃OH maser spectra in one and three maser features toward ON1 and W75N, respectively. We determined that the two maser emissions likely probe the same magnetic field but at different densities. Indeed, a direct comparison of the magnetic field strength and the Zeeman splitting as measured with the excited OH and CH₃OH maser spots, respectively, provided values of the Zeeman splitting coefficient (α_Z) for the 6.7 GHz CH₃OH maser that do not match with any of the table values present in the literature.

Conclusions. We are not able to uniquely identify the dominating hyperfine transition; however, through density considerations we can narrow the choice down to three hyperfine transitions: 3 → 4, 6 → 7 A, and 7 → 8. Furthermore, we support the previously proposed idea that the favored hyperfine transition is not always the same, but that in different high-mass young stellar objects, the dominating one can be any of these three hyperfine transitions.