声触发两相星际介质中稳定振荡小质量核的坍缩
Sound-Triggered Collapse of Stably Oscillating Low-Mass Cores in a Two-Phase Interstellar Medium
摘要：
灵感来自Barnard 68，一个博克球，经历稳定的振荡，我们进行多相流体动力学模拟来分析博克球的稳定性。我们表明，一个高密度的软分子核心，绝热指数$γ$ = 0.7嵌入在一个温暖的等温扩散气体，必须有一个小的密度梯度，以保持稳定性。尽管是稳定的，分子核心仍然可以自发地坍缩，因为它将在几十次振荡或几$10^7$年后松弛以形成足够大的密度梯度。然而，在其松弛过程中，核心可能会突然崩溃的小振幅，长波长（$\sim$ 6 $-$ 36 pc）声波在温暖的气体中的冲击触发。这种触发塌陷机制类似于声致发光现象，其中水下超声波可以驱动气泡聚结。塌缩配置被发现是不同的从内到外和从外到内的低质量恒星形成模型，但质量流量是接近的预测的由内到外的模型。确定了这种堆芯坍塌机制的条件和有效性。一般来说，必须满足宽带谐振条件，其中核心振荡频率和波频率应在几倍内相互匹配。我们的研究结果的一个后果预测传播低质量恒星形成的可能性，其中崩溃的核心，在质量范围内短的一个数量级，发生顺序跟踪波前在一个区域的几十个pc超过$10^7$年。
英文摘要：
Inspired by Barnard 68, a Bok globule, that undergoes stable oscillations, we perform multi-phase hydrodynamic simulations to analyze the stability of Bok globules. We show that a high-density soft molecular core, with an adiabatic index $γ$ = 0.7 embedded in a warm isothermal diffuse gas, must have a small density gradient to retain the stability. Despite being stable, the molecular core can still collapse spontaneously as it will relax to develop a sufficiently large density gradient after tens of oscillations, or a few $10^7$ years. However, during its relaxation, the core may abruptly collapse triggered by the impingement of small-amplitude, long-wavelength ($\sim$ 6 $-$ 36 pc) sound waves in the warm gas. This triggered collapse mechanism is similar to a sonoluminescence phenomenon, where underwater ultrasounds can drive air bubble coalescence. The collapse configuration is found to be different from both inside-out and outside-in models of low-mass star formation; nonetheless the mass flux is close to the prediction of the inside-out model. The condition and the efficiency for this core collapse mechanism are identified. Generally speaking, a broad-band resonance condition must be met, where the core oscillation frequency and the wave frequency should match each other within a factor of several. A consequence of our findings predicts the possibility of propagating low-mass star formation, for which collapse of cores, within a mass range short of one order of magnitude, takes place sequentially tracing the wave front across a region of few tens of pc over $10^7$ years.
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