Molecular Clouds

It is proposed that molecular clouds are in a state of gravitational collapse. The coupled equations of statistical equilibrium and radiative transfer from diatomic molecules in a collapsing cloud are solved for arbitrary optical depths in the rotational lines. It is shown that most of the observed CS and SiO lines and the stronger CO lines are optically thick. In this limit the emitted intensities are independent of the molecular dipole moments. The rate at which energy is radiated in the CO lines is found to exceed the rate at which work is done by the adiabatic compression of the collapsing gas. This result implies the existence of an energy source which maintains the temperature of the gas against the cooling due to radiative energy losses. It is suggested that collisions between gas molecules and warm dust grains transfer energy to the gas. The dust grains are heated by radiation from H ii regions and protostars in the center of the molecular cloud. This picture is supported by the detection of copious far infrared fluxes from many molecular clouds. The rate of energy transfer from the dust to the gas is calculated to be sufficient to maintain the gas at temperatures deduced from observations of CO lines if NH2 > 10 . Subject headings: molecules, interstellar - nebulae