分子cloudS（GEMS）III中的气相元素丰度。解锁CS化学：CS + O反应
Gas-phase Elemental abundances in Molecular cloudS (GEMS) III. Unlocking the CS chemistry: the CS+O reaction
摘要：
CS是冷暗分子云中最丰富的气相含硫分子之一。它在毫米波长范围内有几个跃迁，很容易观察到，并已被广泛用作我们银河系和外部星系中星际介质气体密度的示踪剂。化学模型无法解释所观察到的CS丰度时，假设硫元素丰度的宇宙值。CS + O-> CO + S反应被认为是低温下CS的一种破坏机制，可以解释模型与观测结果之间的差异。它的反应速率已在150 - 400 K的温度下实验测量，但外推到更低的温度是值得怀疑的。本文计算了星际介质中普遍存在的温度<150 K时的CS + O反应速率。我们进行了从头计算，以获得三个最低的PES的CS + O系统。这些PES用于研究反应动力学，使用几种方法最终计算CS + O热反应速率。我们比较了我们的理论计算结果为150 - 400 K与那些在实验室中获得的。我们对CS + O反应进行了详细的理论研究，在150 - 400 K的温度范围内与实验数据一致，证明了我们方法的可靠性。在较低温度下仔细分析后，我们发现，在10 K的速率常数是可以忽略不计的，这是一致的外推的实验数据使用的Arrhenius表达式。我们使用最新的化学网络，在TMC 1硫化学建模的基础上，从GEMS项目观测确定的分子丰度。在我们的模型中，我们考虑到宇宙射线电离率沿云的预期下降。当假设硫丰度的宇宙值时，CS的丰度仍然被高估。
英文摘要：
CS is among the most abundant gas-phase S-bearing molecules in cold dark molecular clouds. It is easily observable with several transitions in the millimeter wavelength range, and has been widely used as a tracer of the gas density in the interstellar medium in our Galaxy and external galaxies. Chemical models fail to account for the observed CS abundances when assuming the cosmic value for the elemental abundance of sulfur. The CS+O -> CO + S reaction has been proposed as a relevant CS destruction mechanism at low temperatures, and could explain the discrepancy between models and observations. Its reaction rate has been experimentally measured at temperatures of 150-400 K, but the extrapolation to lower temperatures is doubtful. Here we calculate the CS+O reaction rate at temperatures <150 K which are prevailing in the interstellar medium. We performed ab initio calculations to obtain the three lowest PES of the CS+O system. These PESs are used to study the reaction dynamics, using several methods to eventually calculate the CS+O thermal reaction rates. We compare the results of our theoretical calculations for 150-400 K with those obtained in the laboratory. Our detailed theoretical study on the CS+O reaction, which is in agreement with the experimental data obtained at 150-400 K, demonstrates the reliability of our approach. After a careful analysis at lower temperatures, we find that the rate constant at 10 K is negligible, which is consistent with the extrapolation of experimental data using the Arrhenius expression. We use the updated chemical network to model the sulfur chemistry in TMC1 based on molecular abundances determined from GEMS project observations. In our model, we take into account the expected decrease of the cosmic ray ionization rate along the cloud. The abundance of CS is still overestimated when assuming the cosmic value for the sulfur abundance.
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