Methanol as a sensitive probe for spatial and temporal variations of the proton-to-electron mass ratio

The 6.7 and 12.2 GHz masers, corresponding to the 5(1) → 6(0)A+ and 2(0) → 3(-1)E transitions in methanol (CH3OH), respectively, are among the brightest radio objects in the sky. We present calculations for the sensitivity of these and other transitions in the ground state of methanol to a variation of the proton-to-electron mass ratio. We show that the sensitivity is greatly enhanced due to a cancellation of energies associated with the hindered internal rotation and the overall rotation of the molecule. We find sensitivities of K(μ) = -42 and K(μ) = -33, for the 5(1) → 6(0)A+ and 2(0) → 3(-1)E transitions, respectively. The sensitivities of other transitions in the different isotopologues of methanol range from -88 to 330. This makes methanol a sensitive probe for spatial and temporal variations of the proton-to-electron mass ratio.