A new and simple approach to determine the abundance of hydrogen molecules on interstellar ice mantles (original) (raw)
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Minerals
The universe is molecularly rich, comprising from the simplest molecule (H2) to complex organic molecules (e.g., CH3CHO and NH2CHO), some of which of biological relevance (e.g., amino acids). This chemical richness is intimately linked to the different physical phases forming Solar-like planetary systems, in which at each phase, molecules of increasing complexity form. Interestingly, synthesis of some of these compounds only takes place in the presence of interstellar (IS) grains, i.e., solid-state sub-micron sized particles consisting of naked dust of silicates or carbonaceous materials that can be covered by water-dominated ice mantles. Surfaces of IS grains exhibit particular characteristics that allow the occurrence of pivotal chemical reactions, such as the presence of binding/catalytic sites and the capability to dissipate energy excesses through the grain phonons. The present know-how on the physicochemical features of IS grains has been obtained by the fruitful synergy of as...
Monte Carlo simulation to investigate the formation of molecular hydrogen and its deuterated forms
New Astronomy, 2015
H 2 is the most abundant interstellar species. Its deuterated forms (HD and D 2) are also significantly abundant. Huge abundances of these molecules could be explained by considering the chemistry occurring on the interstellar dust. Because of its simplicity, Rate equation method is widely used to study the formation of grain-surface species. However, since recombination efficiency of formation of any surface species are heavily dependent on various physical and chemical parameters, Monte Carlo method would be best method suited to take care of randomness of the processes. We perform Monte Carlo simulation to study the formation of H 2 , HD and D 2 on interstellar ices. Adsorption energies of surface species are the key inputs for the formation of any species on interstellar dusts but binding energies of deuterated species are yet to known with certainty. A zero point energy correction exists between hydrogenated and deuterated species which should be considered while modeling the chemistry on the interstellar dusts. Following some earlier studies, we consider various sets of adsorption energies to study the formation of these species in diverse physical circumstances. As expected, noticeable difference in these two approaches (Rate equation method and Monte Carlo method) is observed for production of these simple molecules on interstellar ices. We introduce two factors, namely, S f and β to explain these discrepancies: S f is a scaling factor, which could be used to correlate discrepancies between Rate equation and Monte Carlo methods. β factor indicates the formation efficiency under various circumstances. Higher values of β indicates a lower production efficiency. We found that β increases with a decrease in rate of accretion from gas phase to grain phase.
Monte Carlo Modeling of Astrophysically-Relevant Temperature-Programmed Desorption Experiments
Proceedings of the International Astronomical Union
The formation of molecules in the interstellar medium is significantly driven by grain chemistry, ranging from simple (e.g. H2) to relatively complex (e.g. CH3OH) products. The movement of atoms and molecules on amorphous ice surfaces is not well constrained, and this is a quintessential component of surface chemistry. We show that ice structure created by utilizing an off-lattice Monte Carlo kinetics model is highly dependent on deposition parameters (i.e. angle, rate, and temperature). The model, thus far, successfully predicts the densities of deposition rate- and temperature-dependent laboratory experiments. The simulations indicate, when angle and deposition rate increase, the density decreases. On the other hand, temperature has the opposite effect and will increase the density. We can make ices with desired densities and monitor how molecules, like CO, percolate through H2O ice pores. The strength of this model lies in the ability to replicate TPD-like experiments by monitori...
2013
Interstellar ices are layers of molecules deposited on ne dust grains in dark and dense molecular cloud cores. Subsurface ice has been considered in a few astrochemical models, which have shown that it can be of great importance. The aim of this work is to introduce an established subsurface ice description into the state-of-the-art astrochemical model ALCHEMIC. The model has been developed by the Heidelberg astrochemistry group. The result is an up-to-date model for interstellar molecular cloud research with possible application for protoplanetary disks.
Modeling the chemistry in the icy mantles of interstellar grains
Proceedings of the International Astronomical Union, 2017
The diffusion and photoprocessing of molecules within interstellar ices has been verified experimentally but often not fully included in astrochemical models. With models that consider photodissociation, binary reactions, and diffusion for molecules on the surface and in bulk ice, we explored the chemistry of interstellar and circumstellar ices in gravitationally contracting low-mass starless and prestellar cores, and a protostellar envelope.Results. Photoprocessing gradually converts mixed H2O and CO ices into CO2 and allows for a late synthesis of icy organic species. Different layers within a single icy mantle favor the synthesis different species. Deuterium-rich molecules are concentrated on the outer surface of ice. Formation of organic molecules in bulk ice lowers their average deuterium content. The abundances of major icy species can be changed by about 25-50 % because of ice photoprocessing. Inter-layer diffusion of icy species allows sequential evaporation in protostellar ...
