WASP-107b’s Density Is Even Lower: A Case Study for the Physics of Planetary Gas Envelope Accretion and Orbital Migration
WASP-107b’s Density Is Even Lower: A Case Study for the Physics of Planetary Gas Envelope Accretion and Orbital Migration
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DOI：
10.3847/1538-3881/abcd3c
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发表时间：
2020-11
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影响因子：
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通讯作者：
C. Piaulet;B. Benneke;R. Rubenzahl;A. Howard;Eve J. Lee;D. Thorngren;R. Angus;Merrin Peterson
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文献类型：
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作者：
C. Piaulet;B. Benneke;R. Rubenzahl;A. Howard;Eve J. Lee;D. Thorngren;R. Angus;Merrin Peterson
WASP-107b 的质量位于海王星范围内，半径与木星相当，对行星形成理论提出了挑战。与此同时，行星的低表面重力和恒星的亮度也使其成为大气特征最有利的目标之一。在这里，我们介绍了 WASP-107 系统为期 4 年的 Keck/HIRES 径向速度 (RV) 后续计划的结果，并提供了控制 WASP-107b 气体包层吸积的物理原理的详细研究。我们发现 WASP-107b 的质量仅为海王星质量的 1.8 个（M b = 30.5 ± 1.7 M ⊕）。由此产生的极低密度表明 WASP-107b 的 H/He 包络质量分数 > 85%，除非其大幅膨胀。 3σ 处相应的核心质量 <4.6 M ⊕ 明显低于传统上假设的触发大量气体包层吸积所需的质量。我们证明，这种大的气体与核心质量比最有可能是由于在 ≳1 au 时开始吸积到低不透明度、无尘大气中以及随后迁移到现在的 a b = 0.0566 ± 0.0017 au 造成的。除了 WASP-107b 之外，我们还在宽偏心轨道 (e c = 0.28 ± 0.07) 上检测到第二颗质量更大的行星 ( )，这可能影响了 WASP-107b 的轨道迁移和自旋轨道错位。总的来说，我们新的 RV 观测和包络吸积模型为了解 WASP-107b 的有趣性质和该系统的形成历史提供了重要的见解。展望未来，WASP-107b 将成为了解气体包层吸积物理的关键行星。
With a mass in the Neptune regime and a radius of Jupiter, WASP-107b presents a challenge to planet formation theories. Meanwhile, the planet’s low surface gravity and the star’s brightness also make it one of the most favorable targets for atmospheric characterization. Here, we present the results of an extensive 4 yr Keck/HIRES radial-velocity (RV) follow-up program of the WASP-107 system and provide a detailed study of the physics governing the accretion of the gas envelope of WASP-107b. We reveal that WASP-107b’s mass is only 1.8 Neptune masses (M b = 30.5 ± 1.7 M ⊕). The resulting extraordinarily low density suggests that WASP-107b has a H/He envelope mass fraction of >85% unless it is substantially inflated. The corresponding core mass of <4.6 M ⊕ at 3σ is significantly lower than what is traditionally assumed to be necessary to trigger massive gas envelope accretion. We demonstrate that this large gas-to-core mass ratio most plausibly results from the onset of accretion at ≳1 au onto a low-opacity, dust-free atmosphere and subsequent migration to the present-day a b = 0.0566 ± 0.0017 au. Beyond WASP-107b, we also detect a second, more massive planet ( ) on a wide eccentric orbit (e c = 0.28 ± 0.07) that may have influenced the orbital migration and spin–orbit misalignment of WASP-107b. Overall, our new RV observations and envelope accretion modeling provide crucial insights into the intriguing nature of WASP-107b and the system’s formation history. Looking ahead, WASP-107b will be a keystone planet to understand the physics of gas envelope accretion.