Species abundances in the interstellar medium (ISM) strongly depend on the chemistry occurring at the surfaces of the dust grains. To describe the complexity of the chemistry, various numerical models have been constructed. In most of these models, the grains are described by a single size of 0.1$μ$m. We study the impact on the abundances of many species observed in the cold cores by considering several grain sizes in the Nautilus multi-grain model. We used grain sizes with radii in the range of $0.005μ$m to $0.25μ$m. We sampled this range in many bins. We used the previously published, MRN and WD grain size distributions to calculate the number density of grains in each bin. Other parameters such as the grain surface temperature or the cosmic-ray-induced desorption rates also vary with grain sizes. We present the abundances of various molecules in the gas phase and also on the dust surface at different time intervals during the simulation. We present a comparative study of results obtained using the single grain and the multi-grain models. We also compare our results with the observed abundances in TMC-1 and L134N clouds. We show that the grain size, the grain size dependent surface temperature and the peak surface temperature induced by cosmic ray collisions, play key roles in determining the ice and the gas phase abundances of various molecules. We also show that the differences between the MRN and the WD models are crucial for better fitting the observed abundances in different regions in the ISM. We show that the small grains play a very important role in the enrichment of the gas phase with the species which are mainly formed on the grain surface, as non-thermal desorption induced by collisions of cosmic ray particles is very efficient on the small grains.
Testing various assumptions for radiolysis, non-diffusive chemistry, and chemical desorption in cold cores
Laboratoire d’astrophysique de Bordeaux, Univ. Bordeaux, CNRS（波尔多天体物理学实验室，波尔多大学，法国国家科学研究中心）；School of Astronomy & Space Science, Nanjing University（南京大学天文与空间科学学院）
Influence of galactic arm scale dynamics on the molecular composition of the cold and dense ISM II. Molecular oxygen abundance
分子氧一直是许多观测研究的主题，因为化学模型预测它是氧气的储存库。虽然它已经在星际介质的两个区域被探测到，但它的稀有性对天体化学模型来说是一个挑战。在本文中，我们结合了计算的物理条件与光滑粒子流体动力学（SPH）模拟与我们的全气体颗粒化学模型鹦鹉螺，研究预测的O2丰度在星际物质形成冷芯。因此，我们遵循的气体和冰的化学演化的物质包裹从弥漫的星际条件冷致密的核心。我们的大多数预测的O2丰度低于1 e-8（相对于总质子密度）和预测的柱密度在模拟冷芯是在最大的几个1 e 14 cm-2，在协议的非检测限。这种低的O2丰度可以通过以下事实来解释，即在大部分星际材料中，在O2可以在气相中形成并且免受UV光解离之前，原子氧被耗尽到晶粒表面上（并且被氢化以形成H2O）。我们之所以能得到这个结果，是因为我们考虑了从弥散介质到冷核的物理条件演化的全部历史。
Influence of galactic arm scale dynamics on the molecular composition of the cold and dense ISM III. Elemental depletion and shortcomings of the current physico-chemical models
本文研究了星际介质中的元素损耗。我们结合了银河系模型的结果，描述了致密核形成过程中的气体物理条件与全气体颗粒化学模型。在扩散和稠密介质之间的过渡期间，最初在气体中的原子元素的储存器逐渐在尘埃颗粒上耗尽（对于那些离子来说，有一个中和阶段）。当密度大于100 cm $^{-3}$时，该过程变得有效。如果致密材料回到扩散状态，这些元素又回到气相，因为冰上分子的光离解，然后从颗粒上热解吸。对于密度低于10 cm $^{-3}$的颗粒或气相中的分子形式，颗粒上没有残留物。一个例外是氯，它在低密度下有效地转化。我们现有的气体颗粒化学模型不能再现在扩散介质中观察到的原子的耗尽，除了Cl，其气体丰度遵循在密度小于10 cm $^{-3}$的介质中观察到的气体丰度。这表明关键的过程（可能涉及化学吸附和/或冰辐射深刻改变冰的性质）缺失。
