Figure 8.1 depicts, in a schematic fashion, the generic molecular cloud that will be our object of study. With reference to Table 3.1, the entity shown could represent an individual dark cloud or a massive clump within a giant complex. The outermost region is the atomic envelope, in which ultraviolet photons from the interstellar radiation field or nearby massive stars promptly dissociate any molecular hydrogen that forms. As we discussed in Chapter 3, 21 cm observations of HI have established such a tenuous component around at least some giant complexes (recall Figure 3.3), but have yet to map the analogous structure on smaller scales. Some clumps that are especially embedded may lack this outer layer. In any case, most of the cloud mass is taken up by the molecular interior. This is the region principally observed through various CO isotopes. Inside it we find dense cores, a few of which are sketched here. Some of these cores, in turn, contain young stars. Our present goal is not to explain this hierarchical structuring, but to explore the interior run of temperature and chemical composition as one passes through regions of successively higher density.
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