Signatures of fast and slow magnetohydrodynamic shocks in turbulent
  molecular clouds
  The character of star formation is intimately related to the supersonic
magnetohydrodynamic (MHD) turbulent dynamics of the molecular clouds in which
stars form. A significant amount of the turbulent energy dissipates in low
velocity shocks. Fast and slow MHD shocks differ in how they compress and heat
the molecular gas, and so their radiative signatures reveal distinct physical
conditions.
  We use a two-fluid model to compare one-dimensional fast and slow MHD shocks
propagating at low speeds (a few km/s). Fast shocks are magnetically driven,
forcing ion species to stream through the neutral gas ahead of the shock front.
This magnetic precursor heats the gas sufficiently to create a large, warm
transition zone where all the fluid variables smoothly change in the shock
front. In contrast, slow shocks are driven by gas pressure, and neutral species
collide with ion species in a thin hot slab that closely resembles an ordinary
gas dynamic shock.
  We consider shocks at velocities $v_s = 2$-$4$ km/s and preshock Hydrogen
nuclei densities $n_\mathrm{H} = 10^2$-$10^4$ cm$^{-3}$. We include a simple
oxygen chemistry and cooling by CO, H$_2$ and H$_2$O. CO rotational lines above
$J = 6\to 5$ are more strongly excited in slow shocks. These slow shock
signatures may have already been observed in infrared dark clouds in the Milky
Way.
