Silicon chemistry in dense clouds

The equilibrium chemistry of silicon in dense interstellar clouds is discussed in terms of both gas phase and grain surface reactions. Unless the metal depletion is very large, the gas phase scheme tends to over-produce SiO and/or SiS when compared to the observations of Sgr B2. The scheme also predicts SiC to be an abundant form of silicon. There is a great need for relevant laboratory data on the reactions used here—of the 35 rate coefficients adopted in the scheme, only three have been measured in the laboratory. Reactions between positively charged gas phase ions and small grains can lead to the formation of SiO and SiS. This type of reaction seems to offer a simple explanation for the observed differences between sulphur and silicon chemistry in dense clouds.     

New models of interstellar gas–grain chemistry – III. Solid CO2

Solid CO₂ is observed to be an abundant interstellar ice component towards both quiescent clouds and active star-forming regions. Our recent models of gas–grain chemistry, appropriate for quiescent regions, severely underproduce solid CO₂ at the single assumed gas density and temperature. In this paper, we investigate the sensitivity of our model results to changes in these parameters. In addition, we examine how the nature of the grain surface affects the results and also consider the role of the key surface reaction between O and CO. We conclude that the observed high abundance of solid CO₂ can be reproduced at reasonable temperatures and densities by models with diffusive surface chemistry, provided that the diffusion of heavy species such as O occurs efficiently.     

Observations of H2S toward OMC-1

Interstellar hydrogen sulfide (H2S) and its isotopic variant (H2(34)S) have been observed toward several positions in OMC-1 via their 1(10)-1(01) transitions near 168 GHz using the FCRAO 14 m telescope. We derive total column densities toward Orion(KL) for the extended ridge, for the plateau, and for the hot core, in addition to values for other positions in OMC-1. The fractional abundance of H2S (approximately 10(-9)) in the quiescent regions of OMC-1 seems to be difficult to explain by currently known ion-molecule reactions. The fractional abundance of H2S relative to H2 is enhanced by a factor of 1000 in the hot core and the plateau relative to the quiescent clouds. This enhancement may be a result of grain surface chemistry and/or of high-temperature gas-phase chemistry. From the nondetection of HDS in its 2(11)-2(12) transition, we estimate the abundance ratio [HDS]/H2S] < or = 0.02 in the hot core.     

On the contribution of grain surface reactions to interstellar molecule densities

Some recent arguments concerning the relevance of grain surface reactions to interstellar molecule formation are shown to be without foundation. It is probable that surface reactions, gas phase ion-molecule and neutral reactions all contribute to interstellar molecular densities.     

Equilibrium temperatures of interstellar iron grains

Because of the relatively low number densities found in typical interstellar clouds, molecules observed there must be produced by a combination of both two-body gas-phase reactions and surface reactions. The latter type includes various catalytic reactions, such as the formation of H₂ on transition metal grains. These reactions are very temperature dependent, the grain temperature appearing in the exponential of the rate equations. Because of the small heat capacities of the grains due to their small sizes, they may be subject to considerable fluctuations in temperature. This problem is examined for iron grains and found to be minimal for sizes greater than 100 Å. Steady-state equilibrium temperatures are then calculated for a size distribution of iron particles ranging from 10³ to 10⁹ atoms per grain by a refined method of an earlier work by one of us (RGT). The results are that iron grain temperatures are significantly greater than those of dielectric grains of comparable size in the same radiation field.     

Probing the surfaces of interstellar dust grains: the adsorption of CO at bare grain surfaces

A solid-state feature was detected at around 2175 cm⁻¹ towards 30 embedded young stellar objects in spectra obtained using the Infrared Spectrometer and Array Camera at the European Southern Observatory Very Large Telescope. We present results from laboratory studies of CO adsorbed at the surface of zeolite wafers, where absorption bands were detected at 2177 and 2168 cm⁻¹ (corresponding to CO chemisorbed at the zeolite surface) and 2130 cm⁻¹ (corresponding to CO physisorbed at the zeolite surface), providing an excellent match to the observational data. We propose that the main carrier of the 2175-band is CO chemisorbed at bare surfaces of dust grains in the interstellar medium. This result provides the first direct evidence that gas–surface interactions do not have to result in the formation of ice mantles on interstellar dust. The strength of the 2175-band is estimated to be  ∼4 × 10⁻¹⁹ cm  molecule⁻¹. The abundance of CO adsorbed at bare grain surfaces ranges from 0.06 to 0.16 relative to H₂O ice, which is, at most, half of the abundance (relative to H₂O ice) of CO residing in H₂O-dominated ice environments. These findings imply that interstellar grains have a large (catalytically active) surface area, providing a refuge for interstellar species. Consequently, the potential exists for heterogeneous chemistry to occur involving CO molecules in unique surface chemistry pathways not currently considered in gas grain models of the interstellar medium.     

