Radiative Transfer, Excitation, and Cooling of Molecular Emission Lines (CO and CS)

Abstract

We consider the radiative transfer of molecular lines in interstellar clouds having flow velocities large compared with random motions. The equilibrium level populations of CO and CS are calculated including the effects of both self-radiation (radiative trapping) and collisions with hydrogen molecules (using recently measured cross-sections). Analytic expressions are also developed for the excitation of a two-level molecule. Because of the velocity gradients in the cloud, the observed emission will originate not only from the near boundary but also from interior regions where excitation is greatly enhanced by scattered photons. This ability to see into the clouds qualitatively accounts for the general similarities of the different line profiles in individual clouds and considerably reduces the density of collisional particles needed to account for the observed excitation. Remarkably, even if A > C but τ > 1, the excitation temperature (and the observed intensity) depends on the molecular density but is totally independent of the spontaneous rate A. The rate of gas cooling by CO molecules in clouds of moderate density (n_(H_2)) ~ 10^3 cm^(-3)) is high (~10^(-22) ergs s^(-1)) even when the important cooling transitions (J = 3 → 2 and higher) are optically thick. These results are applicable to either cloud collapse or expansion.