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 5₁→6₀A⁺ and 2₀→3_-1E 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 5₁→6₀A⁺ and 2₀→3_-1E 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.