Laboratory annealing experiments of refractory silicate grain analogs using differential scanning calorimetry

Abstract– Exothermic reactions during the annealing of laboratory synthesized amorphous magnesium‐bearing silicate particles used as grain analogs of cosmic dust were detected by differential scanning calorimetry (DSC) in air. With infrared spectroscopy and transmission electron microscopy, we show that cosmic dust could possibly undergo fusion to larger particles, with oxidation of magnesium silicide and crystallization of forsterite as exothermic reactions in the early solar system. The reactions begin at approximately 425, approximately 625, and approximately 1000 K, respectively, and the reaction energies (enthalpies) are at least 727, 4151, and 160.22 J g⁻¹, respectively. During the crystallization of forsterite particles, the spectral evolution of the 10 μm feature from amorphous to crystalline was observed to begin at lower temperature than the crystallization temperature of 1003 K. During spectral evolution at lower temperature, nucleation and/or the formation of nanocrystallites of forsterite at the surface of the grain analogs was observed.     

On water ice formation in interstellar clouds

A model is proposed for the formation of water ice mantles on grains in interstellar clouds. This occurs by direct accretion of monomers from the gas, be they formed by gas or surface reactions. The formation of the first monolayer requires a minimum extinction of interstellar radiation, sufficient to lower the grain temperature to the point where thermal evaporation of monomers is just offset by monomer accretion from the gas. This threshold is mainly determined by the adsorption energy of water molecules on the grain material; for hydrocarbon material, chemical simulation places this energy between 0.5 and 2 kcal mol⁻¹, which sets the (true) visible extinction threshold at a few magnitudes. However, realistic distributions of matter in a cloud will usually add to this an unrelated amount of cloud core extinction, which can explain the large dispersion of observed (apparent) thresholds. Once the threshold is crossed, all available water molecules in the gas are quickly adsorbed, because the grain cools down and the adsorption energy on ice is higher than on bare grain. The relative thickness of the mantle, and, hence, the slope of  τ₃( A _v)  depend only on the available water vapour, which is a small fraction of the oxygen abundance. Chemical simulation was also used to determine the adsorption sites and energies of O and OH on hydrocarbons and study the dynamics of formation of water molecules by surface reactions with gaseous H atoms, as well as their chances to stick in situ.     

Interstellar chemistry recorded by nitrogen isotopes in Solar System organic matter

The relationship between the organic and D/H ratios in small Solar System bodies (meteorites, interplanetary dust and comets) suggests that isotopic exchange reactions taking place at various temperatures are at the origin of the observed variations. These relationships are used to determine the exothermicity (ΔE) of ion-molecule reactions that fractionated the nitrogen isotopic ratio in the presolar molecular cloud; that is ΔE=43±10 K. Comparison with current models of interstellar chemistry suggests that such a value could be achieved by condensation of ¹⁵N-rich gas-phase precursors onto grain surfaces and their further isolation from the gas by incorporation into large macromolecular structures.     

Understanding radical chemistry in the marine boundary layer

This paper discusses the development of modelling techniques to understand radical chemistry in the troposphere. Data from recent field campaigns, combined with a detailed chemical mechanism, are used to construct box models to simulate radical concentrations. Comparisons with measurements are discussed. These mechanisms are typically of the order of 1700 reactions and 500 species, and are too large to gain an understanding of which reactions are driving the chemistry. Sensitivity analysis has been used to reduce the mechanisms. For reduced mechanisms that can predict the daytime concentrations of OH and HO₂ to within 20% of the full mechanism, we can represent the chemistry of a remote coastal site in the marine boundary layer with only 25 reactions and 17 species. On a semi-polluted day, when the same site was also affected by isoprene from a local biogenic source, the chemistry can be represented by 64 reactions and 42 species. Algebraic steady-state solutions have been derived based on the reduced mechanisms. A local sensitivity analysis has been carried out to identify reactions and input concentrations that lead to the highest uncertainties in the model output. A global sensitivity analysis suggests that the 2σ uncertainty in model [OH] is ±42% and for model [HO₂] is ±25%.     

Kinetic Monte Carlo method for simulating astrochemical kinetics: Hydrogen chemistry in diffuse clouds

We perform numerical simulations of the molecular hydrogen production on the surface of interstellar dust grains and its dissociation by the ultraviolet background in conditions typical for the interstellar medium. The kinetic version of the Monte Carlo method is used for the modeling of the catalytic chemical reactions on the surface of the dust fraction and in the surrounding medium. Our simulations show the importance of the interstellar dust particles for hydrogen chemistry in diffuse molecular clouds.     

