Upon analyzing the CO 7-6 transition maps obtained toward the luminous star-formation regions W51 and DR 21 in the context of existing information, it emerges that three distinct molecular cloud components contribute to this emission. The first of these is a 100-250 K, dynamically active material accounting for 75 percent of the velocity-integrated CO 7-6 emission from the two sources. The remaining two are a 100-500 K, quiescent material associated with the H II regions in each scource, and a cool, 20-40 K quiescent gas overlying some of the hot material; this cool gas is probably part of the more extended molecular envelope around the luminous cloud core.
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Jaffe et al. (1989) studied this question.