Gas-grain chemistry in cold interstellar cloud cores with a microscopic Monte Carlo approach to surface chemistry

AIM: We have recently developed a microscopic Monte Carlo approach to study surface chemistry on interstellar grains and the morphology of ice mantles. The method is designed to eliminate the problems inherent in the rate-equation formalism to surface chemistry. Here we report the first use of this method in a chemical model of cold interstellar cloud cores that includes both gas-phase and surface chemistry. The surface chemical network consists of a small number of diffusive reactions that can produce molecular oxygen, water, carbon dioxide, formaldehyde, methanol and assorted radicals. METHOD: The simulation is started by running a gas-phase model including accretion onto grains but no surface chemistry or evaporation. The starting surface consists of either flat or rough olivine. We introduce the surface chemistry of the three species H, O and CO in an iterative manner using our stochastic technique. Under the conditions of the simulation, only atomic hydrogen can evaporate to a significant extent. Although it has little effect on other gas-phase species, the evaporation of atomic hydrogen changes its gas-phase abundance, which in turn changes the flux of atomic hydrogen onto grains. The effect on the surface chemistry is treated until convergence occurs. We neglect all non-thermal desorptive processes. RESULTS: We determine the mantle abundances of assorted molecules as a function of time through 2x10^5 yr. Our method also allows determination of the abundance of each molecule in specific monolayers. The mantle results can be compared with observations of water, carbon dioxide, carbon monoxide, and methanol ices in the sources W33A and Elias 16. Other than a slight underproduction of mantle CO, our results are in very good agreement with observations.Comment: 13 pages, 7 figures, to be published in A. &

H2 reformation in post-shock regions

H2 formation is an important process in post-shock regions, since H2 is an active participant in the cooling and shielding of the environment. The onset of H2 formation therefore has a strong effect on the temperature and chemical evolution in the post shock regions. We recently developed a model for H2 formation on a graphite surface in warm conditions. The graphite surface acts as a model system for grains containing large areas of polycyclic aromatic hydrocarbon structures. Here this model is used to obtain a new description of the H2 formation rate as a function of gas temperature that can be implemented in molecular shock models. The H2 formation rate is substantially higher at high gas temperatures as compared to the original implementation of this rate in shock models, because of the introduction of H atoms which are chemically bonded to the grain (chemisorption). Since H2 plays such a key role in the cooling, the increased rate is found to have a substantial effect on the predicted line fluxes of an important coolant in dissociative shocks [O I] at 63.2 and 145.5 micron. With the new model a better agreement between model and observations is obtained. Since one of the goals of Herschel/PACS will be to observe these lines with higher spatial resolution and sensitivity than the former observations by ISO-LWS, this more accurate model is very timely to help with the interpretation of these future results.Comment: 12 pages, 3 figures, 1 table, accepted in MNRAS Letter

A kinetic Monte Carlo study of desorption of H2 from graphite (0001)

The formation of H2 in the interstellar medium proceeds on the surfaces of silicate or carbonaceous particles. To get a deeper insight of its formation on the latter substrate, this letter focuses on H2 desorption from graphite (0001) in Temperature-Programmed-Desorption Monte-Carlo simulations. The results are compared to experimental results which show two main peaks and an intermediate shoulder for high initial coverage. The simulation program includes barriers obtained by ab-initio methods and is further optimised to match two independent experimental observations. The simulations reproduce the two experimental observed desorption peaks. Additionally, a possible origin of the intermediate peak is given.Comment: 9 pages, 5 figures, Chem. Phys. Lett. in pres

MONTE CARLO SIMULATIONS OF H2_{2} FORMATION ON GRAINS OF VARYING SURFACE ROUGHNESS

{V. Pirronello \emph{et. al.{N. Katz \emph{et. al.{A. Li and B. Draine \textit{Astrophys. J.Author Institution: Department of Physics, The Ohio State University, Columbus, OH 43210; Departments of Physics, Astronomy, and Chemistry, The Ohio State University, Columbus, OH 43210Although the formation of molecular hydrogen in diffuse and dense regions of the neutral interstellar medium occurs by the recombination of hydrogen atoms on the surfaces of dust particles, the detailed mechanisms by which this process occurs are still in doubt. Experimental studies using a technique known as temperature-programmed desorption (TPD) indicate that for olivine and amorphous carbon, H$_{2}$ is formed by the so-called Langmuir-Hinshelwood mechanism, which involves the diffusion of one or both H atoms to find one another on a given granular surface} \textit{Astrophys. J.} \underline{\textbf{483}}(L131), 1997, V. Pirronello \emph{et. al.} \textit{Astrophys. J.} \underline{\textbf{475}}(L69), 1997, V. Pirronello \emph{et. al.} \textit{Astron. Astrophys.} \underline{\textbf{344}}(681), 1999}. Based on these measurements and a flux of H atoms relevant for diffuse interstellar clouds, it was deduced that the surface temperature range over which efficient H$_{2}$ formation occurs is very small for olivine (6-10 K) and for amorphous carbon (13-17 K)} \textit{Astrophys. J.} \underline{\textbf{522}}(305), 1999}. Considering that the surface temperature for interstellar grains in unshielded regions is probably closer to 20 K} \underline{\textbf{554}}(778), 2001}, it appears that if the experimental results and inferences are correct, then olivine and amorphous carbon are not realistic candidates for granular surfaces in diffuse clouds, where H$_{2}$ formation is known to be efficient. These models, however, all assume flat surfaces characterised by single values of the energy parameters for hydrogen-atom adsorbates. We performed continuous-time random-walk Monte Carlo simulations of H$_{2}$ formation on a variety of grain surfaces of varying roughness based on olivine and amorphous carbon. With these inhomogeneous surfaces, we find that the temperature range over which efficient H$_{2}$ formation occurs in the ISM is much larger than it is for flat surfaces. Our results show, in particular, that the formation of H$_{2}$ on all but the smoothest interstellar grains occurs efficiently at typical surface temperatures in diffuse interstellar clouds

