Close-in Giant Planets via In-situ Gas Accretion and Their Natal Disk Properties

Aims. The origin of close-in Jovian planets is still elusive. We examine the in-situ gas accretion scenario as a formation mechanism of these planets. Methods. We reconstruct natal disk properties from the occurrence rate distribution of close-in giant planets, under the assumption that the occurrence rate may reflect the gas accretion eciency onto cores of these planets. Results. We find that the resulting gas surface density profile becomes an increasing function of the distance from the central star with some structure at r ' 0:1 au. This profile is quite dierent from the standard minimum-mass solar nebula model while our profile leads to better reproduction of the population of observed close-in super-Earths based on previous studies. We compute the resulting magnetic field profiles and find that our profiles can be fitted by stellar dipole fields (/ r􀀀3) in the vicinity of the central star and large-scale fields (/ r􀀀2) at the inner disk regions, either if the isothermal assumption breaks down or if non-ideal MHD eects become important. For the both cases, the transition between these two profiles occur at r ' 0:1 au, which corresponds to the period valley of giant exoplanets. Conclusions. Our work therefore provides the opportunity of testing the in-situ gas accretion scenario against disk quantities, which may constrain the gas distribution of the minimum-mass extrasolar nebula.

研究目标：近距离气态巨行星的起源至今仍未明晰。我们将原位气体吸积场景作为这类行星的形成机制开展研究。
研究方法：我们基于近距离巨行星的出现率分布，重构原行星盘的物理属性，前提假设为该出现率可反映这类行星核的气体吸积效率。
研究结果：我们发现，得到的气体面密度分布随与中心恒星的距离增大而递增，且在r≈0.1天文单位处存在结构特征。该分布与标准的最小质量太阳星云模型存在显著差异，但结合既往研究来看，我们的分布能更好地复现观测到的近距离超级地球族群特征。我们还计算了对应的磁场分布，发现在中心恒星附近，该分布可通过恒星偶极场（∝r⁻³）拟合；而在原行星盘内区，分布则符合大尺度磁场（∝r⁻²），这两种情形分别对应等温假设失效或非理想磁流体动力学（MHD）效应占据主导的场景。两类分布的过渡区域均位于r≈0.1天文单位处，该位置恰好对应系外巨行星的周期谷。
研究结论：综上，本研究提供了通过盘体属性检验原位气体吸积场景的可行路径，可对最小质量系外星云的气体分布形成约束。