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arXiv 1701.04892astro-ph.EP
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原行星盘冷却时间对引力不稳定性形成气体巨行星的影响

The Effect of Protoplanetary Disk Cooling Times on the Formation of Gas Giant Planets by Gravitational Instability

Alan P. Boss

摘要:

有观测证据表明,在年轻恒星周围的宽轨道上,由于盘引力不稳定性而形成了气体巨行星,但盘不稳定性和核心吸积在形成短周期轨道上的气体巨行星中的作用还不太清楚。争议延伸到人口合成模型的系外行星人口和流体动力学模型的碎裂过程。后者主要是指在三维(3D)流体动力学模型中处理辐射转移,控制重力不稳定盘的加热和冷却过程,从而形成致密的团块。这里介绍了一套使用$β$冷却近似的模型。初始盘的质量为0.091 $M_\odot$,围绕1 $M_\odot$的原恒星从4到20 AU延伸。初始最小Toomre $Q_i$值的范围为1.3至2.7,而$β$的范围为1至100。我们表明,选择的$Q_i$是同等重要的$β$值假设:高$Q_i$磁盘可以稳定的小$β$,当初始磁盘温度作为一个下限,而低$Q_i$磁盘可以碎片高$β$。这些结果意味着,在评估磁盘碎片化的可能性时,必须考虑磁盘向低$Q_i$的演变,至少在内部磁盘中,即,大约20 AU。这些模型表明,如果低$Q_i$盘能够形成,那么在大约6 AU到16 AU之间,应该有大量尚未被发现的气体巨星围绕G矮星运行。

英文摘要:

Observational evidence exists for the formation of gas giant planets on wide orbits around young stars by disk gravitational instability, but the roles of disk instability and core accretion for forming gas giants on shorter period orbits are less clear. The controversy extends to population synthesis models of exoplanet demographics and to hydrodynamical models of the fragmentation process. The latter refers largely to the handling of radiative transfer in three dimensional (3D) hydrodynamical models, which controls heating and cooling processes in gravitationally unstable disks, and hence dense clump formation. A suite of models using the $β$ cooling approximation is presented here. The initial disks have masses of 0.091 $M_\odot$ and extend from 4 to 20 AU around a 1 $M_\odot$ protostar. The initial minimum Toomre $Q_i$ values range from 1.3 to 2.7, while $β$ ranges from 1 to 100. We show that the choice of $Q_i$ is equal in importance to the $β$ value assumed: high $Q_i$ disks can be stable for small $β$, when the initial disk temperature is taken as a lower bound, while low $Q_i$ disks can fragment for high $β$. These results imply that the evolution of disks toward low $Q_i$ must be taken into account in assessing disk fragmentation possibilities, at least in the inner disk, i.e., inside about 20 AU. The models suggest that if low $Q_i$ disks can form, there should be an as yet largely undetected population of gas giants orbiting G dwarfs between about 6 AU and 16 AU.

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