Molecular Tracers of Turbulent Shocks in Giant Molecular Clouds

TL;DR

This paper models C-type shocks in giant molecular clouds to identify key cooling processes, showing shock emission dominates CO transitions and contributes significantly to energy dissipation in turbulent molecular gas.

Contribution

It introduces detailed shock models for molecular clouds and quantifies their role in energy dissipation and cooling, highlighting the importance of CO emission and magnetic field compression.

Findings

Shock dissipation mainly through CO rotational transitions.

Shock emission surpasses unshocked gas emission at high CO J levels.

Turbulent energy dissipation rate is comparable to cosmic-ray heating.

Abstract

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.