At low temperatures(10 K),hydrogen atoms can diffuse quickly on grain ice mantles and frequently encounter hydrogen molecules,which cover a notable fraction of grain surface.The desorption energy of H atoms on H2 subs

Astrochemistry has made great progress in recent years. Especially the grain surface chemistry played important roles in the explanation of the formation of the interstellar molecules. In this review, we will discuss the progress, including the different numerical methods to simulate the ice mantles in the astrochemical models. We will also introduce the laboratory astrochemical experimental results, and their contributions to the grain surface chemistry in the review.

The E-isomer of cyanomethanimine (HNCHCN) was first identified in Sagittarius B2(N) (Sgr B2(N)) by a comparison of the publicly available Green Bank Telescope (GBT) PRIMOS survey with laboratory rotational spectra. Recently, Z-cyanomethanimine was detected in the quiescent molecular cloud G+0.693-0.027 with the IRAM30-m telescope. Cyanomethanimine is a chemical intermediate in the proposed synthetic routes of adenine, and may play an important role in forming biological molecules in the interstellar medium. Here we present a new modelling study of cyanomethanimine, using the NAUTILUS gas-grain reaction network and code with the addition of over 400 chemical reactions of the three cyanomethanimine isomers and related species. We apply cold isothermal core, hot core, and C-type shock models to simulate the complicated and heterogeneous physical environment in and in front of Sgr B2(N), and in G+0.693-0.027. We identify the major formation and destruction routes of cyanomethanimine, and find that the calculated abundances of the cyanomethanimine isomers and the ratio of Z-isomer to E-isomer are both in reasonable agreement with observations for selected environments. In particular, we conclude that these isomers are most likely formed within or near the hot core without the impact of shocks, or in the cold regions with shocks.

Astrochemistry has made great progress in recent years.Especially the grain surface chemistry played important roles in the explanation of the formation of the interstellar molecules.In this review,we will discuss the

近年来天体化学在解释星际分子的形成过程中起重要作用的尘埃表面化学取得了长足的发展,本文综述了在天体化学模拟中用到的各种数值计算方法,以及尘埃冰幔的化学模型.同时也介绍在实验室天体化学方面获得的结果,及其对尘埃表面化学所起到的促进作用.

The kinetic Monte Carlo simulation is a rigorous numerical approach to study the chemistry on dust grains in cold dense interstellar clouds. By tracking every single reaction in chemical networks step by step, this approach produces more precise results than other approaches but takes too much computing time. Here we present a method of a new data structure, which is applicable to any physical conditions and chemical networks, to save computing time for the Monte Carlo algorithm. Using the improved structure, the calculating time is reduced by 80 percent compared with the linear structure when applied to the osu-2008 chemical network at 10 K. We investigate the effect of the encounter desorption in cold cores using the kinetic Monte Carlo model with an accelerating data structure. We found that the encounter desorption remarkably decreases the abundance of grain-surface H-2 but slightly influences the abundances of other species on the grain.

Context. Abundant carbon-chain species have been observed towards lukewarm corinos L1527, B228, and L483. These carbon-chain species are believed to be synthesized in the gas phase after CH4 desorbs from the dust grain surface at the temperature around 30 K.Aims. We investigate carbon-chain species formation in lukewarm corinos using a more rigorous numerical method and advanced surface chemical models. We also pay attention to the significance of the finite size effect.Methods. We used the macroscopic Monte Carlo method in our simulations. In addition to the two-phase model, the basic multi-phase model and the new multi-phase models were used for modelling surface chemistry on dust grains. All volatile species can sublime at their sublimation temperatures in the two-phase model while most volatile species are frozen in the ice mantle before water ice sublimes in the basic and the new multi-phase models. The new multi-phase models allow more volatile species to sublime at their sublimation temperatures than the basic multi-phase model does.Results. The significance of the finite size effect is dependent on the duration of the cold phase. The discrepancies between the rate equation approach and the Monte Carlo method decrease as the duration of the cold phase increases. When T similar to 30 K, the abundances of gaseous CH4 and CO in the two-phase model are the highest while the basic multi-phase model predicts the lowest CO and CH4 abundances among all models. The abundances of carbon-chain species in the basic and the new multi-phase models are lower than that in the two-phase model when T similar to 30 K because CH4 is crucial for the synthesis of carbon-chain species. However, because the abundance of electrons increases as the abundance of H3O+ decreases, some carbon-chain species abundances predicted by the basic multi-phase model may not be lower than that in the new multi-phase models. The two-phase model performs best in predicting carbon-chain species abundances to fit observations while the basic multi-phase model works the worst. The abundances of carbon-chain species predicted by the new multi-phase models agree reasonably well with observations.Conclusions. The amount of CH4 can diffuse inside the ice mantle, thus sublime upon warm-up plays a crucial role in the synthesis of carbon-chain species in the gas phase. The carbon-chain species observed in lukewarm corinos may be able to gauge surface chemical models.

We investigate the chemical evolution of a collapsing core that starts from a hydrostatic core and finally forms a low-mass protostar. New multiphase gas-grain models that include bulk diffusion and photon penetration are simulated by the macroscopic Monte Carlo method in order to derive the chemical evolution. There are two types of species in the ice bulk in the new multiphase models: interstitial species can diffuse and sublime at their own sublimation temperatures, while normal species are locked in the ice bulk. Photodissociation rates of icy species are reduced by the exponential decay of UV flux within the ice mantle. Two-phase models and basic multiphase models without bulk diffusion and photon penetration are also simulated for comparison. Our physical model for the collapsing core is based on a 1D radiation hydrodynamics model. Abundant icy radicals are produced at around 10 K in the new multiphase models. Interstitial radicals can diffuse inside ice mantles to form complex organic molecules (COMs) upon warming up. Thus, COMs produced by radical recombination at higher temperatures in the new multiphase models are more than one order of magnitude higher than those in the two-phase and basic multiphase models. Moreover, COMs produced at around 10 K in the new multiphase models are about one order of magnitude higher than those in the two-phase model. Our model shows a reasonable agreement with observations toward low-mass protostars. Molecular oxygen abundances predicted by our new multiphase models agree reasonably well with those found in cometary materials.

The temperatures of dust grains play important roles in the chemical evolution of molecular clouds. Unlike large grains, the temperature fluctuations of small grains induced by photons may be significant. Therefore, if the grain size distribution is included in astrochemical models, the temperatures of small dust grains may not be assumed to be constant. We simulate a full gas-grain reaction network with a set of dust grain radii using the classical MRN grain size distribution and include the temperature fluctuations of small dust grains. Monte Carlo method is used to simulate the real-time dust grain's temperature fluctuations which is caused by the external low-energy photons and the internal cosmic ray induced secondary photons. The increase of dust grains radii as ice mantles accumulate on grain surfaces is also included in our models. We found that surface CO2 abundances in models with grain size distribution and temperature fluctuations are more than one order of magnitude larger than those with single grain size. Small amounts of terrestrial complex organic molecules (COMs) can also be formed on small grains due to the temperature spikes induced by external low-energy photons. However, cosmic ray induced secondary photons overheat small grains so that surface CO sublime and less radicals are formed on grains surfaces, thus the production of surface CO2 and COMs decreases by about one order of magnitude. The overheating of small grains can be offset by grain growth so that the formation of surface CO2 and COMs become more efficient.