This review is concerned with the formation of molecules in the interstellar medium (ISM), which is composed mainly of regions of gas and dust known as interstellar clouds, ranging in size from a few ...to 100's of light years in extent. Upwards of 200 different molecules have been observed spectroscopically in these objects, with a significant fraction of them 'large' by astronomical standards; i.e. containing six or more atoms. Interstellar clouds are of interest to chemists because of the exotic molecules and chemistry that occur in these sources, while they are of interest to astronomers because these clouds are the only known birthplaces of new stars and extrasolar planets. The formation of stars and planets from portions of dense interstellar clouds is a complex evolutionary process with multiple stages dependent upon the mass of the collapsing object. For low-mass stars such as our sun, the process is reasonably well understood and proceeds through the following intermediate stages: cold dense cores, pre-stellar cores, hot cores, and protoplanetary discs. For high-mass stars, the process is significantly less well understood because these objects are rare and are formed through evolutionary stages that are short in duration, at least astronomically speaking. Molecules are found in all of these stages, in the gas phase and often in the solid phase, with the chemistry dependent upon the physical conditions and their history. Indeed, the many molecules detected have helped significantly to unravel much of the complexity involved in stellar and planetary formation. This review is divided into sections in which, following an introduction, we discuss the different types of chemical reactions that synthesise large molecules, starting with cold dense cores of temperature 10 K and gas density
, and proceeding through the various stages of low-mass star formation through protoplanetary discs. Several other types of sources are discussed briefly. We then review some recent progress that has occurred within the last several years in improving our knowledge of the chemistry in this fast-growing and rapidly evolving field of research. We end with a brief discussion of the detailed chemical simulations employed to follow the chemistry in the various sources in the ISM.
The interdisciplinary science of astrochemistry is 45 years of age, if we pinpoint its origin to have occurred when the first polyatomic molecules were detected in the interstellar gas. Since that ...time, the field has grown remarkably from an esoteric area of research to one that unites scientists around the globe. Almost 200 different molecules have been detected in the gas-phase of interstellar clouds, mainly by rotational spectroscopy, while dust particles and their icy mantles in colder regions can be probed by vibrational spectroscopy. Astrochemistry is exciting to scientists in a number of different fields. Astronomers are interested in molecular spectra from the heavens because such spectra are excellent probes of the physical conditions where molecules exist, while chemists are interested in the exotic molecules, their spectra, and the unusual chemical processes that produce and destroy them under conditions often very different from those on our home planet. Chemical simulations involving thousands of reactions are now used to calculate concentrations and spectra of interstellar molecules as functions of time. Even biologists share an interest in the subject, because the interstellar clouds of gas and dust, portions of which collapse to form stars and planetary systems, contain organic molecules that may become part of the initial inventory of new planets and may indeed be the precursors of life. An irresistible subject to its practitioners, astrochemistry is proving to be exciting to a much wider audience. In this perspective article, the field is first introduced, and the emphasis is then placed on the three environments in which chemistry occurs in the interstellar medium: the gas phase, the surfaces of bare dust particles, and the ice mantles that cover bare grains in cold dense interstellar clouds. What we do know and what we do not know is distinguished. The status of chemical simulations for a variety of interstellar sources having to do with stellar and planetary evolution is surveyed. An optimistic view of the future of astrochemistry ends the article.
Chemistry occurs on bare dust particle cores and on the ice mantles surrounding them via a variety of mechanisms.
The chemistry that occurs in interstellar clouds consists of both gas-phase processes and reactions on the surfaces of dust grains, the latter particularly on and in water-dominated ice mantles in ...cold clouds. Some of these processes, especially at low temperature, are very unusual by terrestrial standards. For example, in the gas-phase, two-body association reactions form a metastable species known as a complex, which is then stabilized by the emission of radiation under low-density conditions, especially at low temperatures. In the solid phase, it has been thought that the major process for surface reactions is diffusive in nature, occurring when two species undergoing random walks collide with each other on a surface that has both potential wells and intermediate barriers. There is experimental evidence for this process, although very few rates at low interstellar temperatures are well measured. Moreover, since dust particles are discrete, modeling has to take account that reactant pairs are on the same grain, a problem that can be treated using stochastic approaches. In addition, it has been shown more recently that surface reactions can occur more rapidly if they undergo any of a number of non-diffusive processes including so-called three-body mechanisms. There is some experimental support for this hypothesis. These and other unusual gaseous and solid-state processes will be discussed from the theoretical and experimental points of view, and their possible role in the synthesis of organic molecules in interstellar clouds explained. In addition, their historical development will be reviewed.
Complex Organic Interstellar Molecules Herbst, Eric; van Dishoeck, Ewine F.
Annual review of astronomy and astrophysics,
09/2009, Letnik:
47, Številka:
1
Journal Article
Recenzirano
Of the over 150 different molecular species detected in the interstellar and circumstellar media, approximately 50 contain 6 or more atoms. These molecules, labeled complex by astronomers if not by ...chemists, all contain the element carbon and so can be called organic. In the interstellar medium, complex molecules are detected in the denser sources only. Although, with one exception, complex molecules have only been detected in the gas phase, there is strong evidence that they can be formed in ice mantles on interstellar grains. The nature of the gaseous complex species depends dramatically on the source where they are found: in cold, dense regions they tend to be unsaturated (hydrogen-poor) and exotic, whereas in young stellar objects, they tend to be quite saturated (hydrogen-rich) and terrestrial in nature. Based on both their spectra and chemistry, complex molecules are excellent probes of the physical conditions and history of the sources where they reside. Because they are detected in young stellar objects, complex molecules are expected to be common ingredients for new planetary systems. In this review, we discuss both the observation and chemistry of complex molecules in assorted interstellar regions in the Milky Way.
