Dynamical Astrochemistry / Edition 1

by David A Williams, Thomas W Hartquist, Jonathan M C Rawlings, Cesare Cecchi-Pestellini, Serena Viti

Overview

Astrochemistry is a well-established interdisciplinary subject and the methods for describing time-dependent chemistry in static or slowly-changing regions of interstellar space have been well-developed over many years. Existing astrochemical books normally describe the subject in terms of chemistry in static or slowly-varying astronomical situations but the most significant astronomical regions are those in which the physical conditions change on timescales that are comparable to or shorter than chemical timescales.

Written by leading experts in this area, this is the first book specifically devoted to the astrochemistry of dynamically evolving astronomical regions. It provides a comprehensive description of this important area of science, stressing in particular the methods that have been developed for specific purposes. It will be of interest to researchers in astrochemistry, including both chemists and physicists and could form the basis of a postgraduate course for research students in chemistry and physics.

Product Details

ISBN-13:	9781782627760
Publisher:	RSC
Publication date:	12/13/2017
Pages:	304
Product dimensions:	6.15(w) x 9.20(h) x (d)

Read an Excerpt

CHAPTER 1

Chemistry and Dynamics in the Interstellar Medium

1.1 Introduction

Eighty years ago, optical spectra of three simple molecular species, CH, CH+, and CN, were observed in interstellar gas along lines of sight towards hot stars. These first detections stimulated detailed investigations of chemical reactions in interstellar space, a subject that has since become known as astrochemistry. The next detections, in 1963, were of maser emission at decimetric wavelengths from another simple hydride, interstellar OH, but the subject was totally transformed when millimetre and sub-millimetre wavebands became available for astronomy from the late 1960s. A flood of discoveries of interstellar molecular species was made in the following years and decades, and a high rate of discovery continues today. Observations confirm that by far the most abundant species of the interstellar molecules is molecular hydrogen; however, that species is difficult to detect except in particular circumstances. The next most abundant species is carbon monoxide, and it is readily detectable in its rotational spectrum. It is widely used to trace interstellar molecular gas in the Milky Way and other galaxies. Such observations led to the discovery of a new gaseous and largely molecular component in the Milky Way galaxy: the giant molecular clouds. These mainly molecular clouds have masses ranging up to one million solar masses (M[??]), and are the main reservoirs of mass for the formation of new stars in the Milky Way.

The number of molecular species detected in interstellar and circumstellar regions of the Milky Way galaxy is currently (in 2017) about 200; about 60 of these species have also been detected in gas in external galaxies (a complete and up-to-date list is maintained by staff at the Universität zu Köln at https://www.astro.uni-koeln.de/cdms/molecules). These totals ignore the very large number of discoveries of isotopologues, i.e., species that differ only in the substitution of isotopes, such as D for H, 13C for 12C, or 17O for 16O. The detected species include a large number of diatomics (more than 40) and triatomics (also more than 40), with numbers of detected species thereafter tending to decline with size. For example, there are currently just four detected molecular species containing 10 atoms, and the same is also true for species containing 11 and 12 atoms. Three large "cage" molecules (the fullerenes C60, C60+, and C70) have been detected in circumstellar regions, and — although no precise detections have yet been made — there is good reason to consider that the interstellar medium is populated by polycyclic aromatic hydrocarbons, each molecule containing several benzenetype rings and including perhaps up to 100 atoms. Evidently, the large masses of molecular gas in the Milky Way and other galaxies are chemically quite complex.

These astronomical observations stimulated intense activity in laboratory studies of gas-phase, surface and solid-state chemistry, and in computational modelling of chemistry in various types of astronomical regions. All these studies helped to establish astrochemistry, i.e., the study of the chemical networks that are believed to lead to the formation of interstellar molecules in interstellar and circumstellar regions, and of the consequent harvest of astronomical understanding that such study made possible. In general terms, these networks are now reasonably well understood for most of the smaller molecular species (say, up to five atoms), although some outstanding problems remain. The chemistry governing the larger species is currently an active area of research. We shall summarize our present understanding of interstellar chemistry in Section 1.2.

