Hyperfine structure of the methanol molecule as traced by Class I methanol masers

We present results on simultaneous observations of Class I methanol masers at 25, 36, and 44 GHz towards 22 Galactic targets carried out with the Effelsberg 100-m telescope. The study investigates relations between the hyperfine (HF) structure of the torsion–rotation transitions in CH₃OH and maser activity. By analysing the radial velocity shifts between different maser lines together with the patterns of the HF structure based on laboratory measurements and quantum-chemical calculations, we find that in any source only one specific HF transition forms the maser emission and that this transition changes from source to source. The physical conditions leading to this selective behaviour are still unclear. Using accurate laboratory rest frequencies for the 25 GHz transitions, we have refined the centre frequencies for the HF multiplets at 36, 44, and 95 GHz: f₃₆ = (36169.2488± 0.0002ₛₜₐₜ ± 0.0004sys) MHz. f₄₄ = (44069.4176± 0.0002ₛₜₐₜ ± 0.0004sys) MHz, and f₉₅ = (95169.4414± 0.0003ₛₜₐₜ ± 0.0004sys) MHz. Comparison with previous observations of 44 GHz masers performed 6–10 yr ago with a Korean 21-m Korean Very Long Baseline Interferometry Network telescope towards the same targets confirms the kinematic stability of Class I maser line profiles during this time interval and reveals a systematic radial velocity shift of 0.013± 0.005 km s⁻¹ between the two telescopes.