Physical conditions in molecular clouds.

Recent developments have complicated the picture of the physical conditions in molecular clouds. The discoveries of widespread emission from high-J lines of CO and 12 μm IRAS emission have revealed the presence of considerably hotter gas and dust near the surfaces of molecular clouds. These components can complicate the interpretation of the bulk of the cloud gas. Commonly assumed relations between column density or mean density and cloud size are called into question by conflicting results and by consideration of selection effects. Analysis of density and density structure through molecular excitation has shown that very high densities (n ≥ 10⁶cm^-3) exist in star formation regions, but unresolved structure and possible chemical effects complicate the interpretation. High resolution far-infrared and submillimeter observations offer a complementary approach and are beginning to test theoretical predictions of density gradients in clouds.