热不平衡对动力学形核的影响
The effect of thermal non-equilibrium on kinetic nucleation
摘要：
核化被认为是渐近巨星分支（AGB）恒星、系外行星和褐矮星大气中尘埃和云形成的第一步。在这些环境中，当气相物质凝结到云凝结核（CCN）上时，尘埃和云粒子会增长到宏观尺寸。了解AGB星中CCN和尘埃的形成过程是很重要的，因为在它们的流出物中形成的物质丰富了星际介质。虽然化学和热平衡条件被广泛使用，但在某些高度动态的天体物理环境中，其有效性是有争议的。我们的目标是推导出一个动力学成核模型，其中包括通过采用不同的温度成核物种的热非平衡的影响，并量化热非平衡动力学成核的影响。作为碰撞动力学成核理论的一部分，推导了向前和向后的速率系数。从热不平衡时的质量作用定律导出了吸热后向速率。我们认为弹性碰撞是热平衡驱动因素。对于均匀的TiO 2成核和气体温度为1250 K，我们发现，小至20 K的动力学簇温度的差异增加了一个数量级以上的形成较大的TiO 2簇。在1000 K的气体温度下，簇温度增加约20 K，可以减少一个数量级以上的更大的TiO 2簇的形成。我们的研究结果证实并量化了以前的热非平衡研究的预测。小的热不平衡可以导致较大的簇的合成中的显著变化。因此，重要的是使用动力学成核模型，包括热不平衡，以描述在环境中，即使是小的热不平衡，可以存在的集群的形成。
英文摘要：
Nucleation is considered to be the first step in dust and cloud formation in the atmospheres of asymptotic giant branch (AGB) stars, exoplanets, and brown dwarfs. In these environments dust and cloud particles grow to macroscopic sizes when gas phase species condense onto cloud condensation nuclei (CCNs). Understanding the formation processes of CCNs and dust in AGB stars is important because the species that formed in their outflows enrich the interstellar medium. Although widely used, the validity of chemical and thermal equilibrium conditions is debatable in some of these highly dynamical astrophysical environments. We aim to derive a kinetic nucleation model that includes the effects of thermal non-equilibrium by adopting different temperatures for nucleating species, and to quantify the impact of thermal non-equilibrium on kinetic nucleation. Forward and backward rate coefficients are derived as part of a collisional kinetic nucleation theory ansatz. The endothermic backward rates are derived from the law of mass action in thermal non-equilibrium. We consider elastic collisions as thermal equilibrium drivers. For homogeneous TiO2 nucleation and a gas temperature of 1250 K, we find that differences in the kinetic cluster temperatures as small as 20 K increase the formation of larger TiO2 clusters by over an order of magnitude. An increase in cluster temperature of around 20 K at gas temperatures of 1000 K can reduce the formation of a larger TiO2 cluster by over an order of magnitude. Our results confirm and quantify the prediction of previous thermal non-equilibrium studies. Small thermal non-equilibria can cause a significant change in the synthesis of larger clusters. Therefore, it is important to use kinetic nucleation models that include thermal non-equilibrium to describe the formation of clusters in environments where even small thermal non-equilibria can be present.
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