Low velocity shocks: signatures of turbulent dissipation in diffuse
  irradiated gas
  We examine the chemical and emission properties of mildly irradiated (G0=1)
magnetised shocks in diffuse media (nH=10^2 to 10^4 /cm3) at low to moderate
velocities (from 3 to 40 km/s). Results: The formation of some molecules relies
on endoergic reactions. In J-shocks, their abundances are enhanced by several
orders of magnitude for shock velocities as low as 7 km/s. Otherwise most
chemical properties of J-type shocks vary over less than an order of magnitude
between velocities from about 7 to about 30 km/s, where H2 dissociation sets
in. C-type shocks display a more gradual molecular enhancement as the shock
velocity increases. We quantify the energy flux budget (fluxes of kinetic,
radiated and magnetic energies) with emphasis on the main cooling lines of the
cold interstellar medium. Their sensitivity to shock velocity is such that it
allows observations to constrain statistical distributions of shock velocities.
We fit various probability distribution functions (PDFs) of shock velocities to
spectroscopic observations of the galaxy-wide shock in Stephan's Quintet (SQ)
and of a Galactic line of sight sampling diffuse molecular gas in Chamaeleon.
In both cases, low velocities bear the greatest statistical weight and the PDF
is consistent with a bimodal distribution. In the very low velocity shocks
(below 5 km/s), dissipation is due to ion-neutral friction which powers H2 low
energy transitions and atomic lines. In moderate velocity shocks (20 km/s and
above), the dissipation is due to viscous heating and accounts for most of the
molecular emission. In our interpretation a significant fraction of the gas on
the line of sight is shocked (from 4% to 66%). For example, C+ emission may
trace shocks in UV irradiated gas where C+ is the dominant carbon species.
