A UNIFIED MONTE CARLO TREATMENT OF GAS–GRAIN CHEMISTRY FOR LARGE REACTION NETWORKS. I. TESTING VALIDITY OF RATE EQUATIONS IN MOLECULAR CLOUDS
A UNIFIED MONTE CARLO TREATMENT OF GAS–GRAIN CHEMISTRY FOR LARGE REACTION NETWORKS. I. TESTING VALIDITY OF RATE EQUATIONS IN MOLECULAR CLOUDS
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DOI：
10.1088/0004-637x/691/2/1459
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发表时间：
2008-10
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通讯作者：
A. Vasyunin;D. Semenov;D. Wiebe;Th. Henning Max Planck Institute for Astronomy;Heidelberg;Germany
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作者：
A. Vasyunin;D. Semenov;D. Wiebe;Th. Henning Max Planck Institute for Astronomy;Heidelberg;Germany
在这项研究中，我们首次证明统一的蒙特卡罗（MC）方法可以应用于大型反应网络中的气体-颗粒化学建模。具体来说，我们建立了一个星际介质的时间依赖性气体-颗粒化学模型，涉及约 6000 个气相和 200 个颗粒表面反应。该模型用于测试致密和半透明分子云模型中标准和修改的速率方程方法的有效性，并指定在哪些条件下需要使用随机方法。考虑两种情况：（1）所有物质的表面迁移率都是由热跳跃引起的； (2) 除了热跳跃之外，还允许 H 和 H2 进行与温度无关的量子隧道效应。采用TMC1云的核心和外部区域的物理条件特征。利用气相速率文件 RATE 06 以及一组扩展的气体-颗粒和表面反应。我们发现，在 25-30 K 的温度下，随机模型中的 H2O、NH3、CO 和许多其他气相和表面物质的气相丰度与确定性模型中的气相丰度相差一个数量级以上，至少在考虑隧道效应和/或扩散能比结合能低三倍时是这样。在这种情况下，涉及轻物质的表面反应比相同物质的吸积进行得更快。相反，在没有隧道效应且具有高结合能的模型中，当表面复合的典型时间尺度大于颗粒上吸积的时间尺度时，我们在相同温度范围内获得了MC结果和确定性计算结果之间几乎完美的一致性。在较低温度（∼10 K）下，气态，特别是最重要分子的表面丰度，受随机过程的影响不大。
In this study, we demonstrate for the first time that the unified Monte Carlo (MC) approach can be applied to model gas–grain chemistry in large reaction networks. Specifically, we build a time-dependent gas–grain chemical model of the interstellar medium, involving about 6000 gas-phase and 200 grain-surface reactions. This model is used to test the validity of the standard and modified rate equation methods in models of dense and translucent molecular clouds and to specify under which conditions the use of the stochastic approach is desirable. Two cases are considered: (1) the surface mobility of all species is due to thermal hopping; (2) in addition to thermal hopping, a temperature-independent quantum tunneling for H and H2 is allowed. The physical conditions characteristic for the core and the outer region of the TMC1 cloud are adopted. The gas-phase rate file RATE 06 together with an extended set of gas–grain and surface reactions is utilized. We found that at temperatures of 25–30 K gas-phase abundances of H2O, NH3, CO, and many other gas-phase and surface species in the stochastic model differ from those in the deterministic models by more than an order of magnitude, at least when tunneling is accounted for and/or diffusion energies are three times lower than the binding energies. In this case, surface reactions, involving light species, proceed faster than accretion of the same species. In contrast, in the model without tunneling and with high binding energies, when the typical timescale of a surface recombination is greater than the timescale of accretion onto the grain, we obtain almost perfect agreement between results of MC and deterministic calculations in the same temperature range. At lower temperatures (∼10 K), gaseous and, in particular, surface abundances of most important molecules are not much affected by stochastic processes.