Molecular cooling and thermal balance of dense interstellar clouds

The role of molecular cooling in establishing the temperature of the gas in dense interstellar clouds is examined. Attention is given to the cooling produced by a number of molecules, each of which is representative of different types in terms of abundances, level spacings, transition strengths, and cross sections. Molecular hydrogen densities in the range from 100 to 1 million per cu cm and kinetic temperatures between 10 and 60 K are considered, radiative transfer is calculated on the basis of the velocity-gradient model, and best estimates of the fractional abundance of species important for cooling are used to construct total cooling curves at each kinetic temperature. The contributions of several possible heating sources are evaluated, including heating by cosmic rays, H2 formation on grains, gravitational contraction, collisions with warm dust grains, and magnetic ion-neutral slip. Rates for each process are derived and equated with the cooling rate to obtain equilibrium temperature-density relations in a number of cases. The results are applied in a general discussion about the thermal properties of dense interstellar clouds without internal stellar or protostellar heat sources.