Heating and Cooling

A proper understanding of molecular clouds must include an account of how they absorb energy and reradiate it into space. Our purpose in this chapter is to elucidate the most important mechanisms. We first cover the heating from interstellar cosmic rays and photons. Included in the latter are both the diffuse background of radiation and the X-rays from pre-main-sequence stars. We next describe how clouds cool themselves through emission from their constituent atoms, molecules, and dust grains.

In each case, our main goal is to provide a physical account of the relevant process, but we also give the reader practical formulas for heating and cooling rates.1 We limit ourselves to the case of quiescent clouds, i. e., we ignore any heating from nearby massive stars or internal, bulk motion of the gas. Both of these topics will be covered in later chapters. For convenient reference, Table 7.1 lists all the thermal processes discussed here.

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