Models of massive molecular clouds

Observations of carbon monoxide lines from massive molecular clouds, such as those near Orion A and Sgr B2, have recently been interpreted in terms of large-scale motions, generally collapse or expansion, that imply rapid evolution of these clouds. An alternative explanation is suggested - namely, the clouds are not dominated by large-scale systematic motions and the CO profiles are determined primarily by local motions. Therefore, it is generally not possible to see through these clouds in optically thick lines of molecules such as CO and HCN. Some consequences and tests of this model are briefly discussed.