星际turbulence中尘埃颗粒的破碎和凝聚
Shattering and coagulation of dust grains in interstellar turbulence
摘要：
本文研究了尘埃颗粒在湍流星际介质中的破碎和凝聚。基于可压缩磁流体动力学湍流中的颗粒动力学理论，计算了不同ISM相下尘埃颗粒的典型速度随颗粒尺寸的变化。在本文中，我们开发了一个方案的颗粒破碎和凝聚，并将其应用到湍流ISM通过使用上述湍流理论预测的颗粒速度。由于大颗粒倾向于获得大的速度分散，如早期的研究所示，大颗粒倾向于破碎。对于半径为$a\ga 10^{-6}$ cm的颗粒，在几百万年内的温暖电离介质（WIM）中确实观察到了大的破碎效应。我们还表明，在温暖的中性介质（WNM）粉碎可以限制ISM（$a\sim 2\times 10^{-5} \mathrm{cm}$）的最大晶粒尺寸。另一方面，凝结倾向于修改小颗粒，因为它只发生在颗粒速度足够小时。凝结显着修改了密集的云（DC），其中大部分的颗粒与$a<10^{-6}$厘米凝结在10百万年的粒度分布。事实上，观测到的银河系消光曲线中的$R_V$、碳凸强度和紫外斜率之间的相关性可以用DC中的凝聚来解释。银河系中的粒度分布可能是由上述所有破碎和凝聚效应的组合决定的。考虑到如果尘气比通常大于银河值的$\sim 1/10$，则湍流中的破碎和凝聚是有效的，贫金属和富金属环境的粒度分布的调节机制应该是不同的。
英文摘要：
We investigate shattering and coagulation of dust grains in turbulent interstellar medium (ISM). The typical velocity of dust grain as a function of grain size has been calculated for various ISM phases based on a theory of grain dynamics in compressible magnetohydrodynamic turbulence. In this paper, we develop a scheme of grain shattering and coagulation and apply it to turbulent ISM by using the grain velocities predicted by the above turbulence theory. Since large grains tend to acquire large velocity dispersions as shown by earlier studies, large grains tend to be shattered. Large shattering effects are indeed seen in warm ionized medium (WIM) within a few Myr for grains with radius $a\ga 10^{-6}$ cm. We also show that shattering in warm neutral medium (WNM) can limit the largest grain size in ISM ($a\sim 2\times 10^{-5} \mathrm{cm}$). On the other hand, coagulation tends to modify small grains since it only occurs when the grain velocity is small enough. Coagulation significantly modifies the grain size distribution in dense clouds (DC), where a large fraction of the grains with $a<10^{-6}$ cm coagulate in 10 Myr. In fact, the correlation among $R_V$, the carbon bump strength, and the ultraviolet slope in the observed Milky Way extinction curves can be explained by the coagulation in DC. It is possible that the grain size distribution in the Milky Way is determined by a combination of all the above effects of shattering and coagulation. Considering that shattering and coagulation in turbulence are effective if dust-to-gas ratio is typically more than $\sim 1/10$ of the Galactic value, the regulation mechanism of grain size distribution should be different between metal-poor and metal-rich environments.
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