Fluctuations of the interstellar medium induced by the stellar system - Mechanical heating and dynamical friction

Equations for the large-scale energy balance of the gravitationally coupled system of stars and interstellar gas are derived, including an expression representing the mechanical heating of the ISM by dissipation of turbulent kinetic energy. The mechanical heating rate of the interstellar gas and the dynamical friction between the gas and the stellar system are evaluated. Analytical expressions are derived in a quasi-linear approximation for the mechanical heating rate and the dynamical friction. In dense molecular clouds, with a rich population of young stars or prototstars embedded within the cloud, the resulting mechanical heating rate is of the order of the cosmic ray heating rate. The time scale of dynamical friction is of the order of the free-fall time of the clouds, indicating that the cloud collapse may be considerably delayed.