From an astronomical perspective, observations of molecular spectra in interstellar and circumstellar regions are immediately useful: the basic observational data on spectral lines, their positions in the sky, relative strengths, and linewidths provide essential insight into the physical conditions of the regions in which the spectra originate by helping to constrain the gas number density and temperature in those regions. Shifts in spectral line positions enable the determination of the relative velocities of discrete structures in the interstellar gas, and linewidths may help to establish the presence and extent of some kind of turbulence in the gas. However, one can extract a much greater range of information from these observational data by using astrochemical models based on an understanding of the appropriate chemistry. As we'll see in the next section, the chemistry is determined not only by the density and temperature in the gas, but by other important astronomical parameters specific to the region in which the molecules are found. These include the local radiation field in various wavebands from UV to X-ray, the local flux of cosmic rays with energies in the MeV range, the local relative elemental abundances, and the physical and chemical nature of the local dust and the dust to gas ratio. Further, since interstellar chemistry is inherently time-dependent, observations may give insight into the timescales of relevant processes of the observed region and may help to describe its physical evolution. Thus, astrochemical models can provide very powerful diagnostics of physical conditions within a molecular region. The physics and chemistry of the interstellar medium are described in important texts by Draine (2011) and by Tielens (2005). An observational perspective of astronomical molecules is given in the textbook by Williams and Viti (2013), and a study of the chemistry of cosmic dust may be found in the monograph by Williams and Cecchi-Pestellini (2016).

Molecular observations and subsequent modelling have revealed a rich variety of molecular regions in the Milky Way and other galaxies. Some of these are quiescent; i.e., essentially motionless, so that any chemistry occurring within them has ample time to reach steady state. Many diffuse clouds in the interstellar medium, in which chemistry is dominated by rapid photoionization and photodissociation by stellar radiation, may be in this condition. Also, some regions change, but so slowly that the chemistry is able to reach steady state in the conditions at any stage of evolution of that region. This may be the case, for example, in the very early stages of gravitational collapse of a diffuse gas cloud. However, such quiescence or quasi-quiescence certainly does not apply everywhere. Some physical changes are rapid. Regions may be affected by forces that cause changes to density, temperature and other important physical parameters on timescales that are rapid compared to the time required for the chemistry to attain steady state. For example, interstellar clouds may collide, and such collisions may lead to the generation of shock waves causing transient heating and compression of the gas, with consequent chemical changes. The later stages of gravitational collapse may accelerate gas to high velocity, generating turbulence that modifies the chemistry. Stellar outflows may impinge at high velocity on nearby gas causing turbulent mixing of gases at different temperatures and pressures. Stellar explosions such as novae and supernovae may accelerate gas to very high velocities with consequent changes in physical conditions and chemistry. In such cases, the chemistry in the gas is unlikely to attain steady state and so the molecules that can be observed are not necessarily those that arise in steady state. We shall briefly summarize the physical properties of various interstellar regions in Section 1.3 and Table 1.3 below.

Regions subject to such dynamical effects are some of the most interesting regions to study through their molecular spectra. Such studies reveal the evolution of intergalactic and interstellar gas during the formation of galaxies, stars, and planets, and the effects of stars on their environments. Studies of this kind cannot be approached through steady state models. The chemical networks that we shall discuss are highly nonlinear, and the approach to steady state is not a straightforward process.

This book is devoted to the astrochemistry of astronomical regions subject to rapid dynamical change and to the methods that may be useful in those studies. It is aimed primarily at astrophysicists who wish to investigate the dynamical interstellar and circumstellar media through their molecular emissions, and at astrochemists who wish to understand the sensitivity of interstellar and circumstellar chemistry to the physical environment.

The astronomical objects offered as examples in this book all belong to the Milky Way galaxy. However, the book's relevance is not limited to our galaxy. The physical and chemical processes that we shall describe are of general application to objects in all galaxies. Indeed, the value of astrochemical modelling is that it shows how chemical processes occur in regions of widely differing physical conditions.

