The Cosmic-Chemical Bond

Chemistry from the Big Bang to Planet Formation

D A Williams, T W Hartquist

• Publisher: Royal Society of Chemistry
• Year: 2015
• ISBN:
  9781782626336
• Edition: 1
• Topic: Sciences physiques
• Subtopic: Spectroscopie et analyse du spectre

Introducing astrochemistry to a wide audience, this book describes how molecules formed in chemical reactions occur in a range of environments in interstellar and circumstellar space, from shortly after the Big Bang up to the present epoch. Stressing that chemistry in these environments needs to be driven, it helps identify these drivers and the various chemical networks that operate giving rise to signature molecules that enable the physics of the region to be better understood. The book emphasises, in a non-mathematical way, the chemistry of the Milky Way Galaxy and its planet-forming regions, describes how other galaxies may have rather different chemistries and shows how chemistry was important even in the Early Universe when most of the elements had yet to be formed. This book will appeal to anyone with a general interest in chemistry, from students to professional scientists working in interdisciplinary areas and non-scientists fascinated by the evolving and exciting story of chemistry in the cosmos.

CHAPTER 1

Introduction to Astrochemistry

1.1 MOLECULES IN SPACE

We live in a molecular Universe. Chemistry is, of course, rampant on Earth, and we have known for a long time that the atmospheres of the planets of our Solar System are almost entirely molecular. But one of the most remarkable discoveries of astronomy in the last half century—in an era of astronomical discovery that is truly astounding—has been the fact that molecules exist in abundance in huge clouds of gas between the stars in our own galaxy, the Milky Way, and in the interstellar space of galaxies beyond the Milky Way. In fact, we didn’t even know of the existence of such ‘giant molecular clouds’ until they were detected by the emission from the carbon monoxide (CO) molecules that they contain. The telescopes that made these detections were similar to radio telescopes but operate at shorter wavelengths, detecting emission in the rotational spectrum of CO at a wavelength of 2.6 mm. These clouds themselves fully deserve the adjective ‘giant’; they may contain up to one million times the mass of the Sun in a single cloud.

1.1.1 Chemical Richness

By similar techniques, these clouds were found to be chemically rich; that is, they contain a variety of simple species in addition to CO, such as hydrogen cyanide (HCN), carbon monosulfide (CS), ethynyl (C₂H), cyclopropenylidene (C₃H₂), and even ions—i.e. molecules with an electron missing—such as the formyl ion (HCO⁺). Denser regions embedded within these giant clouds are even richer in their chemistry. For example, very small and dense regions close to young massive stars were found to contain molecular species that are larger and more complex than those found in the extended regions of the giant clouds. These denser and more localised regions contain molecular species such as methanol (CH₃OH), ethanol (CH₃CH₂OH), propanal (CH₃CH₂CHO), and dimethyl ether (CH₃OCH₃).

Some carbon chain molecules such as cyanodecapentayne (HC₁₀CN) were detected first in interstellar and circumstellar space, and later confirmed in the laboratory. Macromolecules such as the cage molecules Buckminster fullerene (C₆₀) and 70-fullerenes (C₇₀) have also been detected in the gaseous envelopes of old stars. There is some evidence that interstellar space includes species known as polycyclic aromatic hydrocarbons (PAHs); these may contain up to a hundred atoms with carbons in hexagonal (i.e. graphitic) arrays terminated by peripheral hydrogen atoms and other structures.

The chemical variety does not end there. Isotopologues of many species have been detected, mostly where hydrogen atoms have been substituted by deuterium. For example, in some places where water (H₂O) has been detected, then the substituted versions HDO and D₂O have also been detected. Similarly, all the possible D-substituted varieties of ammonia have been detected, from NH₃ through NH₂D and NHD₂ to ND₃. These hydrogen isotopologues are often surprisingly abundant, given that the cosmic deuterium abundance relative to hydrogen is in the order of one part in one hundred thousand. Other isotopologues involve carbon and oxygen isotopes. For example, all six versions of CO involving ¹²C and ¹³C and ¹⁶O, ¹⁷O, and ¹⁸O have been detected.

1.1.2 A Variety of Sources

Having noted that the giant molecular clouds—and the star-forming regions within them—were chemically rich, astronomers also turned their attention to other objects in the Milky Way galaxy and detected molecular emissions in many other regions. Evolved cool stars have extended envelopes that drift out into interstellar space. These envelopes show delightfully precise patterns of chemistry, with a system of ‘parent’ molecules close to the star giving rise to ‘daughter’ species in the envelope. These cool stars and envelopes evolve after ten thousand years or so to become the beautiful planetary nebulae, which display their own unique chemistries. Stars near the ends of their lives may explode in novae or supernovae, and the ejecta even in these apparently hostile environments display molecular emissions of various species. Evidently, if chemistry can possibly occur in astronomy, it will! Perhaps the most extreme conditions in which molecules are found are in a stellar atmosphere. Although the atmospheres of massive stars are simply too hot for molecules to exist, even common stars like our Sun—whose atmosphere has a temperature of about 5800 K—show molecular spectra of CO and of H₂ in sunspots, where the temperature is slightly lower at about 4000 K.

