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arXiv 1306.2514astro-ph.EP
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引力不稳定间隙引起的巨行星轨道迁移:行星质量的影响

Orbital migration of giant planets induced by gravitationally unstable gaps: the effect of planet mass

Ryan Cloutier, Min-Kai Lin

摘要:

有人认为长周期巨行星,如HD 95086 b和HR 8799 bcde,可能是由原行星盘的引力不稳定性形成的。然而,自引力盘-卫星相互作用可能导致引力不稳定间隙的形成。这样的不稳定性显着影响的轨道迁移的间隙开放大规模磁盘中的扰动。我们使用二维流体动力学模拟来研究行星质量对间隙的引力稳定性及其对轨道迁移的影响的作用。我们考虑行星与恒星质量比q=0.0003到q=0.003的巨行星,它们位于一个自引力盘中,盘与恒星质量比M_d/M_*=0.08,长宽比h=0.05,开普勒托姆雷参数Q = 1.5,初始轨道半径为2.5倍。固定轨道模拟表明,我们认为所有的行星质量都打开了引力不稳定的缺口,但随着行星质量的增加,不稳定性更强,发展得更快。随着行星质量的增加,行星盘上的扭矩通常变得更正。在自由迁移的模拟中,我们观察到随着行星质量的增加,向外迁移的速度更快,但仅限于能够在早期打开不稳定间隙的行星质量。对于q=0.0003,行星在打开间隙之前经历了快速向内的III型迁移。对于q=0.0013,我们发现有可能通过不稳定间隙产生的正力矩来平衡向内迁移的趋势,但仅限于几十个轨道周期。我们发现不稳定的外间隙边缘可以触发向外的III型迁移,在动力学时间尺度上将行星发送到其初始轨道半径的两倍。这种引发的III型迁移可能是将巨行星带到大轨道距离的一种方式。另一方面,我们的研究结果也意味着间隙开放不是一个充分的条件,在现场形成的巨行星在宽轨道上通过磁盘碎片。

英文摘要:

It has been suggested that long-period giant planets, such as HD 95086b and HR 8799bcde, may have formed through gravitational instability of protoplanetary discs. However, self-gravitating disc-satellite interaction can lead to the formation of a gravitationally unstable gap. Such an instability significantly affects the orbital migration of gap-opening perturbers in massive discs. We use 2D hydrodynamical simulations to examine the role of planet mass on the gravitational stability of gaps and its impact on orbital migration. We consider giant planets with planet-to-star mass ratio q=0.0003 to q=0.003, in a self-gravitating disc with disc-to-star mass ratio M_d/M_*=0.08, aspect ratio h=0.05, and Keplerian Toomre parameter Q = 1.5 at 2.5 times the planet's initial orbital radius. Fixed-orbit simulations show that all planet masses we consider open gravitationally unstable gaps, but the instability is stronger and develops sooner with increasing planet mass. The disc-on-planet torques typically become more positive with increasing planet mass. In freely-migrating simulations, we observe faster outward migration with increasing planet mass, but only for planet masses capable of opening unstable gaps early on. For q=0.0003, the planet undergoes rapid inward type III migration before it can open a gap. For q=0.0013 we find it is possible to balance the tendency for inward migration by the positive torques due to an unstable gap, but only for a few 10's of orbital periods. We find the unstable outer gap edge can trigger outward type III migration, sending the planet to twice its initial orbital radius on dynamical timescales. This triggered type III migration may be a way of bringing giant planets to large orbital distances. On the other hand, our results also imply gap-opening is not a sufficient condition for the in situ formation of giant planets on wide orbits through disc fragmentation.

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