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arXiv 1710.09124astro-ph.SR
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无星核中子结构的形成

Sub-structure formation in starless cores

C. Toci, D. Galli, A. Verdini, L. Del Zanna, S. Landi

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8 pages, 4 figures
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摘要：

受最近对无星分子云核心子结构观测研究的启发，我们采用与膨胀宇宙和太阳风相同的形式主义，研究了嵌入收缩等温云中的比金斯长度小的尺度上密度扰动的演化。我们发现，最初的小振幅，琼斯稳定的扰动（在没有磁场的情况下作为声波传播），在收缩过程中被放大，近似保持波作用密度，直到它们变得非线性和陡峭的冲击在一个时间$t_{\rm nl}$，或成为引力不稳定时，琼斯长度减少到低于扰动的规模在一个时间$t_{\rm gr}$。我们分析评估的时间$t_{\rm nl}$在扰动进入非线性阶段，使用Burgers方程的方法，我们验证数值计算，这个时间标志着开始的阶段的快速耗散的扰动的动能。然后，我们表明，在分子云核心的均方根马赫数的典型值，$t_{\rm nl}$是小于$t_{\rm gr}$，因此，密度扰动可能会消散之前，成为引力不稳定。螺线管模式的增长速度快于可压缩模式，并可能最终通过形成旋涡结构促进碎片化。

英文摘要：

Motivated by recent observational searches of sub-structure in starless molecular cloud cores, we investigate the evolution of density perturbations on scales smaller than the Jeans length embedded in contracting isothermal clouds, adopting the same formalism developed for the expanding Universe and the solar wind. We find that initially small amplitude, Jeans-stable perturbations (propagating as sound waves in the absence of a magnetic field), are amplified adiabatically during the contraction, approximately conserving the wave action density, until they either become nonlinear and steepen into shocks at a time $t_{\rm nl}$, or become gravitationally unstable when the Jeans length decreases below the scale of the perturbations at a time $t_{\rm gr}$. We evaluate analytically the time $t_{\rm nl}$ at which the perturbations enter the non-linear stage using a Burgers' equation approach, and we verify numerically that this time marks the beginning of the phase of rapid dissipation of the kinetic energy of the perturbations. We then show that for typical values of the rms Mach number in molecular cloud cores, $t_{\rm nl}$ is smaller than $t_{\rm gr}$, and therefore density perturbations likely dissipate before becoming gravitational unstable. Solenoidal modes grow at a faster rate than compressible modes, and may eventually promote fragmentation through the formation of vortical structures.

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