New models of interstellar gas–grain chemistry – I. Surface diffusion rates

In recent years it has become evident that large differences can exist between model results of grain-surface chemistry obtained from a rate equation approach and from a Monte Carlo technique. This dichotomy has led to the development of a modified rate equation method, in which a key element is the artificial slowing down of the diffusion rate of surface hydrogen atoms. Recent laboratory research into the surface diffusion rate of atomic hydrogen suggests that atomic hydrogen moves more slowly on grains than heretofore assumed. This research appears to lessen the need for modifications to the rate equation method. Based on the new laboratory work, we have developed appropriate models of gas-phase and grain-surface chemistry in quiescent dense cloud cores to examine the chemical effects of slowing down the rate at which atomic H can scan over dust surfaces. Furthermore, we have investigated the effect of slowing down the rate at which all species can move over grain surfaces.     

Chemical evolution of the gas in C-type shocks in dark clouds

A magnetohydrodynamic model of a steady, transverse C-type shock in a dense molecular cloud is presented. A complete gas–grain chemical network is taken into account: the gas-phase chemistry, the adsorption of gas species on dust grains, various desorption mechanisms, the grain surface chemistry, the ion neutralization on dust grains, the sputtering of grain mantles. The population densities of energy levels of ions CI, CII and OI and molecules H₂, CO, H₂O are computed in parallel with the dynamical and chemical rate equations. The large velocity gradient approximation is used in the line radiative transfer calculations. The simulations consist of two steps: (i) modelling of the chemical and thermal evolution of a static molecular cloud and (ii) shock simulations. A comparison is made with the results of publicly available models of similar physical systems.The focus of the paper is on the chemical processing of gas material and ice mantles of dust grains by the shock. Sputtering of ice mantles takes place in the shock region close to the temperature peak of the neutral gas. At high shock speeds, molecules ejected from ice mantles are effectively destroyed in hot gas, and their survival time is low—of the order of dozens of years. After a passage of high-speed C-type shock, a zone of high abundance of atomic hydrogen appears in the cooling postshock gas that triggers formation of complex organic species such as methanol. It is shown that abundances of some complex organic molecules (COMs) in the postshock region can be much higher than in the preshock gas. These results are important for interpretation of observations of COMs in protostellar outflows.     

Silica grain catalysis of methanol formation

The specific catalytic effect of a silica grain on the formation of methanol via the sequential addition of H atoms to CO adsorbed on the surface is investigated. A negatively charged defect on a siliceous edingtonite surface is found to reduce the gas phase barriers for the H + CO_ads and H + H₂C=O_ads reactions by 770 and 399 K, respectively, when compared to the same reactions in the gas phase. The catalytic effect of negatively charged surface sites could also be applicable to the hydrogenation of other adsorbed unsaturated species. However, the activation energies on the surface defect are still too large (1150 and 2230 K) for CH₃OH to form efficiently at 10–20 K in the interstellar medium via a classical mechanism. It is therefore suggested that quantum mechanical tunnelling through the activation barrier is required for these hydrogen addition reactions to proceed at such temperatures. The calculations show that because the adsorption energies of CO and H₂C=O on the negatively charged defect are substantial, CH₃OH may form efficiently during the warm-up period in star-forming regions.     

The charge state in a highly dense molecular cloud

We have calculated the desorption rates of both physisorbed and chemisorbed ions from grain surfaces, due to the temperature increase at densities higher than 10^–13 g cm^–3. It has been found that physisorbed ions desorb from grain surfaces at neutral densities ofn>1.3×10¹¹ cm^–3, assuming that the desorption energyD is equal to 0.1 eV, while the desorption of chemisorbed ions from grain surface can only occur at neutral densities ofn>10¹⁵ cm^–3, at which point thermal ionization becomes more dominant.The electrons are assumed to be emitted from grain surfaces in a manner similar to the thermonic emission from heated solid surfaces. It was found that the temperature at which electrons are emitted from negatively charged grains depends on the value of the work function of the material of the grain.The charge state has been calculated for two limiting cases. Neglecting the grain surface reactions in case 1, the resulting relative charge density represents an upper limit, such that the electrical conductivity remains high. In this situation the magnetic flux dissipation is mainly contributed by ambipolar diffusion. In the second case, it has been assumed that the charged particles are chemically adsorbed on grain surfaces such that their desorption is negligible. In this case the charge density decreases sharply with increase of neutral density. Therefore, the electrical conductivity decreases sufficiently and Ohmic dissipation becomes effective.     

