[论文解读] The New Generation Planetary Population Synthesis (NGPPS) II. Planetary population of solar-like stars and overview of statistical results

Alexandre Emsenhuber, C. Mordasini+4 more|arXiv (Cornell University)|Jul 10, 2020

Astro and Planetary Science被引用 16

一句话总结

本研究提出新一代行星系合成模型（NGPPS），利用第三代伯尔尼模型模拟类太阳恒星周围行星系的形成与演化。通过模拟每盘1–100颗原行星胚胎的五组群体，发现至少含10颗胚胎的系统可重现类木行星的统计特征，而仅100颗胚胎的群体达到巨行星撞击阶段，平均每系统产生8颗质量大于1 M⊕的行星，其中18%的系统拥有类木行星，1.6%的系统在10 au以外拥有类木行星。

ABSTRACT

Context. Planetary formation and evolution is a combination of multiple interlinked processes. Constraining the mechanisms observationally requires statistical comparison to a large diversity of planetary systems. Aims. We want to understand global observable consequences of different physical processes (accretion, migration, and interactions) and initial properties (like disc masses and metallicities) on the demographics of the planetary population. We also want to study the convergence of our scheme with respect to one initial condition, the initial number of planetary embryo in each disc. Methods. We selected distributions of initial conditions that are representative of known protoplanetary discs. Then, we used the Generation III Bern model to perform planetary population synthesis. We synthesise five populations with each a different initial number of Moon-mass embryos per disc: 1, 10, 20, 50, and 100. The last is our nominal population consisting of 1000 stars (systems) that was used for an extensive statistical analysis of planetary systems around 1 M⊙ stars. Results. The properties of giant planets do not change much as long as there are at least ten embryos in each system. The study of giants can thus be done with simulations requiring less computational resources. For inner terrestrial planets, only the 100-embryos population is able to attain the giant-impact stage. In that population, each planetary system contains, on average, eight planets more massive than 1 M⊕. The fraction of systems with giants planets at all orbital distances is 18%, but only 1.6% are at >10 au. Systems with giants contain on average 1.6 such planets. The planetary mass function varies as M−2 between 5 and 50 M⊕. Both at lower and higher masses, it follows approximately M−1. The frequency of terrestrial and super-Earth planets peaks at a stellar [Fe/H] of −0.2 and 0.0, respectively, being limited at lower [Fe/H] by a lack of building blocks, and by (for them) detrimental growth of more massive dynamically active planets at higher [Fe/H]. The frequency of more massive planets (Neptunian, giants) increases monotonically with [Fe/H]. The fast migration of planets in the 5–50 M⊕ range is reduced by the presence of multiple lower-mass inner planets in the multi-embryos populations. To assess the impact of parameters and model assumptions, we also study two non-nominal populations: insitu formation without gas-driven migration, and a different initial planetesimal surface density. Conclusions. We present one of the most comprehensive simulations of (exo)planetary system formation and evolution to date. For observations, the syntheses provides a large data set to search for comparison synthetic planetary systems that show how these systems have come into existence. The systems, including their full formation and evolution tracks are available online. For theory, they provide the framework to observationally test the global statistical consequences of theoretical models for specific physical processes. This is an important ingredient towards the development of a standard model of planetary formation and evolution.

研究动机与目标

• 理解物理过程（吸积、迁移、相互作用）与初始条件（盘质量、金属丰度）对行星系统计特征的全局可观测影响。
• 评估行星系合成模型在初始胚胎数量方面的收敛性。
• 确定捕捉关键行星系特性（尤其是类木行星）的最小模拟需求。
• 提供全面、公开可获取的合成数据集，用于与观测对比及模型验证。
• 支持建立行星系形成与演化标准模型。

提出的方法

• 模拟每原行星盘含1、10、20、50和100颗月球质量胚胎的五组行星系群体。
• 使用第三代伯尔尼模型，在10 Myr内执行完整的N体动力学与盘-行星相互作用模拟。
• 应用具有代表性的初始盘质量、金属丰度与星子面密度分布。
• 分析统计结果，包括行星多星度、轨道分布、质量函数及金属丰度依赖性。
• 在不同胚胎群体间比较结果，以评估收敛性及对初始条件的敏感性。
• 开展两项非标准模拟：无气态驱动迁移的原位形成模拟，以及初始星子面密度更陡的模拟。

实验结果

研究问题

• RQ1 行星系是否随初始胚胎数量收敛，特别是对类木行星而言？
• RQ2 重现观测到的行星系统计特征所需的最少胚胎数量是多少？
• RQ3 动力学相互作用与迁移如何影响行星系的最终结构？
• RQ4 行星质量函数与发生率如何随恒星金属丰度变化？
• RQ5 迁移假设与初始星子密度等模型假设如何影响群体结果？

主要发现

• 含100颗胚胎的平均行星系包含8颗质量大于1 M⊕的行星，其中18%的系统拥有类木行星，1.6%的系统在10 au以外拥有类木行星。
• 行星质量函数在5至50 M⊕之间遵循M−2关系，在质量更低和更高区域则近似为M−1关系。
• 类地行星与超级-Earth行星的频率分别在恒星[Fe/H] = -0.2与0.0处达到峰值，原因是在低金属丰度下构建块稀缺，而在高金属丰度下发生动力学扰动。
• 类木行星的发生率随[Fe/H]单调增加，与观测结果一致。
• 多颗低质量内侧行星的存在可抑制5–50 M⊕范围内的快速迁移。
• 质量≥10 M⊕的行星结果在含≥10颗胚胎的群体中具有鲁棒性，使气体吸积与类木行星比例研究得以实现资源高效模拟。