The isomerization of hydrogen cyanide to hydrogen isocyanide on icy grain surfaces is investigated by an accurate composite method (jun-Cheap) rooted in the coupled cluster ansatz and by density functional approaches. After benchmarking density functional predictions of both geometries and reaction energies against jun-Cheap results for the relatively small model system HCN•••(H 2 O) 2 the best performing DFT methods are selected. A large cluster containing 20 water molecules is then employed within a QM/QM approach to include a realistic environment mimicking the surface of icy grains. Our results indicate that four water molecules are directly involved in a proton relay mechanism, which strongly reduces the activation energy with respect to the direct hydrogen transfer occurring in the isolated molecule. Further extension of the size of the cluster up to 192 water molecules in the framework of a three-layer QM/QM /MM model has a negligible effect on the energy barrier ruling the isomerization. Computation of reaction rates by transition state theory indicates that on 1
Modeling the processing of interstellar ices by energetic particles
Astronomy & Astrophysics, 2013
Context. Interstellar ice is the main form of metal species in dark molecular clouds. Experiments and observations have shown that the ice is significantly processed after the freeze-out of molecules onto grains. The processing is caused by cosmic-ray particles and cosmic-ray-induced UV photons. These transformations are included in current astrochemical models only to a very limited degree. Aims. We aim to establish a model of the "cold" chemistry in interstellar ices and to evaluate its general impact on the composition of interstellar ices. Methods. The ice was treated as consisting of two layers -the surface and the mantle (or subsurface) layer. Subsurface chemical processes are described with photodissociation of ice species and binary reactions on the surfaces of cavities inside the mantle. Hydrogen atoms and molecules can diffuse between the layers. We also included deuterium chemistry.
SPH simulations of clumps formation by dissipative collision of molecular clouds
2000
Computer experiments of interstellar cloud collisions were performed with a new smoothed-particlehydrodynamics (SPH) code. The SPH quantities were calculated by using spatially adaptive smoothing lengths and the SPH fluid equations of motion were solved by means of a hierarchical multiple timescale leapfrog. Such a combination of methods allows the code to deal with a large range of hydrodynamic quantities. A careful treatment of gas cooling by H, H 2 , CO and H ii, as well as a heating mechanism by cosmic rays and by H 2 production on grains surface, were also included in the code. The gas model reproduces approximately the typical environment of dark molecular clouds. The experiments were performed by impinging two dynamically identical spherical clouds onto each other with a relative velocity of 10 km s −1 but with a different impact parameter for each case. Each object has an initial density profile obeying an r −1-law with a cutoff radius of 10 pc and with an initial temperature of 20 K. As a main result, cloud-cloud collision triggers fragmentation but in expense of a large amount of energy dissipated, which occurred in the head-on case only. Offcenter collision did not allow remnants to fragment along the considered time (∼ 6 Myr). However, it dissipated a considerable amount of orbital energy. Structures as small as 0.1 pc, with densities of ∼ 10 4 cm −3 , were observed in the more energetic collision.
Formation of molecular hydrogen on analogues of interstellar dust grains: experiments and modelling
Journal of Physics: Conference Series, 2005
Molecular hydrogen has an important role in the early stages of star formation as well as in the production of many other molecules that have been detected in the interstellar medium. In this review we show that it is now possible to study the formation of molecular hydrogen in simulated astrophysical environments. Since the formation of molecular hydrogen is believed to take place on dust grains, we show that surface science techniques such as thermal desorption and time-of-flight can be used to measure the recombination efficiency, the kinetics of reaction and the dynamics of desorption. The analysis of the experimental results using rate equations gives useful insight on the mechanisms of reaction and yields values of parameters that are used in theoretical models of interstellar cloud chemistry.
ACS earth and space chemistry, 2017
Carbon monoxide (CO) is the second most abundant gas-phase molecule after molecular hydrogen (H 2) of the Interstellar Medium (ISM). In Molecular Clouds (MCs), an important component of the ISM, it adsorbs at the surface of core grains, usually made of Mg/Fe silicates, and originates Complex Organic Molecules (COMs) through the catalytic power of active sites at the grain surfaces. To understand the atomistic, energetic and spectroscopic details of the CO adsorption on core grains, we resorted to Density Functional Theory based on the hybrid B3LYP-D* functional inclusive of dispersion contribution. We modeled the complexity of interstellar silicate grains by studying adsorption events on a large set of infinite extended surfaces cut out from the bulk Mg 2 SiO 4 forsterite, the Mg end-member of olivines (Mg 2x Fe 2-2x SiO 4), also a very common mineral on Earth crust. Energetic and structural features indicate that CO get exclusively physisorbed, with binding energy values in the 23-68 kJ mol-1 range. Detailed analysis of data revealed that dispersive interactions are relevant together with the important electrostatic contribution due to the quadrupolar nature of the CO molecule. We performed a full thermodynamic treatment of the CO adsorption at the very low temperature typical of the ISM as well as a full spectroscopic characterization of the CO stretching frequency, which we prove to be extremely sensitive to the local nature of the surface-active site of adsorption. We also performed a detailed kinetic analysis of CO desorption from the surface models at different temperatures characterizing the colder regions of the ISM. Our computed data could be incorporated in the various astrochemical models of interstellar grains developed so far and thus contribute to improve the description of the complex chemical network occurring at their surfaces.