Book contents
- Frontmatter
- Contents
- Preface
- List of constants
- List of conversion factors
- 1 The galactic ecosystem
- 2 Gas cooling
- 3 Gas heating
- 4 Chemical processes
- 5 Interstellar dust
- 6 Interstellar polycyclic aromatic hydrocarbon molecules
- 7 HII regions
- 8 The phases of the ISM
- 9 Photodissociation regions
- 10 Molecular clouds
- 11 Interstellar shocks
- 12 Dynamics of the interstellar medium
- 13 The lifecycle of interstellar dust
- 14 List of symbols
- Index of compounds
- Alphabetic list of molecular species
- Index of molecules
- Index of objects
- Index
10 - Molecular clouds
Published online by Cambridge University Press: 05 June 2012
- Frontmatter
- Contents
- Preface
- List of constants
- List of conversion factors
- 1 The galactic ecosystem
- 2 Gas cooling
- 3 Gas heating
- 4 Chemical processes
- 5 Interstellar dust
- 6 Interstellar polycyclic aromatic hydrocarbon molecules
- 7 HII regions
- 8 The phases of the ISM
- 9 Photodissociation regions
- 10 Molecular clouds
- 11 Interstellar shocks
- 12 Dynamics of the interstellar medium
- 13 The lifecycle of interstellar dust
- 14 List of symbols
- Index of compounds
- Alphabetic list of molecular species
- Index of molecules
- Index of objects
- Index
Summary
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.
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- The Physics and Chemistry of the Interstellar Medium , pp. 348 - 395Publisher: Cambridge University PressPrint publication year: 2005