[论文解读] Origin of CH+ in diffuse molecular clouds warm H2 and ion-neutral drift

Valeska Valdivia, B. Godard+4 more|arXiv (Cornell University)|Dec 9, 2016

Astrophysics and Star Formation Studies参考文献 60被引用 12

一句话总结

本研究通过结合理想磁流体动力学（MHD）模拟与自定义化学求解器，探究了弥散分子云中CH+的起源，评估了暖H2和离子-中性粒子漂移的作用。研究发现，CH+在H2质量分数为1%–10%、温度高于300 K的团块外缘高效形成，此时暖的、非平衡态H2可使CH+柱密度提升高达十倍，尽管仍低于观测值；由于漂移速度较低且极少出现，离子-中性粒子漂移的影响微乎其微。

ABSTRACT

This paper assesses the roles of the presence of warm H2, and the increased formation rate due to the ion-neutral drift. We performed ideal MHD simulations that include the heating and cooling of the multiphase ISM, and where we treat dynamically the formation of H2. In a post-processing step we compute the abundances of species at chemical equilibrium. We show that CH+ is efficiently formed at the edge of clumps, in regions where the H2 fraction is low, but nevertheless higher than its equilibrium value, and where the gas temperature is high. We show that warm and out of equilibrium H2 increases the integrated column densities of CH+ by one order of magnitude, up to values still 3-10 times lower than those observed in the diffuse ISM. We balance the Lorentz force with the ion-neutral drag to estimate the ion-drift velocities (vd). We find that the vd distribution peaks around 0.04 km s-1, and that high vd are too rare to have a significant statistical impact on the abundances of CH+. Compared to previous works, our multiphase simulations reduce the spread in vd, and our self-consistent treatment of the ionisation leads to much reduced vd. Nevertheless, our resolution study shows that this velocity distribution is not converged: the ion-neutral drift has a higher impact on CH+ at higher resolution. On the other hand, our ideal MHD simulations do not include ambipolar diffusion, which would yield lower drift velocities. Within these limitations, we conclude that warm H2 is a key ingredient in the efficient formation of CH+ and that the ambipolar diffusion has very little influence on the abundance of CH+, mainly due to the small drift velocities obtained. However, we point out that small-scale processes and other non-thermal processes not included in our MHD simulation may be of crucial importance, and higher resolution studies with better controlled dissipation processes are needed.

研究动机与目标

• 理解弥散分子云中CH+起源这一长期存在的天体化学难题。
• 评估暖H2和离子-中性粒子漂移是否能解释观测到的CH+丰度。
• 评估湍流混合与非平衡态H2形成在增强CH+生成中的作用。
• 量化离子-中性粒子漂移速度对CH+形成速率的影响。
• 识别可能使模拟CH+丰度与观测值一致的缺失物理过程。

提出的方法

• 在多相星际介质中进行三维理想MHD模拟，包含加热、冷却及动态H2形成过程。
• 采用后处理化学求解器，基于局部物理条件计算物种丰度，假设达到化学平衡。
• 追踪H2分数与温度，识别有利于吸热CH+形成的区域。
• 通过平衡洛伦兹力与离子-中性粒子阻力，从MHD模拟中估算离子漂移速度。
• 计算漂移速度分布并评估其对CH+形成的统计影响。
• 进行分辨率研究，以检验离子漂移速度分布的收敛性。

实验结果

研究问题

• RQ1 暖的、非平衡态H2能否解释弥散分子云中观测到的增强CH+丰度？
• RQ2 在真实的MHD模拟中，离子-中性粒子漂移在CH+形成中贡献多大程度？
• RQ3 湍流混合与多相结构如何影响CH+的空间分布与丰度？
• RQ4 为何模拟得到的CH+柱密度仍显著低于观测值？
• RQ5 分辨率与数值耗散如何影响离子漂移速度的估算及其对化学的影响？

主要发现

• CH+在H2质量分数为1%–10%且温度超过300 K的团块外缘高效形成，表明暖的、非平衡态H2起关键作用。
• 暖H2可使积分CH+柱密度提升高达十倍，使模拟结果更接近观测值，但仍存在3–10倍的差距。
• 离子-中性粒子漂移速度峰值约为~0.04 km/s，高速度（>1 km/s）极为罕见，因此对CH+丰度的统计影响可忽略不计。
• 所用分辨率下离子漂移速度分布未收敛，表明更高分辨率模拟可能揭示更强的漂移扩散效应。
• MHD模拟中自洽处理电离过程导致漂移速度显著低于以往研究，从而削弱了漂移在CH+形成中的作用。
• 由于高速度稀少且数值极低，漂移扩散不太可能显著影响CH+丰度。