MONTE CARLO SIMULATIONS ON THE FORMATION OF INTERSTELLAR ICE

Author Institution: Department of Physics, The Ohio State University, Columbus, OH 43210; Departments of Physics, Astronomy, and Chemistry, The Ohio State University, Columbus, OH 43210Formation and destruction of ice mantles play an important role in many astrophysical environments including regions where new stars and planets are born and on icy bodies in our own solar system such as comets. Although we have a general picture about how the mantles form and desorb again, little is known about the basic physical processes. Even the formation of the most abundant ice -- H$_2$O -- is not fully understood. Observations indicate that in dense clouds dust particles are covered by several to hundreds of monolayers of water ice. In diffuse clouds, however, the water ice mantles constitute less than one monolayer, the current detection limit. We studied the formation of ice mantles under several conditions, both in diffuse and dense regions using the continuous-time, random-walk Monte Carlo method. We considered a set of surface reactions with reactants that either accrete onto a grain surface or are products of other surface reactions that remain on the grain. We further considered photodissociation processes for surface species caused by ultra-violet photons. In diffuse areas, these photons are mainly those of the external radiation field and photodissociation is the main destruction route for ice. In dense sources, the much smaller flux of photons arises indirectly from cosmic ray bombardment of H$_{2}$, which produces ions and electrons. We will present our results and comment on how they relate to current observations

Reaction Networks For Interstellar Chemical Modelling: Improvements and Challenges

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 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-catalysed 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.Comment: Accepted for publication in Space Science Review

Quantification of segregation dynamics in ice mixtures

(Abridged) The observed presence of pure CO2 ice in protostellar envelopes is attributed to thermally induced ice segregation, but a lack of quantitative experimental data has prevented its use as a temperature probe. Quantitative segregation studies are also needed to characterize diffusion in ices, which underpins all ice dynamics and ice chemistry. This study aims to quantify the segregation mechanism and barriers in different H2O:CO2 and H2O:CO ice mixtures covering a range of astrophysically relevant ice thicknesses and mixture ratios. The ices are deposited at 16-50 K under (ultra-)high vacuum conditions. Segregation is then monitored at 23-70 K as a function of time, through infrared spectroscopy. Thin (8-37 ML) H2O:CO2/CO ice mixtures segregate sequentially through surface processes, followed by an order of magnitude slower bulk diffusion. Thicker ices (>100 ML) segregate through a fast bulk process. The thick ices must therefore be either more porous or segregate through a different mechanism, e.g. a phase transition. The segregation dynamics of thin ices are reproduced qualitatively in Monte Carlo simulations of surface hopping and pair swapping. The experimentally determined surface-segregation rates for all mixture ratios follow the Ahrrenius law with a barrier of 1080[190] K for H2O:CO2 and 300[100] K for H2O:CO mixtures. During low-mass star formation H2O:CO2 segregation will be important already at 30[5] K. Both surface and bulk segregation is proposed to be a general feature of ice mixtures when the average bond strengths of the mixture constituents in pure ice exceeds the average bond strength in the ice mixture.Comment: Accepted for publication in A&A. 25 pages, including 13 figure

Water formation at low temperatures by surface O2 hydrogenation I: characterization of ice penetration

Water is the main component of interstellar ice mantles, is abundant in the solar system and is a crucial ingredient for life. The formation of this molecule in the interstellar medium cannot be explained by gas-phase chemistry only and its surface hydrogenation formation routes at low temperatures (O, O2, O3 channels) are still unclear and most likely incomplete. In a previous paper we discussed an unexpected zeroth-order H2O production behavior in O2 ice hydrogenation experiments compared to the first-order H2CO and CH3OH production behavior found in former studies on hydrogenation of CO ice. In this paper we experimentally investigate in detail how the structure of O2 ice leads to this rare behavior in reaction order and production yield. In our experiments H atoms are added to a thick O2 ice under fully controlled conditions, while the changes are followed by means of reflection absorption infrared spectroscopy (RAIRS). The H-atom penetration mechanism is systematically studied by varying the temperature, thickness and structure of the O2 ice. We conclude that the competition between reaction and diffusion of the H atoms into the O2 ice explains the unexpected H2O and H2O2 formation behavior. In addition, we show that the proposed O2 hydrogenation scheme is incomplete, suggesting that additional surface reactions should be considered. Indeed, the detection of newly formed O3 in the ice upon H-atom exposure proves that the O2 channel is not an isolated route. Furthermore, the addition of H2 molecules is found not to have a measurable effect on the O2 reaction channel.Comment: 1 page, 1 figur

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