THE TEMPERATURE OF INTERSTELLAR CLOUDS FROM TURBULENT HEATING

Abstract

To evaluate the effect of turbulent heating in the thermal balance of interstellar clouds, we develop an extension of the log-Poisson intermittency model to supersonic turbulence. The model depends on a parameter, d, interpreted as the dimension of the most dissipative structures. By comparing the model with the probability distribution of the turbulent dissipation rate in a simulation of supersonic and super-Alfvénic turbulence, we find a best-fit value of d = 1.64. We apply this intermittency model to the computation of the mass-weighted probability distribution of the gas temperature of molecular clouds, high-mass star-forming cores, and cold diffuse H i clouds. Our main results are: (1) the mean gas temperature in molecular clouds can be explained as the effect of turbulent heating alone, while cosmic-ray heating may dominate only in regions where the turbulent heating is low; (2) the mean gas temperature in high-mass star-forming cores with typical full width at half-maximum of ∼ 6 km s−1 (corresponding to a one-dimensional rms velocity of 2.5 km s−1) may be completely controlled by turbulent heating, which predicts a mean value of approximately 36 K, two to three times larger than the mean gas temperature in the absence of turbulent heating; and (3) the intermittency of the turbulent heating can generate enough hot regions in cold diffuse H i clouds to explain the observed CH+ abundance, if the rms velocity on a scale of 1 pc is at least 3 km s−1, in agreement with previous results based on incompressible turbulence. Because of its importance in the thermal balance of molecular clouds and high-mass star-forming cores, the process of turbulent heating may be central in setting the characteristic stellar mass and in regulating molecular chemical reactions.