Introduction

The interior of a dense molecular cloud is efficiently shielded from the ultraviolet photons of the interstellar radiation field by the grains. Accordingly, the effects of the ultraviolet photons on the physical and chemical states of dense clouds are in general, neglected. However, several internal sources of ultraviolet photons may be present, including the emission from embedded stars and internal shocks driven by mass loss from young stars. Here the discussion is focused on a diffuse source of internal ultraviolet photons arising from energetic cosmic rays capable of penetrating dense clouds. Cosmic ray particles with energies between 10 and 100 MeV ionize molecular hydrogen in the interior of the clouds and generate secondary electrons with a mean energy of around 30 eV (Cravens and Dalgarno 1978). Because the fractional ionization is generally low, the secondary electrons degrade mainly through excitations of various electronic states of H2. Typical excitation energies are 10–15 eV. Ionization of H2 by the secondary electrons may also occur. The subsequent decays of the electronically excited states of H2 produce ultraviolet photons within the clouds.

The idea of molecular hydrogen emission inside dense clouds was invoked by Prasad and Tarafdar (1983) to explain the large abundance of atomic carbon which exists in several molecular clouds. Observations of emission in the 3P1–3P0 and 3P2–3P1 fine-structure lines of neutral atomic carbon in dense interstellar clouds (Phillips and Huggins 1981, Keene et al. 1985, Zmuidzinas, Betz and Goldhaber 1986) have demonstrated the existence of C with abundances relative to CO exceeding 0.1.

Introduction

It's a great pleasue for me to contribute to this volume in honor of Alex Dalgarno's 60th birthday. I first met Alex when I came to Harvard as an assistant professor in 1968. From these turbulent times up to now, Alex has been a very good friend and mentor to me. From Alex I gained an interest in atomic and molecular processes that has influenced my research ever since. My writing skills improved considerably by virtue of working with him. Most important, I am grateful to Alex for setting an example of what a good professor should be, not only as a scholar and teacher, but also as a generous and loyal friend to his students and colleagues.

With Supernova 1987A (February 23, 1987), nature has provided some birthday fireworks that will be a festive reminder of Alex's many important contributions to astrophysics. The brightest supernova since SN1604 (Kepler's supernova), SN1987A is the first one that has been observed in every electromagnetic wavelength band and it is the first one that will remain observable for several years as the debris clears away to allow a detailed view of its interior. Thus, SN1987A offers an unprecedented opportunity to infer details of supernova explosion dynamics and nucleosynthesis. This task presents fascinating and challenging problems in atomic and molecular astrophysics because, as I will describe, SN1987A is remarkably cool (≥7000 K) throughout its interior and there is good evidence that CO and SiO molecules have already formed there.

Introduction

The subject of this article is dust in dense interstellar clouds – its composition and its chemical evolution. The relevant astronomical data include extinction and polarization in the infrared, visible and ultraviolet spectral ranges. Identification of the carriers of the observed spectral features is a non-trivial task. The features appear to be notoriously non-unique; significant fractions of them have been assigned to two or more dissimilar materials. Also, it now appears that grains are made largely of highly disordered and/or composite materials. In disordered mixtures spectral features of molecules can be altered considerably with respect to known spectra of pure crystalline materials, which also complicates the identification.

Clearly, additional sources of information are needed. The sources which I chose to employ are the recently available data on the composition of Halley's comet, and on the structure, the composition and the spectral properties of the interplanetary dust particles (IDPs). I thus assume that cometary and IDP materials preserve many of the characteristics of the dust in the original interstellar cloud. As a rationale I quote a recent review of Geiss (1987) on the results of exploration of Halley's comet: (a)‘… The abundance data show that a large fraction of material in Halley's nucleus condensed at very low temperature’ (b)‘… comets are regular members of the solar system which have preserved the original charactistics of the condensed and accreted matter better than other bodies in this system.’

