[论文解读] The VLT/NaCo large program to probe the occurrence of exoplanets and brown dwarfs at wide orbits. IV. Gravitational instability rarely forms wide, giant planets

A. Vigan, M. Bonavita+35 more|arXiv (Cornell University)|Mar 15, 2017

Stellar, planetary, and galactic studies参考文献 122被引用 7

一句话总结

本研究结合VLT/NaCo和12项以往的直接成像调查，分析距离100 pc内的199颗FGK型恒星周围宽轨道类木行星（1–75 M_Jup，≥20 AU）的出现频率。通过蒙特卡洛模拟和星体合成模型，发现引力不稳定性（GI）可解释宽轨道伴星的质量与分离距离分布，仅1.0–8.6%的恒星拥有由GI形成的行星系统，表明GI虽罕见但与观测结果一致。

ABSTRACT

Understanding the formation and evolution of giant planets ($\ge$1 $M_{Jup}$) at wide orbital separation ($\ge$5 AU) is one of the goals of direct imaging. Over the past 15 years, many surveys have placed strong constraints on the occurrence rate of wide-orbit giants, mostly based on non-detections, but very few have tried to make a direct link with planet formation theories. In the present work, we combine the results of our previously published VLT/NaCo large program with the results of 12 past imaging surveys to constitute a statistical sample of 199 FGK stars within 100 pc, including 3 stars with sub-stellar companions. Using Monte Carlo simulations and assuming linear flat distributions for the mass and semi-major axis of planets, we estimate the sub-stellar companion frequency to be within 0.75-5.7% at the 68% confidence level (CL) within 20-300 AU and 0.5-75 $M_{Jup}$, which is compatible with previously published results. We also compare our results with the predictions of state-of-the-art population synthesis models based on the gravitational instability (GI) formation scenario by Forgan & Rice (2013), with and without scattering. We estimate that in both the scattered and non-scattered populations, we would be able to detect more than 30% of companions in the 1-75 $M_{Jup}$ range (95% CL). With the 3 sub-stellar detections in our sample, we estimate the fraction of stars that host a planetary system formed by GI to be within 1.0-8.6% (95% CL). We also conclude that even though GI is not common, it predicts a mass distribution of wide-orbit massive companions that is much closer to what is observed than what the core accretion scenario predicts. Finally, we associate the present paper with the release of the Direct Imaging Virtual Archive (DIVA, http://cesam.lam.fr/diva/), a public database that aims at gathering the results of past, present, and future direct imaging surveys.

研究动机与目标

• 利用直接成像数据，确定附近FGK型恒星周围宽轨道类木行星（≥1 M_Jup，≥20 AU）的出现频率。
• 检验引力不稳定性（GI）形成机制是否能解释观测到的大质量、宽轨道亚恒星伴星群体。
• 将观测约束与基于GI和核心吸积（CA）情景的最新星体合成模型进行比较。
• 评估GI模型与观测到的伴星检测率和质量分布之间的统计一致性。
• 通过发布公开数据库（DIVA）支持未来调查，整合过去和未来的直接成像结果。

提出的方法

• 整合VLT/NaCo大型项目（184.C-0157）和12项先前针对100 pc内199颗FGK型恒星的直接成像调查的数据。
• 采用蒙特卡洛模拟，假设行星质量与半长轴呈线性平坦分布，以估算亚恒星伴星的频率。
• 利用各次调查的探测极限，模拟在参数空间（1–75 M_Jup，20–300 AU）内伴星的可探测性。
• 使用95%置信水平（CL）的统计框架，将模拟探测率与实际观测结果进行比较。
• 评估Forgan & Rice（2013）星体合成模型中GI形成伴星（有无散射）的预测，分析可探测性阈值。
• 发布直接成像虚拟档案（DIVA），统一整合过去、当前和未来直接成像调查的结果。

实验结果

研究问题

• RQ1 基于直接成像调查，附近FGK型恒星周围宽轨道类木行星（1–75 M_Jup，≥20 AU）的出现频率是多少？
• RQ2 引力不稳定性（GI）形成情景能否解释观测到的大质量、宽轨道伴星群体？
• RQ3 GI形成伴星的预测可探测性与样本中的实际探测率相比如何？
• RQ4 观测到的宽轨道伴星质量与分离距离分布更符合GI还是核心吸积（CA）形成模型？
• RQ5 目前的调查在多大程度上探测到了预期的GI形成行星群体，这对形成理论有何启示？

主要发现

• 在20–300 AU范围内、质量为0.5–75 M_Jup的参数区间内，亚恒星伴星频率估计为68%置信水平下的0.75–5.70%。
• 在样本中观测到三个亚恒星伴星，因此由引力不稳定性形成的行星系统所占恒星比例估计为95%置信水平下的1.0–8.6%。
• GI模型预测的宽轨道大质量伴星质量分布，与观测结果相比显著更接近。
• 即使在无散射的GI群体中，质量在1–75 M_Jup范围内的伴星仍有超过30%可被探测到（95% CL），与实际探测率一致。
• 结果与Forgan & Rice（2013）的星体合成模型兼容，验证了此类模型在统计推断中的适用性。
• 本研究支持发布直接成像虚拟档案（DIVA），即一个整合多个直接成像调查结果的公开数据库，以支持未来统计分析。