AI 中文总结
本研究通过模拟毫米级颗粒在压力凸起中的行为，发现凸起增强时间尺度决定流不稳定性能否形成星子，弱增强时满足停留时间准则，促进星子形成。
AI 中文摘要
我们提出证据表明，流不稳定性（SI）能够在轴对称压力凸起内从毫米级颗粒形成星子，前提是凸起未被过度增强。我们在振幅 $A = 0.6$ 的高斯压力凸起中，以 $160/H$ 的分辨率对毫米级颗粒进行了三维局部剪切盒模拟。我们仅改变牛顿增强时间尺度 $t_{\ m reinf}$，即施加的凸起抵抗颗粒反作用而恢复的时间尺度。我们的参考运行 T1 使用 $t_{\ m reinf} = 1\,Ω^{-1}$。颗粒在最小迎风点处狭窄的轴对称带中堆积，跨越洛希密度，并通过直接引力不稳定性（GI）坍缩。在我们弱增强的运行 T100 中，$t_{\ m reinf} = 100\,Ω^{-1}$，结果不同。颗粒穿过凸起的速度大约慢三倍，并且在达到洛希密度之前，致密的类 SI 细丝在广泛的径向区域发展。这些结果与停留时间准则一致，该准则指出，要使 SI 增长，需满足 $t_{\ m cross} > t_{\ m grow}$，其中 $t_{\ m cross}$ 是颗粒在固体丰度 $Z = Σ_p/Σ_g$ 与迎风参数 $Π= Δv/c_{\ m s}$ 之比 $Z/Π$ 足够高以实现强团簇的区域中花费的时间，$t_{\ m grow}$ 是 SI 增长时间。T1 违反此准则，而 T100 满足此准则。我们的结果表明，增强时间尺度对毫米级颗粒很重要，因为它有助于设定穿越时间，从而决定 SI 是否有时间增长。然而，我们提醒，我们的增强方案是一种理想化的数值构造，而非物理性的凸起形成机制（如行星），对这一图景的更确定性测试将需要采用更具物理动机的凸起进行模拟。
英文摘要
We present evidence that the streaming instability (SI) can form planetesimals from millimeter grains inside axisymmetric pressure bumps, provided the bump is not too strongly reinforced. We conducted three-dimensional local shearing-box simulations of millimeter grains in a Gaussian pressure bump of amplitude $A = 0.6$, at a resolution of $160/H$. We varied only the Newtonian reinforcement timescale $t_{\rm reinf}$, the timescale on which the imposed bump is restored against particle back-reaction. Our reference run T1 uses $t_{\rm reinf} = 1\,Ω^{-1}$. Particles pile up in a narrow axisymmetric band at the minimum-headwind point, cross the Roche density, and collapse through direct gravitational instability (GI). In our weakly reinforced run T100 with $t_{\rm reinf} = 100\,Ω^{-1}$, the outcome is different. Particles drift through the bump roughly three times more slowly, and dense SI-like filaments develop across a wide radial region well before the Roche density is reached. These results are consistent with a residence-time criterion, which states that for the SI to grow, $t_{\rm cross} > t_{\rm grow}$, where $t_{\rm cross}$ is the time a particle spends inside the region where the ratio $Z/Π$ of the solid abundance $Z = Σ_p/Σ_g$ to the headwind parameter $Π= Δv/c_{\rm s}$ is high enough for strong clumping, and $t_{\rm grow}$ is the SI growth time. T1 violates this criterion, and T100 satisfies it. Our results suggest that the reinforcement timescale matters for millimeter grains because it helps set the crossing time, and therefore whether the SI has time to grow. We caution, however, that our reinforcement scheme is an idealised numerical construction rather than a physical bump-forming mechanism such as a planet, and a more definitive test of this picture will require simulations with a more physically motivated bump.
Comments11 pages, 9 figures, submitted to MNRAS
