Helium-enhanced planets along the upper edge of the radius valley
Helium-enhanced planets along the upper edge of the radius valley
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DOI：
10.1038/s41550-022-01823-8
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发表时间：
2022-11
期刊：
影响因子：
14.1
通讯作者：
Isaac Malsky;L. Rogers;E. Kempton;N. Marounina
中科院分区：
文献类型：
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作者：
Isaac Malsky;L. Rogers;E. Kempton;N. Marounina
开普勒巡天发现，次海王星的半径分布是双峰的：在1.5到2.0R <$之间的行星很少。然而，形成山谷的机制尚不清楚。次海王星的低平均密度意味着它们在几百万年内形成，并吸积了原始的包层。由于这些行星接收的X射线和紫外线通量与其外壳的引力结合能相当，因此它们的大气层容易受到质量损失的影响。我们模拟的热量和成分演变的亚海王星进行逃逸与扩散分离之间的氢和氦，并表明，优先损失的氢可以改变其大气成分。半径在1.6到2.5R之间的行星可以通过数十亿年的光蒸发获得超过40%的大气氦质量分数。这种增强可以通过透射光谱检测到，提供了一种新的观测测试，以确定是否大气逃逸创建半径谷。
The Kepler survey revealed that the radius distribution of sub-Neptunes is bimodal: there is a scarcity of planets between 1.5 and 2.0R⊕. However, the mechanism that creates the valley is unknown. The low mean densities of sub-Neptunes imply that they formed within a few million years and accreted primordial envelopes. Because these planets receive X-ray and UV fluxes comparable to the gravitational binding energy of their envelopes, their atmospheres are susceptible to mass loss. We model the thermal and compositional evolution of sub-Neptunes undergoing escape with diffusive separation between hydrogen and helium and show that preferential loss of hydrogen can change their atmospheric compositions. Planets with radii between 1.6 and 2.5R⊕can obtain atmospheric helium mass fractions in excess of 40% from billions of years of photoevaporation. Such enhancement can be detected through transmission spectroscopy, providing a novel observational test to determine whether atmospheric escape creates the radius valley.