Editorial pubs.acs.org/CR
Introduction: Astrochemistry 10−102 cm−3, where interstellar chemistry begins. Although this chemistry is not as rich as detected in dense clouds, some molecules do exist, the most abundant of which is molecular hydrogen, which is roughly of equal abundance to atomic hydrogen. Further gravitational contraction leads to dense clouds, of average gas density 103 cm−3, where star formation begins. Dense clouds come in several varieties and are quite heterogeneous. The gas-phase chemistry of low-mass star formation begins in “cold” prestellar cores, where the temperature can be as low as 5−10 K and the gas density is 104 cm−3. Under these conditions, only binary reactions without activation energy occur appreciably; such constraints are met mainly by exothermic ion-neutral reactions. The ionization stems mainly from collisions with cosmic rays, which are high-energy nuclei, whizzing around at a speed near that of light. The chemistry leads to an assortment of molecular species, including relatively common terrestrial molecules such as CO, which is the second most abundant molecule in the gas, with a concentration of 10−4 that of H2. But, in addition, a variety of exotic species are produced, including radicals (e.g., C6H), molecular cations (e.g., H3+, HCO+), anions (e.g., C6H−), metastable isomers (e.g., HNC, H2CCCC), and unsaturated long carbon chains (e.g., HC11N). The predicted concentrations of these and other species obtained from chemical simulations involving thousands of chemical reactions are often quite accurate by astronomical standards; viz., within a factor of 3 of values obtained from observation of spectra. Chemical simulations are also used to predict concentrations of molecules that comprise the ice mantles of cold dust grains; these species are, with the notable exception of CO, formed by chemical processes rather than simple accretion from the gas. The initial collapse of prestellar cores leading to low-mass star formation is isothermal in nature because of cooling caused by the emission of radiation from collisionally excited atoms and molecules. Over time, a dense central condensation builds up, out of which radiation cannot escape. As the continuing collapse starts to heat up the central condensation and material collapsing toward it, the region now becomes what is known as a protostar. When the collapsing gas and dust reach temperatures of 100−300 K and become a “hot core”, the chemistry changes dramatically; many common organic species can now be observed, including alcohols, esters, acids, aldehydes, etc. In addition, violent shocks blow out material and, perhaps most importantly, disks of gas and dust begin to form and rotate around the young protostar. In the disks, the dust particles start to coalesce into macroscopic bodies, eventually to become comets, meteors, and even extrasolar planets. At the same time, much of the natal cloud is blown away, and what is left is a young star surrounded by a dense protoplanetary disk, which can eventually evolve into a planetary system. Our understanding of the formation of
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strochemistry is the study of the rich and diverse chemistry that occurs throughout the universe. The field of astrochemistry is truly interdisciplinary and involves a variety of scientists, including those who study relevant physical and chemical processes in the laboratory and on the computer, those who observe molecular spectra through large telescopes in assorted regions of the spectrum, and those who simulate the chemistry of individual regions with large kinetic models somewhat akin to those used in combustion studies. Much if not most of the visible matter in the universe is divided into galaxies, which contain up to billions of stars along with interstellar matter. The mass is divided among the many elements, with hydrogen the most abundant, helium lower by a factor of 10 by number, and the chemically important elements C, N, and O lower by a factor of 103−4. The interstellar material resides mainly in the form of clouds, consisting of gas and tiny dust particles. These clouds are the only known birthplaces of stars and planets. The clouds, which can extend up to 100 pc, or 300 light years, in size, range from diffuse objects, through which the light of background stars can be seen, to very dense objects, which extinguish all background visible and ultraviolet light, but can be studied at longer wavelengths from the infrared to the radio spectral range. The field of astrochemistry started to achieve its current status in 1968, when ammonia was first detected by Townes and co-workers toward the center of our galaxy via its inversion transitions. Since that time, almost 200 different gas-phase molecules have been detected in the so-called interstellar medium, mainly in dense interstellar clouds via rotational emission spectroscopy in the millimeter and submillimeter portions of the spectrum. The molecules detected by this technique, which range in size up to 13 atoms, are overwhelmingly organic in nature. Not only do interstellar spectra tell us what molecules are present and how abundant they are, but they also tell us the physical conditions of the environment in which they are formed. Despite difficulties caused by water vapor absorption in our own atmosphere, the infrared region of the spectrum has been used to study the vibrations of interstellar molecules both in absorption, with either background stars or protostars, which are stars in the act of formation, as lamps, and in emission. Infrared absorption studies have been used to probe both the interstellar gas and the molecules that comprise interstellar dust particles, while emission studies have detected very large molecules; namely, polycyclic aromatic hydrocarbons (PAHs) and the fullerenes C60+, C60, and C70. Unlike the gas phase, the molecules comprising dust particles are not exotic: these particles are known to have cores of silicates or amorphous carbon and, in dense cold regions, to have mantles of ices, mainly consisting of water, CO, CO2, and methanol. The history of interstellar molecules begins with generations of old stars, which emit gas and dust either explosively, as in supernovae, or more gently, in expanding stellar envelopes. The atoms and dust particles collect under the influence of gravity into diffuse clouds, of temperature 50−100 K and gas density © 2013 American Chemical Society