1.2 Interstellar and Circumstellar Chemistry — A Brief Summary

There are three main types of chemical process occurring in interstellar and circumstellar regions, as physical conditions permit. These are:

(i) gas phase processes between atoms, ions and molecules, promoted by photoionization, photodissociation, and cosmic ray ionization;

(ii) reactions on the surfaces of bare grains, with either prompt or delayed ejection of products into the gas phase;

(iii) accumulation of molecular ices on the surfaces of dust inside dark clouds, with activation of these ices by cosmic rays or photons and subsequent reactions between the radicals so generated to form species of greater complexity.

An enormous amount of attention has been given to gas-phase schemes, i.e., type (i), although one could properly argue that the truly fundamental process of interstellar chemistry in the local Universe (where dust is abundant) is of type (ii) in the formation of H2. As we shall see in Section 1.2.1, this molecule is essential in all gas phase schemes based on type (i) chemistries, apart from those operating in the ejecta of novae and supernovae (see Chapter 6). Systems of type (ii) depend sensitively on the physical and chemical nature of the surface of the grains. The specific case of H2 formation has recently received considerable attention in the laboratory and through ab initio calculations. It appears from recent research that the more realistic the physical description, the more favourable the H2 formation process is predicted to be, and that the reaction is likely to occur over a significant temperature range. Type (iii) chemistries are now benefitting from detailed experimental and theoretical attention, and while there is a general consensus that schemes of this type can generate chemical complexity from simple initial compositions, there is as yet no agreement on the details.

The presence of dust grains involved in types (ii) and (iii) chemistries is inferred primarily from observational studies of interstellar extinction. These show that a range of sizes and compositions of dust grains must be present, and in particular that there are many more small grains than large. A commonly adopted size distribution for assumed spherical grains of radius a is that dn(a)/da varies as a-3.5, within adopted minimum and maximum radii. An early model of this type for grains of silicates and graphite is called the Mathis-Rumpl-Nordsieck (or MRN) model. It was shown to be capable of fitting extinction data from the infrared to UV, and it has since been substantially modified and extended by Draine, (see also ref. 4).

1.2.1 Gas-phase Chemistries

The basic chemical problem is to understand how the available elements O, C, N, S, etc., as trace atoms in a gas of H and H2, can combine to form molecules (see Table 1.1 for relative elemental abundances in the Sun, often used as a standard, and taken from Asplund et al. (2009); slightly different values taken from studies of nearby B stars are also used — see Nieva and Przybilla 2012). Most of the interstellar gas is at a sufficiently low temperature that energy barriers in, for example, the O–H2 potential energy surface inhibit direct reactions between O and H2 to form OH, which could undergo further reaction with H2 to form H2O. In localized regions of higher temperature, however, such reactions may proceed efficiently at nearly the total collisional rates. Direct atom-exchange reactions of this type are important in regions of elevated temperature that may arise in circumstellar regions, and may account for the abundances of hydrides in such regions.

Circumstellar regions may also be very dense compared to interstellar densities. At sufficiently high densities, above ~1010 molecules cm-3, three body reactions become important. In a system of three species, A + B + C, if a third body, A, collides with an already colliding pair, BC*, it may remove energy (indicated by *) from the pair, leaving it in a bound state, BC. Such reactions are generally efficient, and were particularly important for generating molecular hydrogen in high density, dust-poor conditions in the Early Universe.

1.2.1.1 Basic Oxygen and Carbon Chemistry

In the bulk of the interstellar gas, however, such reactions are inefficient, and the gas needs to be energized in some way. The principal energy sources available in interstellar clouds are cosmic rays and stellar UV radiation. X-rays may also be very important in localized circumstellar regions. Cosmic rays penetrate most interstellar regions, whereas stellar UV radiation is confined to diffuse clouds, where the visual extinction caused by interstellar dust is less than a few magnitudes, and to the peripheries of dark clouds. Cosmic rays ionize hydrogen atoms and molecules to provide H+ and (mainly) H2+. This hydrogen molecular ion extracts a hydrogen atom from H2 to create H3+, a stable triangular ion — protonated dihydrogen — which happens to have low proton affinity so it is capable of donating a proton to many other species, for example:

H3+ + O -> OH+ + H2.

