Context. Hydrogenation reactions of CO in inter-and circumstellar ices are regarded as an important starting point in the formation of more complex species. Previous laboratory measurements by two groups of the hydrogenation of CO ices provided controversial results about the formation rate of methanol. Aims. Our aim is to resolve this controversy by an independent investigation of the reaction scheme for a range of H-atom fluxes and different ice temperatures and thicknesses. To fully understand the laboratory data, the results are interpreted theoretically by means of continuous-time, random-walk Monte Carlo simulations. Methods. Reaction rates are determined by using a state-of-the-art ultra high vacuum experimental setup to bombard an interstellar CO ice analog with H atoms at room temperature. The reaction of CO + H into H 2 CO and subsequently CH 3 OH is monitored by a Fourier transform infrared spectrometer in a reflection absorption mode. In addition, after each completed measurement, a temperature programmed desorption experiment is performed to identify the produced species according to their mass spectra and to determine their abundance. Different H-atom fluxes, morphologies, and ice thicknesses are tested. The experimental results are interpreted using Monte Carlo simulations. This technique takes into account the layered structure of CO ice. Results. The formation of both formaldehyde and methanol via CO hydrogenation is confirmed at low temperature (T = 12−20 K). We confirm that the discrepancy between the two Japanese studies is caused mainly by a difference in the applied hydrogen atom flux, as proposed by Hidaka and coworkers. The production rate of formaldehyde is found to decrease and the penetration column to increase with temperature. Temperature-dependent reaction barriers and diffusion rates are inferred using a Monte Carlo physical chemical model. The model is extended to interstellar conditions to compare with observational H 2 CO/CH 3 OH data.

The results of the sixth blind test of organic crystal structure prediction methods are presented and discussed, highlighting progress for salts, hydrates and bulky flexible molecules, as well as on-going challenges.

Although still poorly understood, the chemistry that occurs on the surfaces of interstellar dust particles profoundly affects the growth of molecules in the interstellar medium. An important set of surface reactions produces icy mantles of many monolayers in cold and dense regions. The monolayers are dominated by water ice, but also contain CO, CO 2 , and occasionally methanol, as well as minor constituents. In this paper, the rate of production of water-ice-dominated mantles is calculated for different physical conditions of interstellar clouds and for the first time images of the morphology of interstellar ices are presented. For this purpose, the continuous-time random-walk Monte Carlo simulation technique has been used. The visual extinction, density, and gas and grain temperatures are varied. It is shown that our stochastic approach can reproduce the important observation that ice mantles only grow in the denser regions.

Even though water is the main constituent in interstellar icy mantles, its chemical origin is not well understood. Three different formation routes have been proposed following hydrogenation of O, O 2 , or O 3 on icy grains, but experimental evidence is largely lacking. We present a solid state astrochemical laboratory study in which one of these routes is tested. For this purpose O 2 ice is bombarded by H or D atoms under ultrahigh vacuum conditions at astronomically relevant temperatures ranging from 12 to 28 K. The use of reflection absorption infrared spectroscopy (RAIRS) permits derivation of reaction rates and shows efficient formation of H 2 O (D 2 O) with a rate that is surprisingly independent of temperature. This formation route converts O 2 into H 2 O via H 2 O 2 and is found to be orders of magnitude more efficient than previously assumed. It should therefore be considered as an important channel for interstellar water ice formation as illustrated by astrochemical model calculations.

The cross-disciplinary field of astrochemistry exists to understand the formation, destruction, and survival of molecules in astrophysical environments. Molecules in space are synthesized via a large variety of gas-phase reactions, and reactions on dust-grain surfaces, where the surface acts as a catalyst. A broad consensus has been reached in the astrochemistry community on how to suitably treat gas-phase processes in models, and also on how to present the necessary reaction data in databases; however, no such consensus has yet been reached for grain-surface processes. A team of ∼25 experts covering observational, laboratory and theoretical (astro)chemistry met in summer of 2014 at the Lorentz Center in Leiden with the aim to provide solutions for this problem and to review the current state-of-the-art of grain surface models, both in terms of technical implementation into models as well as the most up-to-date information available from experiments and chemical computations. This review builds on the results of this workshop and gives an outlook for future directions.

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.

Context. Methanol and its precursor formaldehyde are among the most studied organic molecules in the interstellar medium and are abundant in the gaseous and solid phases. We recently developed a model to simulate CO hydrogenation via H atoms on interstellar ice surfaces, the most important interstellar route to H 2 CO and CH 3 OH, under laboratory conditions. Aims. We extend this model to simulate the formation of both organic species under interstellar conditions, including freeze-out from the gas and hydrogenation on surfaces. Our aim is to compare calculated abundance ratios with observed values and with the results of prior models. Methods. Our model utilises the continuous-time, random-walk Monte Carlo method, which -unlike other approaches -is able to simulate microscopic grain-surface chemistry over the long timescales in interstellar space, including the layering of ices during freeze-out. Results. Simulations under different conditions, including density and temperature, have been performed. We find that H 2 CO and CH 3 OH form efficiently in cold dense cores or the cold outer envelopes of young stellar objects. The grain mantle is found to have a layered structure with CH 3 OH on top. The species CO and H 2 CO are found to exist predominantly in the lower layers of ice mantles where they are not available for hydrogenation at late times. This finding is in contrast with previous gas-grain models, which do not take into account the layering of the ice. Some of our results can be reproduced by a simple quasi-steady-state analytical model that focuses on the outer layer. Conclusions. Observational solid H 2 CO/CH 3 OH and CO/CH 3 OH abundance ratios in the outer envelopes of an assortment of young stellar objects agree reasonably well with our model results, which also suggest that the large range in CH 3 OH/H 2 O observed abundance ratios is due to variations in the evolutionary stages. Finally, we conclude that the limited chemical network used here for surface reactions apparently does not alter the overall conclusions.

Water is abundantly present in the Universe. It is the main component of interstellar ice mantles and a key ingredient for life. Water in space is mainly formed through surface reactions. Three formation routes have been proposed in the past: hydrogenation of surface O, O 2 , and O 3 . In a previous paper [Ioppolo et al., Astrophys. J, 2008, 686, 1474 we discussed an unexpected nonstandard zeroth order H 2 O 2 production behaviour in O 2 hydrogenation experiments, which suggests that the proposed reaction network is not complete, and that the reaction channels are probably more interconnected than previously thought. In this paper we aim to derive the full reaction scheme for O 2 surface hydrogenation and to constrain the rates of the individual reactions. This is achieved through simultaneous H-atom and O 2 deposition under ultra-high vacuum conditions for astronomically relevant temperatures. Different H/O 2 ratios are used to trace different stages in the hydrogenation network. The chemical changes in the forming ice are followed by means of Reflection Absorption Infrared Spectroscopy (RAIRS). New reaction paths are revealed as compared to previous experiments. Several reaction steps prove to be much more efficient (H+O 2 ) or less efficient (H+OH and H 2 +OH) than originally thought. These are the main conclusions of this work and the extended network concluded here will have profound implications for models that describe the formation of water in space. Laboratory experiments show that the formation of water in space is much more complex (green + red arrows) than previously thought (just green).Due to a copyright agreement we are not allowed to publish the full paper on arXiv.org. Please look here for the paper. We apologise for any inconvenience.