REACTIVITY OF OH AND CH₃OH BETWEEN 22 AND 64 K: MODELING THE GAS PHASE PRODUCTION OF CH₃O IN BARNARD 1b

In recent years, ultra-low temperature chemical kinetic experiments have demonstrated that some gas-phase reactions are much faster than was previously thought. One example is the reaction between OH and CH₃OH, which has recently been found to be accelerated at low temperatures yielding CH₃O as its main product. This finding raised the question of whether or not the CH₃O observed in the dense core Barnard 1b could be formed by the gas-phase reaction of CH₃OH and OH. Several chemical models including this reaction and grain-surface processes have been developed to explain the observed abundance of CH₃O, but they have met with little success. Here, we report for the first time the rate coefficients for the gas-phase reaction of OH and CH₃OH down to a temperature of 22 K, which is very close to the temperature in cold interstellar clouds. Two independent experimental set-ups based on the supersonic gas expansion technique coupled to the pulsed laser photolysis laser-induced fluorescence technique were used to determine the rate coefficients in the temperature range 22–64 K. The temperature dependence obtained in this work can be expressed as k(22–64 K) = ${(3.6\pm 0.1)\times {10}^{-12}\left(T/300K\right)}^{-(1.0\pm 0.2)}$ cm³ molecule⁻¹ s⁻¹. Implementing this expression in a chemical model of a cold, dense cloud results in CH₃O/CH₃OH abundance ratios similar to or slightly lower than the value of ∼3 × 10⁻³ observed in Barnard 1b. This finding confirms that the gas-phase reaction between OH and CH₃OH is an important contributor to the formation of interstellar CH₃O. The role of grain-surface processes in the formation of CH₃O, although it cannot be fully neglected, remains controversial.