REACTIVITY OF OH AND CH 3 OH BETWEEN 22 AND 64 K: MODELING THE GAS PHASE PRODUCTION OF CH 3 O IN BARNARD 1b

Abstract

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 CH 3 OH, which has recently been found to be accelerated at low temperatures yielding CH 3 O as its main product. This finding raised the question of whether or not 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, 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 CH 3 OH 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 ± 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 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 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.