2024MNRAS.527.2750V

We present results on numerical calculations of the sensitivity coefficients, Q_μ, of microwave molecular transitions in ¹³CH₃OH and ${\rm CH_3\, ^{18}OH}$ to the hypothetical variation in the fundamental physical constant μ - the electron-to-proton mass ratio. The invariability of μ in time and space is one of the basic assumptions of the Standard Model of particle physics which can be tested at cosmological scales by means of astronomical observations in the Galaxy and external galaxies. Our calculations show that these two methanol isotopologues can be utilized for such tests since their microwave transitions from the frequency interval 1-100 GHz exhibit a large spread in Q_μ values which span a range of -109 ≲ Q_μ ≲ 78. We show that the thermal emission lines of ¹³CH₃OH observed in the star-forming region NGC 6334I constrain the variability of μ at a level of 3 × 10^–8 (1σ), which is in line with the most stringent upper limits obtained previously from observations of methanol (CH₃OH) and other molecules in the Galaxy.