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arXiv 1607.04079astro-ph.GA
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为什么有些核心仍然没有恒星？

Why do some cores remain starless ?

S. Anathpindika

备注：
10 Figures, 3 Tables; To appear in the Publications of the Australian Society of Astronomy
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摘要：

能够使核心无星（在本地宇宙中）的物理条件是这项工作的研究主题。为此，我们研究了四个无星核心的演化：B68、L 694 -2、L1517 B、L1689和L1521 F。一个典型的核心，从早期的模拟研究核心形成的分子云的密度分布被用于此目的。我们证明-（一）核心收缩在准静态的方式在一个时间尺度上的顺序为$\sim 10^{5}$年。那些保持无星没有短暂地获得一个中心集中的密度配置，模仿不稳定的Bonnor Ebert球的密度分布反弹之前，（ii）我们的三个测试核心，即L 694 -2，L1689-SMM 16和L1521 F仍然无星，尽管成为热超临界。相反，B68和L1517 B仍然是次临界的; L1521 F只有在通过增加尘埃颗粒的尺寸来增强气体冷却时才塌陷成为VeLLO。这一结果是可靠的，因为其他核心，即B68，L 694 -2，L1517 B和L1689，以前保持无星也可能类似地导致崩溃。我们的主要结论是：（a）获得热超临界状态并不能确保核心一定会成为原恒星，（b）潜在的恒星形成核心（这里是VeLLO L1521 F）可能会经历尘埃颗粒的凝聚，这必须增强气体-尘埃耦合，从而降低气体温度，从而有助于坍缩。这一假设似乎有一些观测支持，（c）取决于它在任何特定时期的动力学状态，核心可能看起来是压力限制的，引力/维里束缚的，这表明核心的引力/维里束缚不足以确保它会形成恒星，尽管收缩核心中的气体有效冷却是至关重要的，这样它就可以进一步坍缩成为原恒星。

英文摘要：

Physical conditions that could render a core starless(in the local Universe) is the subject of investigation in this work. To this end we studied the evolution of four starless cores, B68, L694-2, L1517B, L1689, and L1521F, a VeLLO. The density profile of a typical core extracted from an earlier simulation developed to study core-formation in a molecular cloud was used for the purpose. We demonstrate - (i) cores contracted in quasistatic manner over a timescale on the order of $\sim 10^{5}$ years. Those that remained starless did briefly acquire a centrally concentrated density configuration that mimicked the density profile of a unstable Bonnor Ebert sphere before rebounding, (ii) three of our test cores viz. L694-2, L1689-SMM16 and L1521F remained starless despite becoming thermally super-critical. On the contrary B68 and L1517B remained sub-critical; L1521F collapsed to become a VeLLO only when gas-cooling was enhanced by increasing the size of dust-grains. This result is robust, for other cores viz. B68, L694-2, L1517B and L1689 that previously remained starless could also be similarly induced to collapse. Our principle conclusions are : (a) acquiring the thermally super-critical state does not ensure that a core will necessarily become protostellar, (b) potentially star-forming cores (VeLLO L1521F here), could be experiencing coagulation of dust-grains that must enhance the gas-dust coupling and in turn lower the gas temperature, thereby assisting collapse. This hypothesis appears to have some observational support, and (c) depending on its dynamic state at any given epoch, a core could appear to be pressure-confined, gravitationally/virially bound, suggesting that gravitational/virial boundedness of a core is insufficient to ensure it will form stars, though it is crucial for gas in a contracting core to cool efficiently so it can collapse further to become protostellar.

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