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Is the Gas-phase OH+H2CO Reaction a Source of HCO in Interstellar Cold Dark Clouds ? A Kinetic, Dynamic and Modelling Study

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Authors : A.J. Ocaña, E. Jiménez, B. Ballesteros, A. Canosa, M. Antiñolo, J. Albaladejo, M. Agúndez, J. Cernicharo, A. Zanchet, P. Del Mazo, O. Roncero, and A. Aguado.
Abstract : The chemical kinetics of neutral–neutral gas-phase reactions at ultralow temperatures is a fascinating research subject with important implications on the chemistry of complex organic molecules in the interstellar medium (T∼10–100 K). Scarce kinetic information is currently available for these kinds of reactions at T<200 K. In this work, we use the Cinétique de Réaction en Ecoulement Supersonique Uniforme (CRESU ; Reaction Kinetics in a Uniform Supersonic Flow) technique to measure for the first time the rate coefficients (k) of the gas-phase OH+H2CO reaction between 22 and 107 K. The k values greatly increase from 2.1×10−11 cm3 s−1 at 107 K to 1.2×10−10 cm3 s−1 at 22 K. This is also confirmed by quasi-classical trajectories (QCT) at collision energies down to 0.1 meV performed using a new full dimension and ab initio potential energy surface that generates highly accurate potential and includes long-range dipole–dipole interactions. QCT calculations indicate that at low temperatures HCO is the exclusive product for the OH+H2CO reaction. In order to revisit the chemistry of HCO in cold dense clouds, k is reasonably extrapolated from the experimental results at 10 (2.6×10−10 cm3 s−1). The modeled abundances of HCO are in agreement with the observations in cold dark clouds for an evolving time of 10^5–10^6 yr. The different sources of production of HCO are presented and the uncertainties in the chemical networks are discussed. The present reaction is shown to account for a few percent of the total HCO production rate. This reaction can be expected to be a competitive process in the chemistry of prestellar cores. Extensions to photodissociation regions and diffuse cloud environments are also addressed.
Journal : Astrophys. J., 850[1], 28 (2017).