Introduction

The ISM contains a number of phases characterized by different temperatures, densities, and ionization fractions (cf. Section 1.2 and Table 1.1). The origin and interrelationship of these phases in the ISM, and their energy and ionization sources, are among the most fundamental subjects of investigation in the field. Quite generally, a new stable phase reflects the onset of a new cooling mechanism or the decline of a heating source. Hence, cold HI clouds and the warm intercloud medium result from the increased importance of [CII] cooling at higher densities and Lyα and [OI] 6300Å cooling at higher temperatures, respectively (cf. Section 2.6.1). The hot phase reflects the recent input of supernova energy (cf. Section 12.3). Cold molecular clouds result from the increased cooling due to rotational transitions in molecules (cf. Section 2.6.2). However, the latter fall somewhat outside this classification scheme since their existence also reflects the importance of self-gravity. Understanding the structure of the ISM thus requires an understanding of its sources of heating and cooling.

Heating and cooling processes have been discussed in Chapters 3 and 2, respectively. Here, we apply these to the neutral diffuse interstellar medium. We will first examine the ionization and energy balance of the cold and warm phases of the interstellar medium (Section 8.2). Then, we will focus on the physical principles that allow several phases to coexist in thermal and pressure equilibrium in the interstellar medium (Section 8.3).

When, upon my return to Holland, I started to teach an advanced course on the interstellar medium in 1998, I quickly realized that there was no suitable textbook available. There is, of course, the incomparable monograph by Spitzer, Physics of the Interstellar Medium (1978, New York: Wiley and Sons). But that book is quite challenging and not very suitable for a student course. Moreover, by now, it is very dated. Over the intervening years, our insights into the basic physics of the interstellar medium have much improved thanks, for example, to the opening up of the infrared and submillimeter windows. In particular, molecules, which we now know to be deeply interwoven into the fabric of the Universe, play only a little role in Spitzer's book. When Eddington made his famous remark, “Atoms are physics but molecules are chemistry,” he merely expressed, on the one hand, the dream of a physicist of a simple universe, which can be caught in a single equation, and, on the other hand, the dread of a reality where solutions are never clean and simple. The latter is of course obvious to a chemist and it is now abundantly clear that Eddington's fear has turned into reality, even for astronomy. Present-day graduate students will require an intimate knowledge of molecular astrophysics in order to be active in the field of the interstellar medium of our own or other galaxies whether it is in the here and now or all the way back in the early Universe.

Introduction

The lifecycle of interstellar dust and the processes that play a role are summarized in Fig. 13.1. Dust is formed at high densities and temperatures in the ejecta from stars. This leads to the formation of high temperature condensates such as silicates, graphite, and carbides. Stardust grains with an isotopic composition betraying their birthsites have been isolated from meteorites (Chapter 5). In the interstellar medium, dust cycles many times between the intercloud and cloud phases. In the warm neutral and ionized intercloud media, dust is processed by strong shocks driven by supernova explosions (Section 12.3). The hot gases in the shock can sputter atoms from the grains. Also, high velocity collisions among grains can lead to vaporization, melting, phase transformation, and shattering of the projectile and target. These processes have been discussed in Section 5.2. In the denser media – diffuse and dense clouds – gas-phase species can accrete onto grains forming a mantle. In diffuse clouds, the accreted species may be predominantly bound by chemisorbed forces – partly because physisorbed species will be rapidly photodesorbed by the high flux of FUV photons (Section 10.6.1). In molecular clouds, the accretion process leads to the formation of an icy mantle consisting of simple molecules such as H2O, CO, CO2, and CH3OH (see Section 10.5). These ices may be processed by UV photons and high-energy cosmic rays into larger, more complex species, which could be more tightly bound to the cores.

In a way, astrochemistry describes a cosmic dance of the elements in which atoms are constantly reshuffled from one species to another. This molecular rearrangement may be effected by gas phase binary collisions where atoms change partner or through recombination on grain surfaces. This “dance” is driven by the action of various energy sources, including photons and cosmic rays. In order to appreciate astrochemistry properly, we first have to get a basic understanding of the “dance” steps involved. This chapter will focus on the basic chemical processes that are of importance. In later chapters, we will then overview the resulting chemical reaction schemes that drive molecular complexity in the Universe.

There is a variety of processes that can lead to the formation of molecules in the interstellar medium, but these can be separated into two broad classes: reactions that occur in the gas phase and reactions that occur on the surfaces of small grains prevalent throughout the interstellar medium. These two classes of reactions are discussed in turn in the two sections of this chapter. The focus is on the formation of relatively simple species. The chemistry of large and complex interstellar molecules is discussed in Chapter 6.

Gas-phase chemical reactions

Gas phase reactions can be divided into different categories depending on their general effects. There are the bond-formation processes, including radiative association (cf. Section 4.2), which link atoms into simple or more complex species.

Introduction

Interstellar dust is an important component of the interstellar medium. Dust provides the dominant opacity source in the interstellar medium for non-ionizing photons and therefore controls the spectral energy distribution of the ISM at all wavelengths longer than 912 Å. Dust grains also lock up a substantial fraction of all heavy elements. Grains provide a surface on which species can accrete, meet, and react – giving rise to an interesting and complex chemistry. This chapter will look at the physical processes involving dust, including their interaction with light – in particular their energy balance and the resulting temperature – and their charge balance. Dust also regulates the gas phase abundances of the elements through accretion and destruction processes. This chapter discusses the physical processes involved in dust destruction. The chemical processes that control accretion and ice mantle formation are described in Section 4.2. The growth and characteristics of interstellar ice mantles are discussed in Sections 10.6 and 10.7.4. We will return to the lifecycle of interstellar dust and the depletion of the elements in Chapter 13. The composition of interstellar dust has been widely debated and silicates and graphite are generally considered the most important interstellar dust components. In this chapter, we have therefore focussed on these compounds. However, the discussion is very general and one might often substitute minerals for silicates and amorphous carbon for graphite in the discussion of the physical processes.

