On the Migration of Protogiant Solid Cores
On the Migration of Protogiant Solid Cores
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关于原始巨星固体核心的迁移
DOI：
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发表时间：
2006
期刊：
影响因子：
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通讯作者：
W. Kley
中科院分区：
文献类型：
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作者：
F. Masset;F. Masset;G. D’Angelo;W. Kley
最近，计算资源的增加使得对嵌入气态原行星盘中的行星进行高分辨率、三维计算成为可能。它们提供了可以与分析预测相比较的行星迁移时间尺度的估计。虽然这些预测可能导致极短的地球质量核心的迁移时间，但最近的数值计算给出了一个意想不到的结果：作用在质量在5到20 M⊕之间的行星的扭矩比解析的线性估计要小得多。这些发现推动了目前的工作，通过二维和三维数值计算的方式，调查这种差异或我们所称的“偏移”的存在和根源。我们证明了偏移量确实是物理的，并且是由同轨道的自转力矩引起的，因为(1)偏移量与圆盘涡度梯度成比例，(2)偏移量的渐近值取决于圆盘的粘性，(3)偏移量与马蹄形区域宽度的过剩有关。我们发现，偏移量对应于围绕行星流动的非线性的开始，这随着行星质量的增加而改变流线拓扑：在低质量时，流动的非线性被限制在行星的邦迪球内，而在较大质量时，流线显示出一幅经典的图像，使人想起受限的三体问题，在“罗氏叶”中有一个前进的环行星圆盘。这一行为对于薄(H/r≲0.06)原行星盘中的亚临界固体核心(M⊕15M≲)特别重要。在表面密度分布较浅的圆盘中，它们的迁移可能会显著减慢或逆转。
The increase of computational resources has recently allowed high-resolution, three-dimensional calculations of planets embedded in gaseous protoplanetary disks. They provide estimates of the planet migration timescale that can be compared to analytical predictions. While these predictions can result in extremely short migration timescales for cores of a few Earth masses, recent numerical calculations have given an unexpected outcome: the torque acting on planets with masses between 5 and 20 M⊕ is considerably smaller than the analytic, linear estimate. These findings motivated the present work, which investigates existence and origin of this discrepancy or "offset," as we shall call it, by means of two- and three-dimensional numerical calculations. We show that the offset is indeed physical and arises from the co-orbital corotation torque, since (1) it scales with the disk vortensity gradient, (2) its asymptotic value depends on the disk viscosity, (3) it is associated to an excess of the horseshoe zone width. We show that the offset corresponds to the onset of nonlinearities of the flow around the planet, which alter the streamline topology as the planet mass increases: at low mass the flow nonlinearities are confined to the planet's Bondi sphere, whereas at larger mass the streamlines display a classical picture reminiscent of the restricted three-body problem, with a prograde circumplanetary disk inside a "Roche lobe." This behavior is of particular importance for the subcritical solid cores (M ≲ 15 M⊕) in thin (H/r ≲ 0.06) protoplanetary disks. Their migration could be significantly slowed down, or reversed, in disks with shallow surface density profiles.