Rate coefficients for interstellar gas-phase chemistry

Abstract

In the early 1970s the first gas-phase models highlighted the ways through which molecules can be synthesized in the interstellar medium. Although it is now obvious that grain surface chemistry plays an active role in the formation of molecules, it is also apparent that gas-phase processes remain a major key to our understanding of interstellar chemistry. In both fields there is still a lack of relevant experimental data, even if the last ten years have brought remarkable breakthroughs concerning reaction rate coefficients for the extreme temperature conditions of interstellar clouds. Pioneering works have also been performed in order to determine the nature of the radicals produced in the dissociative recombination (DR) of molecular ions. At the same time the DR rate coefficients (α) have been the subject of much controversy, especially for H⁺₃ ions which play a central role in interstellar chemistry.

The most recent laboratory results obtained at our laboratory concerning DR processes and chemistry at very low temperatures are reviewed. The apparatus used for DR studies is described and α measurements for several ions, including H⁺₃, HCO⁺, HCS⁺ as well as some polycyclic species are presented and compared to previous experimental results and theoretical predictions. The problem of vibrational excitation which has been overlooked in the past is discussed. The new CRESU facilities for chemistry at very low temperatures built at Rennes are described together with the results obtained on the CN + O₂ reaction down to 26 K.