由气体阻力辅助的固体的重力不稳定性：由湍流质量扩散率减慢
The Gravitational instability of solids assisted by gas drag: slowing by turbulent mass diffusivity
摘要：
Goldreich和Ward（1973）在Toomre参数为$Q_T \equiv c_p Ω_0 / πG Σ_p < 1$（$c_p$为粒子弥散速度）时，薄粒子层的（轴对称）引力不稳定性发生。Ward（1976，2000）通过加入气体对粒子的拖曳效应扩展了这一分析，并发现即使当$Q_T > 1$时，足够长的波总是不稳定的。Youdin（2005 a，b）进行了详细的分析，并表明不稳定性使球粒大小（$\sim 1 $ mm）的颗粒在合理的生长时间内发生径向结块，即使在存在中等量湍流搅拌的情况下也是如此。Youdin的分析包括湍流在设定尘埃层厚度和在动量方程中产生湍流粒子压力方面的作用。然而，他忽略了湍流质量扩散系数对扰动波的影响。在这里，我们表明，包括这种效果的增长率显着降低，由一个量，取决于湍流的水平，并减少湍流的最大强度的不稳定性可以承受1至3个数量级。不稳定性是可行的，只有当湍流是非常弱的，固体到气体的粒子层的表面密度是大大提高了最小质量星云的值。一个简单的力学解释的不稳定性表明，如何的方位角分量的阻力促进不稳定性，而径向分量阻碍它。一个重力扩散过稳定性也是可能的，但从来没有实现在星云模型。
英文摘要：
The Goldreich and Ward (1973) (axisymmetric) gravitational instability of a razor thin particle layer occurs when the Toomre parameter $Q_T \equiv c_p Ω_0 / πG Σ_p < 1$ ($c_p$ being the particle dispersion velocity). Ward(1976,2000) extended this analysis by adding the effect of gas drag upon particles and found that even when $Q_T > 1$, sufficiently long waves were always unstable. Youdin (2005a,b) carried out a detailed analysis and showed that the instability allows chondrule-sized ($\sim 1 $ mm) particles to undergo radial clumping with reasonable growth times even in the presence of a moderate amount of turbulent stirring. The analysis of Youdin includes the role of turbulence in setting the thickness of the dust layer and in creating a turbulent particle pressure in the momentum equation. However, he ignores the effect of turbulent mass diffusivity on the disturbance wave. Here we show that including this effect reduces the growth-rate significantly, by an amount that depends on the level of turbulence, and reduces the maximum intensity of turbulence the instability can withstand by 1 to 3 orders of magnitude. The instability is viable only when turbulence is extremely weak and the solid to gas surface density of the particle layer is considerably enhanced over minimum-mass-nebula values. A simple mechanistic explanation of the instability shows how the azimuthal component of drag promotes instability while the radial component hinders it. A gravito-diffusive overstability is also possible but never realized in the nebula models.
机构、总结、中文摘要均由AI分析而来，可能存在错误，请以论文正文为准。
