Infrared pumping and rotational excitation of molecules in interstellar clouds

Abstract

We investigate the effect of infrared radiation, at the wavelength of the lowest vibrational transition, on the rotational excitation of diatomic molecules. We use a simplified model in which the infrared source is characterized by a temperature, emissivity, and geometrical filling factor, as seen by the molecules, to derive upper limits to the size of the region affected by an infrared source embedded in an interstellar cloud. Numerical calculations for the CS molecule, which appears to be a relatively good candidate for infrared pumping, indicate that, over appreciable regions near the hot sources M17a/b and W3-IRS 2/2a, the rotational population will be appreciably perturbed and hydrogen densities derived neglecting this effect will be significantly overestimated.