Modeling sulfur depletion in interstellar clouds
Modeling sulfur depletion in interstellar clouds
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DOI：
10.1051/0004-6361/201834446
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发表时间：
2019-03
影响因子：
6.5
通讯作者：
J. Laas;P. Caselli
中科院分区：
文献类型：
--
作者：
J. Laas;P. Caselli
上下文。星际硫在气相中的元素耗竭一直是天体化学模型面临的一个反复出现的挑战。观测表明，在星际分子云的扩散和半透明阶段，硫相对于其宇宙价值来说仍然是相对不耗尽的，但它的原子和分子气相成分不能解释这种宇宙价值，因为它的视线包含高密度的环境。目标。我们试图通过模拟星际云从原始状态的扩散原子云到密度更高的分子环境的演变来解决这个问题，使用气粒天体化学代码和增强的硫反应网络。方法。在前人文献和硫模型的基础上，系统更新和扩展了常见的气-颗粒天体化学反应网络，重点关注颗粒化学和过程。使用一个简单的天体化学模型对网络更新结果进行基准测试，并将该模型的结果与来自文献的典型天文观测结果进行比较。结果。我们新的气粒天体化学模型能够重现硫的元素消耗，即硫可以从气相中被消耗两个数量级，并且如果云的化学年龄至少为106年，则这一过程可能发生在黑云条件下。由此产生的颗粒上含硫物质的混合物涵盖了所有最常见的化学元素（H/C/N/O），与在彗星环境中观察到的分子没有什么不同。值得注意的是，与目前所有其他天体化学模型不同，这种混合物并不仅仅由H2S主导。结论。尽管我们的物理模型相对简单，但大多数已知的气相含硫分子丰度在致密条件下都能准确再现，然而，它们并不是硫的主要分子汇。我们的模型预测，大多数“丢失”的硫是以有机硫的形式存在的，它们被困在谷物上。
Context. The elemental depletion of interstellar sulfur from the gas phase has been a recurring challenge for astrochemical models. Observations show that sulfur remains relatively non-depleted with respect to its cosmic value throughout the diffuse and translucent stages of an interstellar molecular cloud, but its atomic and molecular gas-phase constituents cannot account for this cosmic value toward lines of sight containing higher-density environments. Aims. We have attempted to address this issue by modeling the evolution of an interstellar cloud from its pristine state as a diffuse atomic cloud to a molecular environment of much higher density, using a gas-grain astrochemical code and an enhanced sulfur reaction network. Methods. A common gas-grain astrochemical reaction network has been systematically updated and greatly extended based on previous literature and previous sulfur models, with a focus on the grain chemistry and processes. A simple astrochemical model was used to benchmark the resulting network updates, and the results of the model were compared to typical astronomical observations sourced from the literature. Results. Our new gas-grain astrochemical model is able to reproduce the elemental depletion of sulfur, whereby sulfur can be depleted from the gas-phase by two orders of magnitude, and that this process may occur under dark cloud conditions if the cloud has a chemical age of at least 106 years. The resulting mix of sulfur-bearing species on the grain ranges across all the most common chemical elements (H/C/N/O), not dissimilar to the molecules observed in cometary environments. Notably, this mixture is not dominated simply by H2S, unlike all other current astrochemical models. Conclusions. Despite our relatively simple physical model, most of the known gas-phase S-bearing molecular abundances are accurately reproduced under dense conditions, however they are not expected to be the primary molecular sinks of sulfur. Our model predicts that most of the “missing” sulfur is in the form of organo-sulfur species that are trapped on grains.