Most of the diffuse interstellar medium is cold, but it must harbor pockets of hot gas to explain the large observed abundances of molecules like CH+, OH and HCO+. Because they dissipate locally large amounts of kinetic energy, MHD shocks and coherent vortices in turbulence can drive endothermic chemical reactions or reactions with large activation barriers. We predict the spectroscopic signatures in the H2 rotational lines of MHD shocks and vortices and compare them to those observed with the ISO-SWS along a line of sight through the Galaxy which samples 20 magnitudes of mostly diffuse gas.

The trigger of hot chemistry in the cold diffuse medium

The large observed abundances of CH, CH+, HCO+, and OH in the (mainly cold) diffuse medium (T ≈ 50 K) imply that activation barriers and endothermicities of several 103 K are overcome. Pockets of hot gas must therefore exist. Large ion-neutral drift speeds, of several km s−1, can equally contribute to triggering certain ion-neutral reactions in cold gas, such as the endothermic reaction C+ + H2 which forms CH+ (ΔE/k=4640 K). Two phenomena, operating at very different scales, are able to reproduce the observed abundances. These are MHD shocks (Flower & Pineau des Forêts, 1998 and references therein) and intense vortices, thought to be responsible for a large fraction of the viscous dissipation of supersonic turbulence (Joulain et al. 1998).