Molecular cooling and thermal balance of dense interstellar clouds

We analyze in detail the cooling produced by line emission from a variety of molecular and atomic species, including those observed as well as theoretically expected in dense interstellar clouds. At molecular hydrogen densities less than 3 x 10/sup 4/ cm/sup -3/ and kinetic temperatures between 10 K and 40 K, /sup 12/CO is the dominant coolant. At n (H/sub 2/) =3 x 10/sup -3/, however, C c, O/sub 2/, and the rarer isotopic species of carbon monoxide together contribute half of the total cooling. As n (H/sub 2/) is increased beyond 3 x 10/sup 4/ cm/sup -3/, a large number of species including water, hydrides, molecular ions, and less abundant diatomic molecules collectively dominate the cooling. The cooling per H/sub 2/ molecule, ..lambda../n (H/sub 2/), is only very weakly density-dependent for n (H/sub 2/) greater than a few times 10/sup 2/ cm/sup -3/. At a density of 4 x 10/sup 3/ cm/sup -3/, ..lambda..=2.6 x 10/sup -26/T/sub kin//sup 2.2/ ergs cm/sup -3/ s/sup -1/. The rate of energy transfer by dust-gas collisions results in infrared emission by dust grains being a significant coolant for the gas only for n (H/sub 2/) >1.5 x 10/sup 4/ cm/sup -3/. We evaluatemore » the contribution of a number of gas heating machanisms which may be present in interstellar clouds including heating by cosmic rays, H/sub 2/ formation, gravitational collapse, and magnetic ion-neutral slip heating. For clouds with kinetic temperatures approx.10 K, cosmic ray heating alone may be sufficient to balance the gas cooling for 3 x 10/sup 2/< or =n (H/sub 2/) < or =10/sup 4/ cm/sup -3/, these conditions being in good agreement with the observationally determined characteristics of dark clouds.« less