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.

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Complex Organic Interstellar Molecules

The most abundant interstellar molecule, H-2, is generally thought to form by recombination of H atoms on grain surfaces On surfaces, hydrogen atoms can be physisorbed and chemisorbed and their mobility can be governed by quantum mechanical tunneling or thermal hopping We have developed a model for molecular hydrogen formation on surfaces This model solves the time-dependent kinetic rate equation for atomic and molecular hydrogen and their isotopes, taking the presence of physisorbed and chemisorbed sites, as well as quantum mechanical diffusion and thermal hopping, into account The results show that the time evolution of this system is mainly governed by the binding energies and barriers against migration of the adsorbed species We have compared the results of our model with experiments on the formation of HD on silicate and carbonaceous surfaces under irradiation by atomic H and D beams at low and at high temperatures This comparison shows that including both isotopes, both physisorbed and chemisorbed wells, and both quantum mechanical tunneling and thermal hopping is essential for a correct interpretation of the experiments This comparison allows us to derive the characteristics of these surfaces For the two surfaces we consider, we determine the binding energy of H atoms and H-2 molecules, as well as the barrier against diffusion for the H atoms to move from one site to another We conclude that molecular hydrogen formation is efficient until quite high (similar to 500 K) temperatures At low temperatures, recombination between mobile physisorbed atoms and trapped chemisorbed atoms dominates At higher temperatures, chemisorbed atoms become mobile, and this then drives molecular hydrogen formation We have extended our model to astrophysically relevant conditions The results show that molecular hydrogen formation proceeds with near unity efficiency at low temperatures ( T less than or equal to 20 K) While the efficiency drops, molecular hydrogen formation in the ISM can be very efficient even at high temperatures, depending on the physical characteristics of the surface

https://iopscience.iop.org/article/10.1086/381775/pdf

Formation on grain surfaces

Context. We describe and benchmark two sophisticated chemical models developed by the Heidelberg and Bordeaux astrochemistry groups.Aims. The main goal of this study is to elaborate on a few well-described tests for state-of-the-art astrochemical codes covering a range of physical conditions and chemical processes, in particular those aimed at constraining current and future interferometric observations of protoplanetary disks.Methods. We considered three physical models: a cold molecular cloud core, a hot core, and an outer region of a T Tauri disk. Our chemical network (for both models) is based on the original gas-phase osu_03_2008 ratefile and includes gas-grain interactions and a set of surface reactions for the H-, O-, C-, S-, and N-bearing molecules. The benchmarking was performed with the increasing complexity of the considered processes: (1) the pure gas-phase chemistry, (2) the gas-phase chemistry with accretion and desorption, and (3) the full gas-grain model with surface reactions. The chemical evolution is modeled within 109  years using atomic initial abundances with heavily depleted metals and hydrogen in its molecular form. Results. The time-dependent abundances calculated with the two chemical models are essentially the same for all considered physical cases and for all species, including the most complex polyatomic ions and organic molecules. This result, however, required a lot of effort to make all necessary details consistent through the model runs, e.g., definition of the gas particle density, density of grain surface sites, or the strength and shape of the UV radiation field. Conclusions. The reference models and the benchmark setup, along with the two chemical codes and resulting time-dependent abundances are made publicly available on the internet. This will facilitate and ease the development of other astrochemical models and provide nonspecialists with a detailed description of the model ingredients and requirements to analyze the cosmic chemistry as studied, e.g., by (sub-) millimeter observations of molecular lines.

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Chemistry in disks - IV. Benchmarking gas-grain chemical models with surface reactions

We survey the current situation regarding chemical modelling of the synthesis of molecules in the interstellar medium. The present state of knowledge concerning the rate coefficients and their uncertainties for the major gas-phase processes—ion-neutral reactions, neutral-neutral reactions, radiative association, and dissociative recombination—is reviewed. Emphasis is placed on those key reactions that have been identified, by sensitivity analyses, as ‘crucial’ in determining the predicted abundances of the species observed in the interstellar medium. These sensitivity analyses have been carried out for gas-phase models of three representative, molecule-rich, astronomical sources: the cold dense molecular clouds TMC-1 and L134N, and the expanding circumstellar envelope IRC +10216. Our review has led to the proposal of new values and uncertainties for the rate coefficients of many of the key reactions. The impact of these new data on the predicted abundances in TMC-1 and L134N is reported. Interstellar dust particles also influence the observed abundances of molecules in the interstellar medium. Their role is included in gas-grain, as distinct from gas-phase only, models. We review the methods for incorporating both accretion onto, and reactions on, the surfaces of grains in such models, as well as describing some recent experimental efforts to simulate and examine relevant processes in the laboratory. These efforts include experiments on the surface-catalyzed recombination of hydrogen atoms, on chemical processing on and in the ices that are known to exist on the surface of interstellar grains, and on desorption processes, which may enable species formed on grains to return to the gas-phase.

