Infrared pumping and rotational excitation of molecules in interstellar clouds

The effects of the rotational excitation of diatomic molecules in interstellar clouds by the absorption of infrared radiation are evaluated. A criterion for infrared pumping to alter significantly populations of the rotational states of ground and first excited vibrational levels is derived, with the effects of collisions neglected, and used to calculate the minimum required filling factor or source temperature for pumping various diatomic molecules. A model infrared source region consisting of a region of warm dust heated by embedded stars or H II regions is then developed which is characterized by an effective temperature, opacity and solid-angle dilution factor as seen by the molecular cloud, and used to derive upper limits to the size of the region affected by the infrared source. Numerical calculations for the case of the CS molecule are presented which indicate that the hot components of sources such as W3-IRS 2/2a and M17a/b emit sufficient infrared flux to perturb significantly the populations of the rotational levels. Such perturbations could lead to erroneous estimates of molecular densities if it is assumed that collisional excitation is the only operative pumping mechanism.