Introduction

A detailed knowledge of collisional excitation processes is important in various aspects of the study of interstellar clouds. Because local thermodynamic equilibrium rarely obtains in such environments, the diagnosis of physical conditions, such as temperature, particle density, and radiation density, requires a quantitative understanding of all microscopic processes (i.e. collisional excitation and deexcitation and radiative decay and absorption) which influence the excitation conditions. Quite often, only rotational excitation of simple molecules need be considered. For instance, dense, cold cloud gas is studied primarily by observing millimeter and submillimeter emission features arising from transitions between different rotational levels of molecules in their ground electronic and vibrational states. In diffuse clouds, simple diatomics such as H2, CN and C2 are observed through electronic absorption transitions involving different rotational levels of the ground vibrational and electronic state. Even some atomic species such as C and C+ are observed in various fine structure states. This information is important for the diagnosis of diffuse clouds. Observable emission from vibrationally excited molecules arises in hotter gas or in regions exposed to a strong ultraviolet radiation field, but the collisional excitation of vibrational states is not well understood for the appropriate temperature range. However, the observed emissions due to the decay of collisionally excited fine structure levels of atomic species, such as C and C+ can be used to investigate clouds.

Second, collisional excitation of atomic and molecular species is always followed by spontaneous radiative emission leading to a loss of energy from the medium which is an important cooling process of the interstellar gas.

The present volume has been designed to be a self-contained introduction to the field of molecular astrophysics. It can serve as the text for a one semester postgraduate course concerning that subject exclusively or as a supplementary text in a postgraduate course on the interstellar medium. It can also be used by research astronomers, atomic and molecular physicists, chemists, and atmospheric scientists who have interests in weakly ionized plasmas and the physical and chemical processes which occur in them and who wish to become familiar with recent work in molecular astrophysics.

Many of the articles concern theoretical studies and modelling. Part I, consisting of two chapters written by observers, provides a general description of the astronomical context within which much of the remainder of the volume should be considered. The contribution by Per Friberg and Åke Hjalmarson reviews briefly our understanding of the global physical properties of the Galactic interstellar medium, describes the wide range of conditions within the dark molecular clouds (those having visual extinctions greater than about 1) and the distribution in the Galaxy of the dark clouds, and compares the chemical contents of different regions in various dark clouds. Leo Blitz has written about observations of molecules in other galaxies including some with active nuclei.

The study of chemistry in diffuse molecular clouds (those having a visual extinction of about 1 or less) constitutes the subject of Part II and is perhaps the most fundamental area in molecular astrophysics.

Introduction

Our understanding of the structure of the interstellar medium has progressed enormously in the last 15 years largely as a result of the development of radio emission and ultraviolet absorption observations. From CO radio surveys we now know that about one-half of the interstellar gas is in the form of giant molecular clouds that are the formation sites of O and B stars, while measurements of highly excited gas, such as hot bubbles and expanding shells, indicate that a significant volume of the interstellar medium contains dilute, hot gas. The physical properties and dynamics of the interstellar gas are strongly influenced by the interaction of the massive O and B stars and supernovae. Supernova shells sweep up interstellar matter creating clouds and also impact them causing compression and fragmentation. The O and B stars produce large HII regions which expand and compress the interstellar gas. Thus on a theoretical basis we expect to find complex velocity fields, shock structures, and density inhomogeneities in the diffuse and translucent component of the interstellar clouds.

Today it is hard to imagine studying the diffuse interstellar clouds without relying on observations of molecular lines. In an excellent review article by Dalgarno and McCray (1972) on the heating, ionization and cooling processes of HI regions, these authors stated that ‘The processes leading to interstellar molecule formation are only partially understood …’. The central role of molecules as both important components and physical probes was just becoming evident.

