CO耗竭：微观视角
CO depletion: a microscopic perspective
摘要：
在恒星形成的区域，密度和温度的变化会导致气体冻结在尘埃颗粒上，形成冰幔，这会影响云的化学成分。本文的目的是详细了解星际尘埃颗粒上CO的消耗（和解吸）。在两种不同的（天体物理学相关的）条件下进行了实验模拟。同时，动力学蒙特卡罗模拟用于模拟实验条件。在我们的实验中，CO分子在低于27 K的温度下附着到水冰上，其沉积速率不依赖于衬底温度。在预热阶段，解吸过程确实表现出细微的差异，表明存在弱结合的CO分子，因此突出了低扩散效率。在TPD期间冰厚度之后的IR测量证实扩散在接近解吸的温度下发生。应用到天体物理条件下，在一个前恒星的核心，CO分子的结合能，范围在300 K和850 K之间，取决于CO已沉积的条件。由于结合能的范围很广，CO的消耗作为AV的函数比最初认为的要重要得多。弱结合的分子很容易通过蒸发释放到气相中，改变了吸积和解吸之间的平衡，这导致在高沸点下更大丰度的CO。此外，弱结合的CO分子也更容易移动，这可能会增加星际冰内的反应性。
英文摘要：
In regions where stars form, variations in density and temperature can cause gas to freeze-out onto dust grains forming ice mantles, which influences the chemical composition of a cloud. The aim of this paper is to understand in detail the depletion (and desorption) of CO on (from) interstellar dust grains. Experimental simulations were performed under two different (astrophysically relevant) conditions. In parallel, Kinetic Monte Carlo simulations were used to mimic the experimental conditions. In our experiments, CO molecules accrete onto water ice at temperatures below 27 K, with a deposition rate that does not depend on the substrate temperature. During the warm-up phase, the desorption processes do exhibit subtle differences indicating the presence of weakly bound CO molecules, therefore highlighting a low diffusion efficiency. IR measurements following the ice thickness during the TPD confirm that diffusion occurs at temperatures close to the desorption. Applied to astrophysical conditions, in a pre-stellar core, the binding energies of CO molecules, ranging between 300 K and 850 K, depend on the conditions at which CO has been deposited. Because of this wide range of binding energies, the depletion of CO as a function of AV is much less important than initially thought. The weakly bound molecules, easily released into the gas phase through evaporation, change the balance between accretion and desorption, which result in a larger abundance of CO at high extinctions. In addition, weakly bound CO molecules are also be more mobile, and this could increase the reactivity within interstellar ices.
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