Special Issue: 2013 Astrochemistry Published: December 11, 2013 8707
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and T. Lamberts, explores the use of kinetic Monte Carlo (KMC) stochastic methods and their relation to astrochemistry. In the next article, Wolf Geppert and Mats Larsson begin with a very useful introduction to gas-phase ion-neutral and ion-electron chemistry throughout the interstellar medium and the outer Solar System, and then they discuss experimental measurements and results for dissociative recombination, ionneutral, and even ion−ion reactions. The role of gas-phase collisions in the interstellar medium is not limited to reactions; inelastic nonreactive collisions are also of great importance because they help to determine internal state distributions in the frequent case in which thermal equilibrium does not adequately describe the system. Evelyne Roueff and François Lique discuss recent advances in the theory of and experiments concerning molecular excitation in the ISM via collisions as well as radiation and chemical processes. The authors speculate that interstellar chemistry will eventually incorporate some of the state-to-state collisions/ reactions now studied in the laboratory and theoretically. The next four papers discuss the chemistry that occurs during star formation, and how it differs from the ion− molecule, ion−electron chemistry that dominates the gas phase of cold interstellar clouds. Robin Garrod and Susanna Widicus Weaver start off this section of the thematic issue with a review of chemical simulations of hot core chemistry using gas−grain networks of chemical reactions. Much of what is detected in the gas phase is thought to derive from radical−radical reactions that occur on warming dust particles, on which molecules heavier than hydrogen can diffuse sufficiently to react, and the products can eventually sublime into the gas as the temperature continues to rise. The story is continued in the article by Yuri Aikawa, who emphasizes the dynamics of the physical collapse from cold core to protoplanetary disk for low-mass star systems and discusses a variety of hydrodynamic treatments. With higher dimensional models, a disk starts to form, although the calculations have so far not been extended sufficiently long to lead to the formation of a true protoplanetary disk. During the early stages of collapse, when the temperature has risen from 10 K to perhaps 30 K, a second phase of ion− molecule chemistry occurs, as discussed by Nami Sakai and Satoshi Yamamoto, who label the phenomenon “warm carbonchain chemistry”. The chemistry starts when volatile hydrocarbons such as methane sublime and act as precursors to a new round of gaseous chemistry that leads, once again, to unsaturated species known as “carbon chains”. The chemistry of protoplanetary disks is reviewed by Thomas Henning and Dmitry Semenov. Chemical simulations normally assume a fixed physical model, in which the density and temperature are functions of height from the midplane of the disk as well as the distance from the central star. The chemistry in the inner disk, where temperatures can range up to 5000 K, is quite different from the chemistry in the cool outer disk, where chemical simulations suggest that the chemistry can be divided into three regions depending upon height. Protoplanetary disks are small, but the spectra of nearby disks will be resolved spatially by the Atacama Large Millimeter/ Submillimeter Array (ALMA), a large interferometer that is undergoing construction in a high desert in Chile Now that the reader has achieved some understanding of the complex nature of the interstellar medium, he/she is ready to read the article by Ewine van Dishoeck, Eric Herbst, and David Neufeld on the abundance and chemistry of one molecule
high-mass stars is very much less advanced than that of the lowmass variety for a number of reasons, including the fact that none of these relatively rare stars is very close to the earth, and they evolve through their assorted evolutionary stages much more quickly. There are many other regions in the universe with unusual chemistry, all of which are studied by astrochemists. In this thematic issue, we present 13 papers on assorted topics of current importance in astrochemistry. These papers can be divided, at least partially, into the following ordered topics: basic interstellar chemistry, databases, and modeling; molecules as probes; chemical and physical processes of relevance to astrochemistry; chemical simulations of evolutionary stages in star formation; and weathering of solar system bodies. The thematic issue begins with an article by Marcelino Agúndez and Valentine Wakelam on the gas-phase chemistry of cold cores in dark interstellar clouds. The chemistry, dominated by ion-neutral reactions, was one of the first to be studied, and dark cloud cores are perhaps the best studied regions of the interstellar medium. The article ends with a lengthy treatment of the basic chemistry that occurs in the gas