Introduction

Astrophysical environments offer chemical modelers the unusual challenge of predicting reaction rate coefficients for temperature ranges well outside those commonly encountered in the laboratory (250–2000 K). The direct extrapolation of thermal laboratory data involves implicit, often incorrect, assumptions about the dynamics of chemical reactions at extreme temperatures. Furthermore, the internal energy distributions of constituent atoms and molecules in astrophysics are frequently nonthermal, and direct application of thermal reaction rate data may not be appropriate. An understanding of the physics of chemical reactions is essential for accurate modeling (Dalgarno 1985). Elucidation of the role of various forms of energy in chemical reactions – translational, vibrational, rotational, and fine structure – now affords improved predictions of rate coefficients for astrophysical situations.

In the low-temperature environment of cold interstellar clouds, the chemical kinetics are dominated by reactions whose potential energy surfaces are without barriers. Such reactions are characterized by large exothermicities and long-range electrostatic interactions that suppress chemical barriers that may occur as collision systems approach short distances. Current models of cold interstellar cloud chemistry illustrate the importance of exothermic ion–molecule reactions (cf. Dalgarno and Black (1976), Black and Dalgarno (1977), van Dishoeck and Black (1986)). Many neutral systems are also likely to react rapidly at very low temperatures and may be important in the chemical kinetics of interstellar clouds (Graff 1989). The following section discusses dynamical characteristics of neutral reactions that are likely to be fast at low temperatures.

The cloudy structure of our Galaxy

The interstellar medium (ISM) in our Galaxy is in a complex state. The temperature and the density vary by about five and ten orders of magnitude respectively. The medium is exposed to cosmic rays and starlight and contains magnetic fields. Its average density is about 10−24 g cm−3, or about one hydrogen atom per cubic centimeter, corresponding to 0.025 M⊙ pc−3. Thus on average one solar mass of the medium is contained in 40 pc3. About 75% of the medium is hydrogen, about 25% is helium, and the remaining part (about 2%) is in heavier atoms. A large fraction of the heavier elements has condensed out as dust grains having an average density of about 0.001 M⊙ pc−3.

The local kinetic gas temperatures range from 10 K to 106 K. These temperatures correspond to energies which are much lower than those typical of cosmic rays. The lower temperatures correspond to energies which are less than those typical for starlight. Cosmic rays and starlight drive processes including ionization; hence, the ionization structure sometimes is very far from that in a gas in thermal equilibrium at the gas kinetic temperature. The energy density of cosmic rays is ≈0.5 eV cm−3, of Galactic magnetic fields ≈0.2 eV cm−3 (≈10−6G), and of diffuse starlight ≈0.5 eV cm−3. Hence there is a rough equipartition of energy among these components. The description of the interaction of matter with radiation is complicated by the huge variation of opacities for different wavelengths.

Introduction

When low and intermediate mass stars evolve off the main sequence, they become red giants. During the first ascent up the red giant branch, the stars lose mass at a relatively modest rate (∼10−9 M⊙ yr−1. Dupree (1986)). However, during the second ascent up the red giant branch, the asymptotic giant branch (AGB), stars lose mass at a much greater rate; up to 10−4 M⊙ yr−1 (Zuckerman 1980, Jura 1986a, Olofsson 1985). These outflows from AGB stars are very cold (T < 1000 K), and they contain large amounts of dust and molecules. The chemistry in these outflows is the topic of this chapter (see also Olofsson (1987), Omont (1987), Millar (1988)).

Stars derive most of their luminosity from nuclear reactions that occur in their interiors. In the red giant phase, material from the interior is mixed with that at the surface and the composition of the photosphere of a red giant can be very different from the star's initial composition. In most cases, it appears that in the atmospheres of the red giants, the major elements are still hydrogen and helium, even though material is mixed from the interiors to the surface. However, the next most common elements, carbon, nitrogen and oxygen, may have very nonstandard compositions, and this can greatly affect the chemistry. In particular, the ratio [C]/[O] is critical. In the usual thermodynamic description for material in cool stellar atmospheres, we expect CO, because of its very high binding energy, to contain as much carbon and oxygen as possible.