2011ApJ...738...26L

The methanol molecule CH₃ OH has a complex microwave spectrum with a large number of very strong lines. This spectrum includes purely rotational transitions as well as transitions with contributions of the internal degree of freedom associated with the hindered rotation of the OH group. The latter takes place due to the tunneling of hydrogen through the potential barriers between three equivalent potential minima. Such transitions are highly sensitive to changes in the electron-to-proton mass ratio, µ = m_e/m_p, and have different responses to µ-variations. The highest sensitivity is found for the mixed rotation-tunneling transitions at low frequencies. Observing methanol lines provides more stringent limits on the hypothetical variation of µ than ammonia observation with the same velocity resolution. We show that the best-quality radio astronomical data on methanol maser lines constrain the variability of µ in the Milky Way at the level of|Δµ/µ| < 28x10^–9 (1σ) which is in line with the previously obtained ammonia result,|Δµ/µ| < 29x10^–9 (1σ). This estimate can be further improved if the rest frequencies of the CH₃ OH microwave lines will be measured more accurately.