Cooling Rates of Molecular Clouds Based on Numerical Magnetohydrodynamic Turbulence and Non-LTE Radiative Transfer

We have computed line-emission cooling rates for the main cooling species in models of interstellar molecular clouds. The models are based on numerical simulations of supersonic magnetohydrodynamic (MHD) turbulence. Non-LTE radiative transfer calculations have been performed to properly account for the complex density and velocity structures in the MHD simulations. Three models are used. Two of the models are based on MHD simulations with different magnetic field strength (one model is super-Alfvénic, while the other has equipartition of magnetic and kinetic energy). The third model includes the computation of self-gravity (in the super-Alfvénic regime of turbulence). The density and velocity fields in the simulations are determined self-consistently by the dynamics of supersonic turbulence. The models are intended to represent molecular clouds with linear size L~6 pc and mean density <n>~300 cm^-3, with the density exceeding 10⁴ cm^-3 in the densest cores. We present ¹²CO, ¹³CO, C¹⁸O, O₂, O I, C I, and H₂O cooling rates in isothermal clouds with kinetic temperatures 10-80 K. Analytical approximations are derived for the cooling rates. The inhomogeneity of the models reduces photon trapping and enhances the cooling in the densest parts of the clouds. Compared with earlier models, the cooling rates are less affected by optical depth effects. The main effects come, however, from the density variation, since cooling efficiency increases with density. This is very important for the cooling of the clouds as a whole, since most cooling is provided by gas with density above the average.