This OH+ ion can react directly and efficiently in successive ion-molecule reactions with H2 to form H2O+ and H3O+:

OH+ + H2 -> H2O+ + H;

H2O+ + H2 -> H3O+ + H.

Exothermic ion-molecule reactions of this type are efficient, occurring in almost every collision of the reacting pair. The ions created in these ion-molecule reactions rapidly react with electrons in dissociative recombination reactions to form O, H, OH, (hydroxyl) and H2O (water). Subsequent reactions may begin to generate chemical complexity; for example, reaction of C+ with H2O generates HCO+ (the formyl ion), which dissociatively recombines with electrons to give CO (carbon monoxide). Both HCO+ and CO are very important observed interstellar species.

H3+ ions also provide an entry into carbon chemistry; the reaction of H3+ with a carbon atom forms CH+, which (similarly to OH+) can successively abstract H atoms from hydrogen molecules to form CH2+ and CH31, while a slow radiative association of CH3+ with H2 forms CH5+. Recombination of these ions with electrons may lead to C, CH (methyladyne), CH2 (methylene), CH3 (methyl), and CH4 (methane), and reactions of these species may lead to further chemical complexity; for example, reaction of CH3 with N atoms can generate hydrogen cyanide, HCN.

1.2.1.2 Basic Nitrogen and Sulfur Chemistry

These straightforward schemes do not operate for all species. Nitrogen atoms have a low proton affinity and cannot accept a proton from H3+, although N2 molecules will do so, creating N2H+ (protonated nitrogen), Which — after dissociatively recombining with electrons — can provide nitrogen hydride, NH. However, in a separate channel, nitrogen ions, N+, created by cosmic ray ionization of N atoms, do react directly with H2, creating NH+ and — after successive H-atom abstractions — other hydride ions, leading to the formation, after recombination, of the hydrides NH, NH2 and NH3 (nitrogen hydride, aminyl, and ammonia).

Like nitrogen, sulfur has a lower proton affinity than H2, so cannot accept a proton from H3+. The main entry to the chemistry of sulfur-bearing interstellar species seems to be in the reaction of S atoms with radicals such as OH and CH. Simple networks of atom exchange reactions generate species such as SO, SO2, CS, HCS, and H2CS (sulfur monoxide and dioxide, carbon monosulfide, thioformyl, and thioformaldehyde, respectively). Reactions of O atoms with thioformyl give carbonyl sulphide, OCS. All of these sulfur-bearing molecules are detected interstellar species.

(Continues…)

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Copyright © 2018 David A. Williams, Thomas W. Hartquist, Jonathan M. C. Rawlings, Cesare Cecchi-Pestellini and Serena Viti.
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Table of Contents

Chemistry and Dynamics in the Interstellar Medium;
Shocks and Turbulence and Their Effects on Chemistry;
Non-Thermal Chemistry in the Interstellar Medium;
Gas Dynamics under Gravity: Star Formation;
Stellar Jets and Outflows;
Outflows and Explosions of Evolved Stars;
Conclusions: where do we go from here?

Editorial Reviews

This book will no doubt stand as the definitive work in this field for some time to come. If you have even a passing interest in the interstellar medium, either in the Milky Way or in other galaxies, you should read it. Furthermore, you will be able to give it to every future PhD student on their first day and say ‘start here’.

The Observatory magazine

This book will no doubt stand as the definitive work in this field for some time to come. If you have even a passing interest in the interstellar medium, either in the Milky Way or in other galaxies, you should read it. Furthermore, you will be able to give it to every future PhD student on their first day and say ‘start here’.

The Observatory magazine, Volume 138, No. 1266 - Derek Ward-Thompson