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Reaction Networks for Interstellar Chemical Modelling: Improvements and Challenges

Abridged: We detail and benchmark two sophisticated chemical models developed by the Heidelberg and Bordeaux astrochemistry groups. The main goal of this study is to elaborate on a few well-described tests for state-of-the-art astrochemical codes covering a range of physical conditions and chemical processes, in particular those aimed at constraining current and future interferometric observations of protoplanetary disks. We consider three physical models: a cold molecular cloud core, a hot core, and an outer region of a T Tauri disk. Our chemical network (for both models) is based on the original gas-phase osu_03_2008 ratefile and includes gas-grain interactions and a set of surface reactions for the H-, O-, C-, S-, and N-bearing molecules. The benchmarking is performed with the increasing complexity of the considered processes: (1) the pure gas-phase chemistry, (2) the gas-phase chemistry with accretion and desorption, and (3) the full gas-grain model with surface reactions. Using atomic initial abundances with heavily depleted metals and hydrogen in its molecular form, the chemical evolution is modeled within 10^9 years. The time-dependent abundances calculated with the two chemical models are essentially the same for all considered physical cases and for all species, including the most complex polyatomic ions and organic molecules. This result however required a lot of efforts to make all necessary details consistent through the model runs, e.g. definition of the gas particle density, density of grain surface sites, the strength and shape of the UV radiation field, etc. The reference models and the benchmark setup, along with the two chemical codes and resulting time-dependent abundances are made publicly available in the Internet: this http URL

/pdf/chemistry-in-disks-iv-benchmarking-gas-grain-chemical-models-2d6ohy0gik.pdf

Chemistry in Disks. IV. Benchmarking gas-grain chemical models with surface reactions

A stochastic model of grain surface chemistry, based on a master equation description of the probability distributions of reactive species on grains, is developed. For an important range of conditions, rates of molecule formation are limited by low accretion rates, so that the probability that a grain contains more than one reactive atom or molecule is small. We derive simple approximate expressions for these circumstances, and explore their validity through comparison with numerical solutions of the master equation for H, O and H, N, O reaction systems. A more detailed analysis of the range of validity of several analytic approximations and numerical solutions, based on exact analytical results for a model in which H and H2 are the only species, is also made. Though the use of our simple approximate expressions is computationally ecient, the solution of the master equation under the assumption that no grain contains more than two particles of each species usually gives more accurate results in the parameter regimes where the deterministic rate equation approach is inappropriate. The implementation of sparse matrix inversion techniques makes the use of such a truncated master equation solution method feasible for considerably more complicated surface chemistries than the ones we have examined here.

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A stochastic approach to grain surface chemical kinetics

A model of the grain surface chemistry involving the accretion of atoms of two dierent elements, X and Y, and their reactions to form species X2 ,X Y, and Y 2 was examined for a wide range of choices for the values of its three free parameters - the accretion rate of X and Y, the desorption rate of X and the grain surface sweeping time of Y, all considered relative to the grain surface sweeping rate of X. Relative production rates of the diatomics were calculated with five methods involving, respectively, a high-order truncation of the master equation, a low-order truncation of the master equation, the standard deterministic rate equation approach, a modified rate equation approach and a set of approximations which are in some cases appropriate for accretion dominated chemistry. The accuracies of the relative production rates calculated with the dierent methods were assessed for the wide range of model parameters. The more accurate of the low-truncation master equation calculations and the standard deterministic rate equation approach gives results which are in most cases within ten or twenty per cent of the results given by the high-truncation master equation calculations. For many cases, the more accurate of the low order truncation and the standard deterministic rate equation approaches is indicated by a consideration of the average number of atoms of the two species on the grain's surface.

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T. Toniazzo

Papers

A stochastic approach to grain surface chemical kinetics

Diffusive grain-surface chemistry involving the atoms and diatomic molecules of two elements