of dark clouds, and the extent of the agreement with observation. The chemistry discussed in the article by Agúndez and Wakelam leads naturally into the following article by Takeshi Oka, which focuses on the basic molecule H3+, its spectroscopy, its interstellar chemistry, and its use as a probe. One of the main uses of this species is to constrain the ionization rate caused by cosmic rays in these diverse regions. The precursor of much of the gas-phase chemistry in the interstellar medium is molecular hydrogen, yet under interstellar conditions, this molecule can only be formed appreciably on the surfaces of dust particles by association of two hydrogen atoms. Although astrochemists have long estimated the rate of the interstellar formation of H2, detailed laboratory studies are more recent. Gianfranco Vidali, who, along with colleagues, pioneered the experimental study of H2 formation on surfaces of interstellar relevance, discusses in detail experiments and their interpretation on the surface formation of H2. One of the results of our increased understanding of interstellar chemistry involving dust grains is the distinction now made between surface chemistry on bare grains and chemistry that occurs both on and in ice mantles. Tetsuya Hama and Naoki Watanabe, leaders in the field of ice chemistry of relevance to the interstellar medium, discuss a number of aspects of this chemistry from the experimental point of view. In particular, they discuss the following processesadsorption, desorption, diffusion, tunneling reactions, and nuclear spin conversion on amorphous solid water, and they relate their experimental work to what occurs in the cold interstellar medium. Among the molecular syntheses featured in the article are the formation of ices of water, methanol, and carbon dioxide. Deuterium enrichment on ice, especially of formaldehyde and methanol, is also a key topic. In gas-grain simulations of interstellar regions, the chemistry in both the gas and on grains is typically treated by kinetic rate equations. Yet there are problems with this basic treatment, especially as regards grain surfaces and grain mantle chemistry, where the small number of reactive species, such as hydrogen atoms on individual grains, and the need to consider irregular surfaces are more accurately treated by stochastic methods. A leader in this field, Herma Cuppen, along with L. Karssemeijer 8708
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waterin a variety of different sources. The authors discuss three different chemical processes that lead to water formation: low temperature ion-neutral chemistry, applicable to the gas phase of cold dense cores; ice chemistry, applicable to the chemistry that occurs on and in granular ice mantles; and hightemperature gas-phase chemistry, which produces water efficiently in regions over 300 K via neutral−neutral reactions. Up to now, this Thematic Issue on Astrochemistry has by and large bypassed planetary science, but we redeem ourselves in the concluding article by Chris Bennett, Claire Pirim, and Thomas Orlando, where chemical and physical processes that cause weathering in solar system bodies are reviewed. The processes are looked at both from the viewpoint of fundamental physics and from that of actual observations and space missions. We hope that you will enjoy the scientific riches in this Thematic Issue and that you will come away with the knowledge that much interest in chemistry occurs outside the bounds of our home planet.
John T. Yates, Jr. received his Ph.D. from MIT. He was an NRC Postdoctoral Fellow at the National Bureau of Standards (now NIST) and then a staff member there for 17 years. During that time he was a Sherman Fairchild Fellow at Caltech for one year. In 1982, he joined the Department of Chemistry at the University of Pittsburgh as R. K. Mellon Professor of Chemistry and as the founding Director of the Surface Science Center. He had a courtesy appointment in Physics at Pitt. In 2006 he moved to the University of Virginia Chemistry Department as Shannon Fellow and Professor of Chemistry. Among his honors are five ACS Awards and the Medard Welch Prize of the American Vacuum Society. He is a Fellow of the American Physical Society, the Institute of Physics, the American Chemical Society, and the American Vacuum Society. He recently became Associate Editor of Chemical Reviews. In 1996 he was elected to the National Academy of Sciences.
Eric Herbst
Department of Chemistry, University of Virginia
John T. Yates, Jr.
Department of Chemistry, University of Virginia
AUTHOR INFORMATION Notes
Views expressed in this editorial are those of the authors and not necessarily the views of the ACS. Biographies
Eric Herbst received his graduate education in physical chemistry at Harvard University, under the direction of William Klemperer, where he first began to study astrochemistry in addition to his major thesis work in molecular-beam spectroscopy. After a post-doctoral stint at Harvard, where he and Klemperer wrote a seminal paper on ion− molecule chemistry in dense interstellar clouds, and a second postdoctoral position at the University of Colorado under W. Carl Lineberger, he held faculty positions at The College of William and Mary in chemistry, at Duke University in physics, and at The Ohio State University in physics and astronomy. He is currently Commonwealth Professor of Chemistry at the University of Virginia, with courtesy appointments in astronomy and physics. A scientific editor of The Astrophysical Journal from 1998−2007, Herbst is a Fellow of the American Physical Society and the Royal Society of Chemistry, from which he received the Centenary Prize in 2004. 8709
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