Gas-grain chemistry in cold interstellar cloud cores with a microscopic Monte Carlo approach to surface chemistry
Gas-grain chemistry in cold interstellar cloud cores with a microscopic Monte Carlo approach to surface chemistry
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DOI：
10.1051/0004-6361:20077423
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发表时间：
2007-04
影响因子：
6.5
通讯作者：
Q. Chang;H. Cuppen;E. Physics;T. O. S. University;Usa Leiden Observatory;Leiden University;T. Chemistry;Astronomy;Usa
中科院分区：
文献类型：
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作者：
Q. Chang;H. Cuppen;E. Physics;T. O. S. University;Usa Leiden Observatory;Leiden University;T. Chemistry;Astronomy;Usa
目标。我们最近开发了一种微观蒙特卡罗方法来研究星际颗粒的表面化学和冰幔的形态。该方法的目的是消除表面化学的速率方程形式主义固有的问题。在这里，我们报告的第一次使用这种方法在化学模型的冷星际云核，包括气相和表面化学。表面化学网络由少量的扩散反应组成，这些反应可以产生分子氧、水、二氧化碳、甲醛、甲醇和各种自由基。方法.通过运行气相模型开始模拟，该气相模型包括颗粒上的吸积，但没有表面化学或蒸发。起始表面由平坦或粗糙的橄榄石组成。我们使用随机技术以迭代方式介绍H、O和CO三种物质的表面化学。在模拟的条件下，只有原子氢可以蒸发到显著的程度。虽然它对其他气相物种的影响很小，但原子氢的蒸发改变了其气相丰度，这反过来又改变了原子氢到晶粒上的流量。处理对表面化学的影响，直到发生收敛。我们忽略所有非热解吸过程。结果我们确定了地幔丰度的分类分子作为时间的函数，通过2 $\× $10 5年。我们的方法还允许确定特定单层中每个分子的丰度。地幔的结果可以与W33 A源和Elias 16源中的水、二氧化碳、一氧化碳和甲醇冰的观测结果进行比较。除了地幔CO的轻微的生产不足，我们的结果是在非常好的协议与观测。
Aims. 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. Methods. 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 2 $\times$ 10 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.