ABSTRACT The recent discovery of methyl formate and dimethyl ether in the gas phase of cold cores with temperatures as cold as 10 K challenges our previous astrochemical models concerning the ...formation of complex organic molecules (COMs). The strong correlation between the abundances and distributions of methyl formate and dimethyl ether further shows that current astrochemical models may be missing important chemical processes in cold astronomical sources. We investigate a scenario in which COMs and the methoxy radical can be formed on dust grains via a so-called chain reaction mechanism, in a similar manner to CO2. A unified gas-grain microscopic-macroscopic Monte Carlo approach with both normal and interstitial sites for icy grain mantles is used to perform the chemical simulations. Reactive desorption with varying degrees of efficiency is included to enhance the nonthermal desorption of species formed on cold dust grains. In addition, varying degrees of efficiency for the surface formation of methoxy are also included. The observed abundances of a variety of organic molecules in cold cores can be reproduced in our models. The strong correlation between the abundances of methyl formate and dimethyl ether in cold cores can also be explained. Nondiffusive chemical reactions on dust grain surfaces may play a key role in the formation of some COMs.
We have studied the formation of fluorine- and chlorine-bearing species for a variety of dense interstellar conditions using a gas-grain network. Our homogeneous models have been constructed for ...low-temperature dense clouds, as well as warm-up regions. In addition to the observed species HF, , HCl, , and , we have included a number of additional halogen-containing molecules, and explored their gas-phase and grain-surface chemistry. These molecules include neutral species such as Cl2, ClO, CCl, and HCCl, as well as the carbon-halogen species CH2Cl and CH3Cl, and ionic species such as , , CH3ClH+, , , and . Predictions are made for the abundances of these species as functions of time, and comparisons are made with the observed abundances obtained for halogen species in dense regions, which include HF, HCl, CH3Cl, and CF+. The peak fractional abundance of the newly detected gas-phase CH3Cl is predicted to be in our warm-up simulations, depending upon density and the age of the pre-warm-up phase after which warm-up begins. These values can be compared with the observed abundance of methyl chloride in the hot corino IRAS 16293-2422 if the abundance of methanol is known.
Abstract
A new, more comprehensive model of gas–grain chemistry in hot molecular cores is presented, in which nondiffusive reaction processes on dust-grain surfaces and in ice mantles are implemented ...alongside traditional diffusive surface/bulk-ice chemistry. We build on our nondiffusive treatments used for chemistry in cold sources, adopting a standard collapse/warm-up physical model for hot cores. A number of other new chemical model inputs and treatments are also explored in depth, culminating in a final model that demonstrates excellent agreement with gas-phase observational abundances for many molecules, including some (e.g., methoxymethanol) that could not be reproduced by conventional diffusive mechanisms. The observed ratios of structural isomers methyl formate, glycolaldehyde, and acetic acid are well reproduced by the models. The main temperature regimes in which various complex organic molecules (COMs) are formed are identified. Nondiffusive chemistry advances the production of many COMs to much earlier times and lower temperatures than in previous model implementations. Those species may form either as by-products of simple-ice production, or via early photochemistry within the ices while external UV photons can still penetrate. Cosmic ray-induced photochemistry is less important than in past models, although it affects some species strongly over long timescales. Another production regime occurs during the high-temperature desorption of solid water, whereby radicals trapped in the ice are released onto the grain/ice surface, where they rapidly react. Several recently proposed gas-phase COM-production mechanisms are also introduced, but they rarely dominate. New surface/ice reactions involving CH and CH
2
are found to contribute substantially to the formation of certain COMs.
Unidentified infrared emission bands are ubiquitous in many astronomical sources. These bands are widely, if not unanimously, attributed to collective emissions from polycyclic aromatic hydrocarbon ...(PAH) molecules, yet no single species of this class has been identified in space. Using spectral matched filtering of radio data from the Green Bank Telescope, we detected two nitrile-group-functionalized PAHs, 1- and 2-cyanonaphthalene, in the interstellar medium. Both bicyclic ring molecules were observed in the TMC-1 molecular cloud. In this paper, we discuss potential in situ gas-phase PAH formation pathways from smaller organic precursor molecules.
Hot Cores in Magellanic Clouds Acharyya, Kinsuk; Herbst, Eric
The Astrophysical journal,
05/2018, Letnik:
859, Številka:
1
Journal Article
Recenzirano
We have studied the chemistry of molecules through complex organic molecules (COMs) in complexity in conditions resembling galactic hot molecular cores in the Large and Small Magellanic Clouds using ...a gas-grain network. To the best of our knowledge, there have been no previous such quantitative studies of hot core chemistry in these low metallicity, dust-poor galaxies. We utilized a physical model that consists of an initial isothermal collapse, followed by a warm-up phase to hot core conditions. Four different temperatures-10, 15, 20, and 25 K-were used for the isothermal collapse phase, considering the fact that these galaxies might have higher dust temperatures in cold regions than observed in the Milky Way. We found that for some abundant species, such as CO and water, hot core abundances are consistent with the reduced elemental abundances of the LMC and SMC. For other less abundant species, such as CH4 and HCN, the calculated abundances are larger when compared with elemental abundances, whereas for species like ammonia they are lower. Our calculations show that some COMs can also be formed in reasonable quantity for hot cores in the Magellanic Clouds when the grain temperature is lower than 25 K. Our results can be compared with recent observations of the hot cores in the high-mass young stellar object (YSO) ST11 and regions A1 and B3 of the star-forming source N113 in the LMC. Model results are in reasonable agreement with the observed abundances and upper limits.
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