3‐D elemental and isotopic composition of presolar silicon carbides

Abstract— Thirteen presolar silicon carbide grains—three of supernova (SN) origin and ten of asymptotic giant branch (AGB) star origin—were examined with time‐of‐flight‐secondary ion mass spectrometry (TOF‐SIMS). The grains had been extracted from two different meteorites—Murchison and Tieschitz—using different acid residue methods. At high lateral resolution of ～300 nm, isotopic and elemental heterogeneities within the micrometer‐sized grains were detected. The trace elemental abundances, when displayed in two‐element correlation plots, of Li, Mg, K, and Ca show a clear distinction between the two different meteoritic sources. The different concentrations might be attributed to differences of the host meteorites and/or of extraction methods whereas the stellar source seems to be less decisive. In one SN grain with ²⁶Mg‐enrichment from extinct ²⁶Al, the acid treatment, as part of the grain separation procedure, affected the Mg/Al ratio in the outer rim and therefore the inferred initial ²⁶Al/²⁷Al ratio. A second SN grain exhibits a lateral heterogeneity in ²⁶Al/²⁷Al, which either is due to residual Al‐rich contamination on the grain surface or to the condensation chemistry in the SN ejecta.     

The effect of grain size distribution on H2 formation rate in the interstellar medium

The formation of molecular hydrogen  (H₂)  in the interstellar medium takes place on the surfaces of dust grains. Hydrogen molecules play a role in gas-phase reactions that produce other molecules, some of which serve as coolants during gravitational collapse and star formation. Thus, the evaluation of the production rate of hydrogen molecules and its dependence on the physical conditions in the cloud are of great importance. Interstellar dust grains exhibit a broad size distribution in which the small grains capture most of the surface area. Recent studies have shown that the production efficiency strongly depends on the grain composition and temperature as well as on its size. In this paper, we present a formula that provides the total production rate of  H₂  per unit volume in the cloud, taking into account the grain composition and temperature as well as the grain size distribution. The formula agrees very well with the master equation results. It shows that for a physically relevant range of grain temperatures, the production rate of  H₂  is significantly enhanced due to their broad size distribution.     

The source and origin of the exotic component and kreep-rich materials on the moon

Trace element concentrations in the lunar soils are inversely correlated to the grain size -i.e., the finer the grain size the higher the trace element content. These enrichments are attributed to the combined effects of comminution of local rocks and a preferential addition of the exotic component to the finer grain sizes. The exotic component appears similar to Fra Mauro materials and there is a strong correlation between the percentage of exotic component and distance from Fra Mauro site. Based on trace element characteristics, it is suggested that exotic component is finely comminuted KREEP and is dispersed on the lunar surface from the Fra Mauro and other high gamma-ray active regions in the Imbrium-Procellarum regions. The major and trace element chemistry of these materials is considered to be due to impact melting of trace-element-rich accessory phases followed by variable dilution with partial melts of the bulk solid phases of plagioclase, low Ca pyroxene and olivine. The generation of KREEP and exotic component is an impact produced local process and need not imply a global occurrence of radioactive crust on the Moon.     

Detection of interstellar hydrogen sulfide in cold, dark clouds

We have detected interstellar hydrogen sulfide (H2S) toward the cold, dark clouds L134N and TMC 1. We derive total column densities of approximately 2.6 x 10(13) cm-2 and approximately 7.0 x 10(12) cm-2 at the SO peak of L134N and at the NH3 peak of TMC 1, respectively. Since the expected gas phase reactions leading to the formation of H2S are thought to be endothermic, grain surface reactions may play a major role in the synthesis of this species in cold, dark clouds. If the carbon abundance is high and grain surface reactions are the dominant formation route, H2CS would be expected to form instead of H2S, and the abundances of H2CS have been observed to be high where those of H2S are low in L134N and TMC 1.     

Insights into secondary reactions occurring during atmospheric ablation of micrometeoroids

Ablation of micrometeoroids during atmospheric entry yields volatile gases such as water, carbon dioxide, and sulfur dioxide, capable of altering atmospheric chemistry and hence the climate and habitability of the planetary surface. While laboratory experiments have revealed the yields of these gases during laboratory simulations of ablation, the reactions responsible for the generation of these gases have remained unclear, with a typical assumption being that species simply undergo thermal decomposition without engaging in more complex chemistry. Here, pyrolysis–Fourier transform infrared spectroscopy reveals that mixtures of meteorite‐relevant materials undergo secondary reactions during simulated ablation, with organic matter capable of taking part in carbothermic reduction of iron oxides and sulfates, resulting in yields of volatile gases that differ from those predicted by simple thermal decomposition. Sulfates are most susceptible to carbothermic reduction, producing greater yields of sulfur dioxide and carbon dioxide at lower temperatures than would be expected from simple thermal decomposition, even when mixed with meteoritically relevant abundances of low‐reactivity Type IV kerogen. Iron oxides were less susceptible, with elevated yields of water, carbon dioxide, and carbon monoxide only occurring when mixed with high abundances of more reactive Type III kerogen. We use these insights to reinterpret previous ablation simulation experiments and to predict the reactions capable of occurring during ablation of carbonaceous micrometeoroids in atmospheres of different compositions.