Published:08 Feb 2023
2023. "Preface", Astrochemistry, David A Williams, Cesare Cecchi-Pestellini
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Astrochemistry is now a well-established subject. For chemists, astrochemistry provides an opportunity to study how chemistry may operate in extreme and hostile environments; it makes huge demands on both laboratory and theoretical chemistry and consequently stimulates new activities in these studies. For astronomers, astrochemistry has revealed previously unknown molecular components of our galaxy, the Milky Way, and of external galaxies; these newly-recognised components play a fundamental role in the evolution of galaxies and have revolutionized our understanding of the Milky Way and of our place in it.
The aim of this book is to introduce some of these exciting ideas to students of chemistry. We describe the processes by which gas phase chemistry occurs in interstellar and circumstellar gases. This chemistry creates specific molecular tracers of those regions, triggers the growth of solid particles allowing surface and solid state chemistry to occur, and generates a remarkably rich range of molecular species which may even have links to astrobiology. The presence of the most abundant molecular species, such as molecular hydrogen and carbon monoxide, influences the evolution of interstellar gas, permitting the formation of dense cores of gas in which the formation of stars, planets, comets and asteroids may occur.
We cannot hope to cover all topics of relevance to astrochemistry in this introductory text. For example, we have not discussed at appropriate length the roles of gas dynamics, the thermal balance in interstellar gas, or the range of timescales that should be considered, although these are important topics. Our intention is to concentrate on the chemical aspects of the subject rather than the physical; current texts generally emphasize the latter. We hope that this book will provide interest and encouragement for some readers to take the subject to a higher level. The structure of the book is described below.
Chapter 1 introduces the astronomical background, while Chapter 2 discusses some (probably familiar) aspects of the interaction of molecules with electromagnetic radiation. In Chapter 3 we describe the types of reactions that are most relevant to astrochemistry, while in Chapter 4 the difficulties of initiating interstellar chemistry are described. The challenges are overcome by the introduction of highly specific initiating chemical pathways. It becomes clear at this point that the reaction network necessary for a comprehensive description of interstellar chemistry can be very large; typically, these networks involve reactions between hundreds of species interacting in thousands of reactions. Fortunately, large libraries of reactions (with measured, computed, or estimated rate coefficients) are available, together with many computer programmes that enable the chemical rate equations to be integrated. Chapter 5 introduces these topics and shows how, with suitable computer programmes, a student may make rapid progress in following the evolution of chemistry in a variety of astronomical environments. Thus, Chapter 5 is an important chapter that draws together all the ideas presented in the book up to this point and should be able to give the student reader an impressive skill in studying astrochemistry.
As mentioned in Chapter 1, interstellar dust is present in all interstellar gas. Dust scatters and absorbs starlight so that the interiors of gas clouds are shielded from stellar radiation (as described in Chapter 5) and molecules survive more easily in those locations. Where and how is dust formed? These are the topics of Chapter 6, which describes how the seemingly unlikely situations in the atmospheres of cool stars and in the apparently even more unlikely envelopes of supernovae ejecta, fairly simple gas phase chemistry can ultimately lead to the growth of dust particles. These particles (or grains) are then ejected into the interstellar medium where they may evolve further. Dust grains do have active chemical roles, beyond the rather passive role of shielding clouds from stellar radiation. These active roles include providing surfaces on which chemistry may occur (described in Chapter 7) and on which mixed molecular ices may in some situations accumulate (Chapter 8). These ices are deposited in rather simple chemical forms, but – as demonstrated in many laboratory experiments – when suitably energized by fast particles or by energetic radiation provide an efficient chemistry leading to the formation of a wide range of relatively large organic molecules, many of which are species that are astronomically detected. Evidently, solid state chemistry plays a key role in astrochemistry. Chapter 9 describes how planet formation is a by-product of star formation and how large organic molecules may be transported onto a planet. We discuss the boundaries between astrochemistry and astrobiology, and the possible relevance that molecules such as amino acids formed in ices may have in abiogenesis. Finally, Chapter 10 looks back to questions raised in Chapter 1, to discuss the extent to which those questions have been answered in the course of this book.
We hope that the user of this book will experience the thrills and enjoyment in learning about a field of study that is interdisciplinary and open-ended, feelings that we ourselves share when thinking about astrochemistry.
David A. Williams
University College London
INAF – Observatory of Palermo