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arXiv 1711.03490astro-ph.EP
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碎屑盘对星子形成的制约

Debris Disc Constraints on Planetesimal Formation

Alexander V. Krivov, Aljoscha Ide, Torsten Löhne, Anders Johansen, Jürgen Blum

摘要:

在过去的几十年里,人们提出了两条形成星子的基本路线。一种是经典的“缓慢增长”情景。另一种是粒子集中模型,在这种模型中,小卵石在局部集中,然后在引力作用下坍缩形成微行星。这两种模型都对新生星子的大小谱和内部结构做出了一定的预测。我们使用这些预测作为输入来模拟气体扩散后留下的碎片盘的碰撞演化。碎片光盘排放作为一个系统的年龄在这些模拟计算的功能进行了比较与几个斯皮策和赫歇尔碎片光盘调查A型恒星周围。我们确认,大多数圆盘的观测亮度演变都可以通过经典模型重现。此外,我们发现,这是同样一致的颗粒浓度模型预测的星子的大小分布-提供的对象是松散的约束“卵石堆”,因为这些模型也预测。不管假设的星子形成机制如何,解释样本中最亮的碎片盘揭示了一个“盘质量问题”。“为了通过碰撞模拟再现这样的圆盘,需要高达1000地球质量的星子的总质量,这超过了碎片圆盘的原行星祖先中可用的固体总质量。这可能表明,在一些明亮的圆盘中,搅拌被延迟,其中一些最近发生了巨大的撞击,一些系统可能比以前认为的更年轻,或者非碰撞过程对尘埃的产生有很大的贡献。

英文摘要:

Two basic routes for planetesimal formation have been proposed over the last few decades. One is a classical "slow-growth" scenario. Another one is particle concentration models, in which small pebbles are concentrated locally and then collapse gravitationally to form planetesimals. Both types of models make certain predictions for the size spectrum and internal structure of newly-born planetesimals. We use these predictions as input to simulate collisional evolution of debris discs left after the gas dispersal. The debris disc emission as a function of a system's age computed in these simulations is compared with several Spitzer and Herschel debris disc surveys around A-type stars. We confirm that the observed brightness evolution for the majority of discs can be reproduced by classical models. Further, we find that it is equally consistent with the size distribution of planetesimals predicted by particle concentration models - provided the objects are loosely bound "pebble piles" as these models also predict. Regardless of the assumed planetesimal formation mechanism, explaining the brightest debris discs in the samples uncovers a "disc mass problem." To reproduce such discs by collisional simulations, a total mass of planetesimals of up to ~1000 Earth masses is required, which exceeds the total mass of solids available in the protoplanetary progenitors of debris discs. This may indicate that stirring was delayed in some of the bright discs, that giant impacts occurred recently in some of them, that some systems may be younger than previously thought, or that non-collisional processes contribute significantly to the dust production.

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