2015ApJ...802...74S

To explore the formation mechanisms of gas phase CH₃ OH in cold starless cores, we have conducted high spectral resolution observations toward the cyanopolyyne peak of Taurus Molecular Cloud-1 (TMC-1 CP) with the IRAM 30 m telescope, the Green Bank Telescope, and the Nobeyama 45 m telescope. The spectral lines of CH₃OH toward TMC-1 CP are found to have a double-peaked profile separated by 0.5 km/s. Since the double-peaked profile is observed for ¹³CH₃OH, it is not due to optical depth and/or self-absorption effects. The spectral line profile of CH₃OH is much different from those of C³⁴S, C₃S, and HC₇N observed toward this source. The H₂densities of the emitting region of CH₃OH for the blueshifted and redshifted components are derived to be (1.7±0.5)x10⁴/cm3 and (4.3±1.2)x10⁴/cm3, respectively. These densities are similar to or slightly lower than those found for the other molecules. These results suggest a chemical differentiation between CH₃OH and the other molecules, which has indeed been confirmed by mapping observations of the CH₃ OH and C³⁴S lines. These results are consistent with the general idea that CH₃OH is formed on dust grains and is liberated into the gas phase by non-thermal desorption. The grain-surface origin of CH₃OH is further confirmed by the CH₃OH/¹³CH₃OH ratio. Weak shocks caused by accreting diffuse gas to the TMC-1 filament, photoevaporation caused by cosmic-ray induced UV radiation, and the desorption of excess reaction energy in the formation of CH₃OH on dust grains are discussed for the desorption mechanisms.