Reactivity of OH and CH 3 OH Between 22 and 64 K: Modelling the Gas Phase Production of CH 3 O in Barnard 1B.

In the last years, ultra-low temperature chemical kinetic experiments have demonstrated that some gas-phase reactions are much faster than previously thought. One example is the reaction between OH and CH ₃ OH, which has been recently found to be accelerated at low temperatures yielding CH ₃ O as main product. This finding opened the question of whether the CH ₃ O observed in the dense core Barnard 1b could be formed by the gas-phase reaction of CH ₃ OH and OH. Several chemical models including this reaction and grain-surface processes have been developed to explain the observed abundance of CH ₃ O with little success. Here we report for the first time rate coefficients for the gas-phase reaction of OH and CH ₃ OH down to a temperature of 22 K, very close to those 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 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 ± 0.1) × 10 ^-12 ( T/ 300 K) ^-(1.0±0.2) cm ³ molecule ^-1 s ^-1 . Implementing this expression in a chemical model of a cold dense cloud results in CH ₃ O/CH ₃ OH abundance ratios similar 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 CH ₃ OH is an important contributor to the formation of interstellar CH ₃ O. The role of grain-surface processes in the formation of CH ₃ O, although it cannot be fully neglected, remains controversial.