Introduction

Molecular clouds differ in a number of important aspects from the atomic clouds discussed in Chapter 8. They tend to be denser and have a higher column density. As a result, the intensity of the dissociating FUV radiation field is lower and gas-phase chemistry in these clouds is primarily driven by cosmic-ray ionization. Accretion on grains is another process that becomes more important at these higher densities and ice mantles – absent in the diffuse ISM – become prevalent inside molecular clouds. In fact, the interaction between the gas phase and the solid state becomes one of the driving forces of molecular diversity in molecular clouds. Molecular clouds are also much cooler than diffuse clouds (≃10K versus ≃100 K) due to the enhanced cooling associated with the small energy spacing of molecular rotational levels and the higher densities of molecular clouds. A final difference with diffuse clouds is the importance of self-gravity in molecular clouds. While gravitational collapse is outside the scope of this book, new stars are only formed in molecular clouds and such embedded protostars can influence their environment through shocks driven by their powerful outflows. Protostars will also heat the surrounding dust and this energy is coupled to the gas through gas–grain collisions. This heating of the dust can evaporate previously accreted ice mantles when the temperature is raised above the sublimation temperature.

In this chapter, we will discuss the physical and chemical processes that dominate the characteristics of molecular clouds.