Molecular Tracers of Turbulent Shocks in Giant Molecular Clouds
  Giant molecular clouds contain supersonic turbulence and simulations of
magnetohydrodynamic turbulence show that these supersonic motions decay in
roughly a crossing time, which is less than the estimated lifetimes of
molecular clouds. Such a situation requires a significant release of energy. We
run models of C-type shocks propagating into gas with densities around 10^3
cm^(-3) at velocities of a few km / s, appropriate for the ambient conditions
inside of a molecular cloud, to determine which species and transitions
dominate the cooling and radiative energy release associated with shock cooling
of turbulent molecular clouds. We find that these shocks dissipate their energy
primarily through CO rotational transitions and by compressing pre-existing
magnetic fields. We present model spectra for these shocks and by combining
these models with estimates for the rate of turbulent energy dissipation, we
show that shock emission should dominate over emission from unshocked gas for
mid to high rotational transitions (J >5) of CO. We also find that the
turbulent energy dissipation rate is roughly equivalent to the cosmic-ray
heating rate and that the ambipolar diffusion heating rate may be significant,
especially in shocked gas.
