Is the dust-to-gas ratio constant in molecular clouds?
Is the dust-to-gas ratio constant in molecular clouds?
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分子云中尘埃与气体的比率恒定吗？
DOI：
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发表时间：
2017
期刊：
影响因子：
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通讯作者：
G. Laibe
中科院分区：
文献类型：
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作者：
T. Tricco;Daniel J. Price;G. Laibe
我们对分子云中的尘埃超音速湍流进行数值模拟。我们以 1:100 的初始粉尘与气体质量比对 0.1、1 和 10 {\mu}m 大小的尘埃颗粒进行建模，求解气体和粉尘组合动力学方程，其中粉尘通过阻力项与气体耦合。我们发现，对于 0.1 和 1 {\mu}m 颗粒，尘埃与气体的比率通常偏离平均值 10-20%，因为与动态时间相比，由于气体阻力而导致的尘埃停止时间很短。与之前的发现相反，我们没有发现 0.1 {\mu}m 颗粒的尘气比出现数量级波动的证据。较大的 10 {\mu}m 尘埃颗粒可能会使尘气比局部增加一个数量级。小颗粒 (0.1 {\mu}m) 和大颗粒 ($\gtrsim$ 1 {\mu}m) 都追踪了气体的大尺度形态，但是我们发现了颗粒“尺寸排序”的证据，其中湍流优先将较大颗粒集中到致密区域。尺寸分类可能有助于解释对暗云中“核心光”的观察，以及为什么与弥漫的星际介质相比，沿着银河系分子云的视线，消光定律会有所不同。
We perform numerical simulations of dusty, supersonic turbulence in molecular clouds. We model 0.1, 1 and 10 {\mu}m sized dust grains at an initial dust-to-gas mass ratio of 1:100, solving the equations of combined gas and dust dynamics where the dust is coupled to the gas through a drag term. We show that, for 0.1 and 1 {\mu}m grains, the dust-to-gas ratio deviates by typically 10-20% from the mean, since the stopping time of the dust due to gas drag is short compared to the dynamical time. Contrary to previous findings, we find no evidence for orders of magnitude fluctuation in the dust-to-gas ratio for 0.1 {\mu}m grains. Larger, 10 {\mu}m dust grains may have dust-to-gas ratios increased by up to an order of magnitude locally. Both small (0.1 {\mu}m) and large ($\gtrsim$ 1 {\mu}m) grains trace the large-scale morphology of the gas, however we find evidence for 'size-sorting' of grains, where turbulence preferentially concentrates larger grains into dense regions. Size-sorting may help to explain observations of 'coreshine' from dark clouds, and why extinction laws differ along lines of sight through molecular clouds in the Milky Way compared to the diffuse interstellar medium.