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arXiv 1106.0487astro-ph.EPastro-ph.SR
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由引力不稳定性形成的行星快速向内迁移

Rapid inward migration of planets formed by gravitational instability

Clément Baruteau, Farzana Meru, Sijme-Jan Paardekooper

摘要:

像HR 8799系统那样,观测到大质量的系外行星与其宿主恒星分离,挑战了行星形成的理论。一个可能的形成机制涉及到大量的自引力盘破碎成团块。虽然已经对碎片化的条件进行了广泛的研究,但对这些巨行星胚胎的后续演变,特别是它们预期的轨道迁移知之甚少。假设一个单一的行星已经形成的碎片,我们调查它的相互作用与gravitoturbulent盘嵌入。二维流体动力学模拟与一个简单的处方光盘冷却。首先建立一个稳定的重力湍流盘,然后重新开始模拟,包括一个质量范围约等于碎片盘中预期的团块初始质量的行星。行星迅速向内迁移,尽管由于湍流密度波动的随机踢。我们发现,迁移时间尺度基本上是I型迁移,行星没有时间打开一个缺口。在其形成位置长宽比约为0.1的圆盘中,质量与木星相当或大于木星的行星可以在短至10000年的时间内迁移,也就是说,在100 AU处大约有10个轨道。由于引力不稳定性,大质量行星在与恒星的距离很大的情况下形成,因此不太可能留在原地,并且应该迅速向原行星盘的内部迁移,无论行星质量如何。

英文摘要:

The observation of massive exoplanets at large separation from their host star, like in the HR 8799 system, challenges theories of planet formation. A possible formation mechanism involves the fragmentation of massive self-gravitating discs into clumps. While the conditions for fragmentation have been extensively studied, little is known of the subsequent evolution of these giant planet embryos, in particular their expected orbital migration. Assuming a single planet has formed by fragmentation, we investigate its interaction with the gravitoturbulent disc it is embedded in. Two-dimensional hydrodynamical simulations are used with a simple prescription for the disc cooling. A steady gravitoturbulent disc is first set up, after which simulations are restarted including a planet with a range of masses approximately equal to the clump's initial mass expected in fragmenting discs. Planets rapidly migrate inwards, despite the stochastic kicks due to the turbulent density fluctuations. We show that the migration timescale is essentially that of type I migration, with the planets having no time to open a gap. In discs with aspect ratio ~ 0.1 at their forming location, planets with a mass comparable to, or larger than Jupiter's can migrate in as short as 10000 years, that is, about 10 orbits at 100 AU. Massive planets formed at large separation from their star by gravitational instability are thus unlikely to stay in place, and should rapidly migrate towards the inner parts of protoplanetary discs, regardless of the planet mass.

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