The discovery of molecules in external galaxies beyond the Milky Way is also an exciting story, and one that is still being explored. Although we cannot resolve the structure of very distant galaxies, and the great distances mean that the emissions are very weak, observations suggest that the wealth of chemistry that we observe in different regions of the Milky Way will be repeated in similar galaxies throughout the Universe. Even more remarkably it is now possible, in certain circumstances, to detect molecular emissions from objects that are so far away that the radiation detected was emitted when the Universe was merely a few percent of its present age of 13.7 billion years. Evidently, chemistry was occurring very early indeed in the history of the Universe.

1.1.3 Common Interests

From the point of view of chemistry, the discovery of these molecules is fascinating, and the demands of astronomy have challenged existing chemical knowledge. Chemists have met that challenge superbly and have been stimulated into enormous efforts in both laboratory and theoretical work. From the point of view of astronomy, the existence of molecules in interstellar and circumstellar gas has opened a new way to study astronomical regions that were previously inaccessible to observations. This new way of studying astronomy has enabled astronomers to find out more about interstellar and circumstellar matter, the formation of stars and galaxies, and the interaction of those stars and galaxies with their environments. From molecular spectra, astronomers can deduce densities, temperatures, elemental abundances, ionisation rates and many other important parameters.

A new subject, astrochemistry, involving chemists and astronomers has developed to exploit and enrich the overlap in interests between these two areas. The Royal Society of Chemistry and the Royal Astronomical Society have combined to encourage this collaboration in the now well-established UK Astrophysical Chemistry Group. However, the subject may have even wider ramifications. Nearly all of the identified interstellar molecules are familiar in organic chemistry. Although very simple in biological terms, some of them can be recognised as the building blocks of larger molecules of relevance to biological chemistry. For example, the amino acid glycine (NH₂CH₂COOH) has been detected in the comet Wild 2 by the NASA spacecraft Starburst, and a related molecule aminoacetonitrile (NH₂CH₂CN) has been detected by conventional radio astronomy in the centre of the Milky Way. These detections have given some support to the idea that life is widespread throughout the Galaxy, and that interstellar organic molecules may provide the feedstock for a complicated pre-biological chemistry when planets form during the process of star formation inside an interstellar cloud. As yet, these ideas remain fascinating speculations.

This book, however, is focused strictly on chemical matters. The main question we want to address is this: what are the processes by which these molecules are formed and destroyed in the various astronomical locations in which they are found? To answer it, we shall need to have some specific details of the nature of those locations. The remainder of this chapter is therefore devoted to a rather general discussion on the relevant astronomical background. More detailed descriptions are given in subsequent chapters.

1.2 THE ASTRONOMICAL BACKGROUND: GAS AND DUST

Most molecular-rich astronomical regions can be described either as interstellar or near-stellar. These regions contain solids in the form of dust particles, as well as gas.

1.2.1 Interstellar Environments

The first point to establish is the elemental composition of the interstellar gas. It is almost entirely hydrogen. Only about one tenth of one percent (by number of atoms, relative to hydrogen) is in the important elements oxygen, carbon and nitrogen, taken together. Other atoms are even less abundant. The relative abundances of the elements can be measured in the Sun and in other stars and ionised regions of space. There is some variation between these measurements; the values for the Sun are often used as a standard, though of course these numbers may not apply everywhere in the Milky Way and almost certainly do not apply in other galaxies. The solar values for the relative abundances of some important elements are shown in Table 1.1.

We see from this information that for every 10 000 H-atoms in the interstellar medium, there will be roughly 6 O-atoms, 3 C-atoms, and 1 N-atom. So the atoms that are needed to make the molecules that have been detected (and to make terrestrial planets—and us!) are really a very minor component of the interstellar medium.

Table 1.1 Solar abundances by number of atoms, relative to hydrogen, of some chemically important species. Relative abundances in the interstellar medium may be rather smaller than these values.

H	1 000 000	Mg	44
He	100 000	Si	36
O	540	Fe	35
C	300	S	15
N	74	Na	2

In fact, from the point of view of chemistry, the situation is even more difficult, in that not all of these atoms in the minor component are actually available to make molecules. Some of them are locked (almost permanently) in interstellar dust. Observations of the Milky Way galaxy show that the interstellar gas is everywhere mixed with dust. The dust is detected be...