Verstraete L. 1, Falgarone E. 2, Pineau des Forêts G. 3, Flower D. 4, & Puget J.-L. 1
1 IAS, Orsay, France
2 ENS and Observatoire de Paris, Paris, France
3 Observatoire de Paris, Meudon, France
4 The University, Durham, UK
The diffuse interstellar medium of our Galaxy appears much more active
chemically than anticipated on the basis of its low density
(
), temperature (100 K) and UV shielding. Molecules such as
CH+, OH and HCO+ are observed in this medium with abundances far in
excess to those predicted by equilibrium chemistry in diffuse gas. The
formation of these molecules requires that activation barriers of
several 1000 K be overcome and endothermic reactions activated. The
cold diffuse medium must therefore harbor pockets of hot gas where
this chemistry can be activated. Magneto hydrodynamic shocks are
possible candidates able to trigger the hot chemistry and typically a
few shocks at 10
per magnitude of gas are required to reproduce
the observed amounts of molecules.
An alternative scenario involves bursts of viscous dissipation of the turbulent energy of the gas, weakly coupled to the magnetic field lines. These bursts occur over sizescales close to the dissipation length of turbulence (a few 10 AU) and their lifetime is so small that all the chemical and thermal processes are out-of-equilibrium. The main differences between the two scenarios are the lack of gas compression in the dissipative structures, which are vorticity extrema, and their much shorter timescales and sizescales than those of MHD shocks. These differences bear signatures in the excitation of the H2 rotational ladder.
We have performed ISO-SWS
observations of the S(0) through S(4) pure rotational lines of 
along a line of sight across a long pathlength of diffuse gas in
the Galaxy, chosen to minimize the contribution of molecular
clouds and star forming regions.
We present the results of these observations and confront them to the
model predictions of MHD shocks and turbulent dissipative structures.