气体-颗粒化学模拟的一种新的修正速率方法
A new modified-rate approach for gas-grain chemical simulations
摘要：
理解晶粒表面过程对于解释ISM的化学性质至关重要。然而，精确的表面化学模型是计算昂贵的，并且难以与气相模拟集成。提出了一种求解颗粒表面化学体系的修正速率法。它的目的是准确地模拟高度复杂系统，否则只能使用有时不充分的速率方程的方法来处理。与以前的速率修改技术相比，表面生产率的函数形式被修改，而不仅仅是速率系数。这种形式是适当的极端“小晶粒”的限制，并可以使用分析主方程的方法进行验证。对这个基本形式进行了各种进一步的修改，以考虑过程之间的竞争，提高表面占据概率的估计，并允许在适用的情况下切换到正常速率方程。新方法进行了测试，对以前使用精确的技术解决的系统。即使是最简单的方法也是相当精确的，并且比速率方程有很大的改进。进一步的修改允许的主方程的结果被准确地再现为甲醇生产系统，在计算精度。小的差异出现时，非零的活化能被假定为甲醇系统，这是由于复杂的反应竞争过程，不能很容易地解决，而不使用精确的方法。在所测试的最复杂的复杂系统中，计算丰度的不准确性从来不会超过百分之几十，通常为百分之一的数量级。在简单的网络，包括氢的系统中，该方法的实施是微不足道的，而结果是高度准确的。
英文摘要：
Understanding grain-surface processes is crucial to interpreting the chemistry of the ISM. However, accurate surface chemistry models are computationally expensive and are difficult to integrate with gas-phase simulations. A new modified-rate method for solving grain-surface chemical systems is presented. Its purpose is accurately to model highly complex systems that can otherwise only be treated using the sometimes inadequate rate-equation approach. In contrast to previous rate-modification techniques, the functional form of the surface production rates was modified, and not simply the rate coefficient. This form is appropriate to the extreme "small-grain" limit, and can be verified using an analytical master-equation approach. Various further modifications were made to this basic form, to account for competition between processes, to improve estimates of surface occupation probabilities, and to allow a switch-over to the normal rate equations where these are applicable. The new method was tested against systems solved previously using exact techniques. Even the simplest method is quite accurate, and a great improvement over rate equations. Further modifications allow the master-equation results to be reproduced exactly for the methanol-producing system, within computational accuracy. Small discrepancies arise when non-zero activation energies are assumed for the methanol system, which result from complex reaction-competition processes that cannot be resolved easily without using exact methods. Inaccuracies in computed abundances are never greater than a few tens of percent, and typically of the order of one percent, in the most complex systems tested. Implementation of the method in simple networks, including hydrogen-only systems, is trivial, whilst the results are highly accurate.
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