In this chapter, we will examine the various line cooling processes of interstellar gas. Together with the heating processes, which are discussed in the next chapter, this will allow us to solve the energy balance for the gas in a wide variety of environments. This will be applied in HII regions (Section 7.3), HI regions (Section 8.2.2), photodissociation regions (Section 9.3), and molecular clouds (Section 10.3). This chapter starts off with a refresher on the concepts of electronic, vibrational, and rotational spectroscopy (Section 2.1). We will then discuss the cooling rate (Section 2.2) with the emphasis on two-level systems (Section 2.3). A two-level system analysis is a powerful tool for understanding the details of gas emission processes and we will encounter this many times in subsequent chapters. We will then consider in detail the cooling processes in ionized (Section 2.4) and neutral (Section 2.5) gas. The last section (Section 2.6) discusses the cooling law.

Spectroscopy

Table 2.1 summarizes typical properties of transitions. These are, of course, directly related to the binding energies of the species involved. Electronic binding energies for atoms increase from left to right in the periodic system from about 5 eV to some 20 eV. For hydrogen and helium the lowest electronic transitions are fairly high (10.2 and 21.3 eV, respectively); a substantial fraction of the ionization energy. Multi-electron systems have electronic orbitals that are low in energy compared with their ionization potentials.

Introduction

Shock waves are common phenomena in the interstellar medium. Shocks occur whenever material moves at velocities exceeding the sound velocity in the surrounding medium and the upstream material cannot dynamically respond to the upcoming material until it arrives. The shock will then compress, heat, and accelerate the medium. The heated material cools through the emission of line photons, further compressing the medium.

In the interstellar medium, two types of shocks are of interest. First, for fast shocks, the gas is so suddenly stopped and heated to a high temperature that insignificant radiative and non-radiative relaxation can take place; e.g., the shock front is much thinner than the postshock relaxation layer. As a result, we can cast the mass, momentum, and energy conservation equations in terms of simple “jump” conditions that relate the preshock (front) and postshock (front) density, temperature, and velocity to each other. These shocks are called J-shocks (J for jump). Second, for weak shocks in a magnetized medium with a low degree of ionization, the shock work is done by trace ions drifting through the predominantly neutral medium. In such shocks, the shock front is much thicker than the cooling length scale and the temperature is set by the balance between heating and cooling. In this case, we cannot use jump conditions but, rather, have to solve the conservation equations for the shock front structure. These shocks are called C-shocks (C for continuous).

Introduction

While earlier suggestions had appeared over the years, the importance of large molecules in space was first realized on the basis of the observed strong mid-infrared emission in the ISM. The Infrared Astronomical Satellite (IRAS) discovered widespread emission at 12 μm in the diffuse ISM – the so-called IR cirrus – where the expected temperature of dust in radiative equilibrium with the stellar radiation field is expected to be too cool to emit at such short wavelengths (cf. Section 5.2.3). This problem had actually already been recognized in connection with the observed mid-IR emission from PDRs far from the illuminating stars, which is also much brighter than expected for radiatively heated dust grains (Section 9.4). It was then quickly realized that very small dust grains with 20–100 C atoms – actually, large molecules – can be transiently heated to high temperatures, because of their limited heat capacity. Such hot species will cool through emission in their mid-IR vibrational modes. The observed interstellar IR spectrum is very characteristic of aromatic species and hence the carriers are really large polycyclic aromatic hydrocarbon molecules (PAHs).

In this chapter, we will discuss the physics and chemistry of such large molecules. The emphasis will be on their interaction with radiation. However, the presence of large molecules in space will also have profound influence on other aspects of the ISM and these will be examined as well.

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.

Nomenclature in astronomy is often very confusing if not intentionally obfuscating. Every subfield arbitrarily introduces its own symbols and a book encompassing many different subfields has to dance a fine line between consistency with common usage in the literature and avoiding internal confusion. I have tried to avoid using the same symbols for more than one quantity as much as possible but did not quite succeed. This glossary provides a list of symbols that are used throughout the book, often with a reference to the equation where the symbol is first introduced or best defined. In those cases where a symbol is used in a different way than listed here – and this is not obvious from the context – I have tried to redefine the symbol consistently in the text.

• Aij Einstein A coefficient (Eq. 2.17)

• Aj The abundance of species j (Eq. 2.37)

• Av The dust extinction at visual wavelength, in magnitudes

• Aλ The dust extinction at wavelength λ, in magnitudes (Eqs. (5.2) and (5.84))

• a The grain size

• B(ν, T) The Planck function at frequency ν and temperature T

• Bij Einstein B coefficient (Eq. (2.18))

• Be (also B, A, C) Rotational constant (Eqs. (2.4), (2.8), (2.9), (2.11))

• B0 The preshock magnetic field strength

• B1 The postshock magnetic field strength

• b The Doppler line-width parameter

• CV The heat capacity of the grain material

• Cs The speed of sound (Eq. (11.8))

• CI The speed of sound in neutral gas

• CII The speed of sound in ionized gas

• c The speed of light […]