Shocks in dense clouds I. Dust dynamics
Shocks in dense clouds I. Dust dynamics
复制标题
浓云中的冲击 I. 尘埃动力学
DOI：
10.1051/0004-6361:20078094
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发表时间：
2007
影响因子：
6.5
通讯作者：
A. Jones
中科院分区：
文献类型：
--
作者：
V. Guillet;G. Forêts;A. Jones
目标。为了模拟C型和J型激波中的尘埃过程，提出了一种研究稠密云中横向激波中尘埃动力学的新的多流体方法。方法：研究方法。我们扩充了现有的稳态激波程序，以包括带有冰壳的颗粒核的MRN尺寸分布的影响。详细考虑了尘埃的电荷分布及其演化，并将其计入电离平衡。确定了二维颗粒动力学，包括颗粒惯性和电荷涨落的影响，特别注意了带电颗粒绕磁力线的旋转和电离态对颗粒动力学的反馈。结果。我们发现，C冲击的临界速度随着气体密度的增加而增加，但它仅与高丰度的多环芳烃和宇宙射线诱导的次级光子对电子的光剥离关系很弱。C激波中详细的尘埃动力学包括两个截然不同的阶段：1)短回转阶段，然后是2)长期漂移阶段。在J激波中，只存在第一个回转阶段。在通过分子云(nH=104 cm-3)传播的C激波中，大颗粒(？100A)在第二阶段仍与磁场耦合。然而，PAHs的高丰度可能导致气体中电子的短缺和激波尾部大颗粒的解耦。在高密度云(nH=106 cm-3)中，大颗粒在整个C激波过程中都与磁场解耦。在所有的C激波中，小颗粒(≃100A)仍然与磁场强烈耦合，而非常小的颗粒(？100A)则服从随机动力学。只要它们是带电的，很小的颗粒就会保持与磁场的强烈耦合，但每次它们变成中性时，它们往往会耦合到中性气体。我们研究了C激波中沿激波方向的电场效应，发现电场并不显著地改变颗粒间的相对速度。所导出的颗粒动力学可用于研究稠密云中C和J激波中的尘埃过程，通过气体-颗粒和颗粒-颗粒碰撞的影响。
Aims. A new multi-fluid approach to the dust dynamics in transverse shocks in dense clouds is presented with the aim of modelling the dust processing in C- and J-type shocks. Methods. We have augmented an existing steady-state shock code to include the effects of an MRN size distribution of grain cores with icy mantles. The dust charge distribution and its evolution is considered in detail and included in the ionization balance. The 2-D grain dynamics are determined, including the effects of grain inertia and charge fluctuations, paying particular attention to the gyration of the charged grains around the magnetic field lines and the feedback of the ionization state on grain dynamics. Results. We find that the critical velocity for C shocks increases with the gas density but that it is only weakly dependent on a high abundance of PAHs and on the photodetachment of electrons by secondary photons induced by cosmic-rays. The detailed dust dynamics in C shocks is shown to comprise two distinct phases: 1) a short gyration phase followed by 2) a long term drift phase. In J shocks only the first gyration phase is present. In C shocks propagating through molecular clouds (n H = 10 4 cm -3 ), large grains (»100 A) remain coupled to the magnetic field during the second phase. However, a high abundance of PAHs can lead to a shortage of electrons in the gas and the decoupling of large grains in the shock tail. Large grains are decoupled from the magnetic field all through the C shock in high density clouds (n H = 10 6 cm -3 ). In all C shocks small grains (≃100 A) remain strongly coupled to the magnetic field, whereas very small grains («100 A) are subject to stochastic dynamics. As long as they are charged very small grains remain strongly coupled to the magnetic field but tend to couple to the neutral gas everytime they become neutral. We have investigated the effects of an electric field along the shock direction in C shocks and find that it does not significantly modify the relative velocities between grains. The derived grain dynamics can be used to study dust processing in C and J shocks in dense clouds through the effects of gas-grain and grain-grain collisions.