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The known populations of super-Earths and sub-Neptunes with orbital periods less than virgul100 days have been shaped by atmospheric evaporation comma leading to the observed Radius Valley near 1.7 Earth radii. However comma it is currently unclear what the dominant mass loss mechanism is for these planets. The two likely culprits are photoevaporation by the host star.s high energy radiation and core-powered evaporation comma where the latent heat of formation in the planet.s core causes the atmosphere to expand and escape. Distinguishing between these two mass loss mechanisms will require much more precise exoplanet radii statistics or comma alternatively comma measurements of mass loss for planets of varying ages and incident high energy flux. We propose to address this critical question by using COS to observe the transit of the virgul20 million year old planet AU Mic b and measure its mass loss rate in UV metal lines of carbon comma silicon comma and oxygen. AU Mic is the brightest pre-main sequence star known to host a transiting planet and offers a unique opportunity to detect the planet.s atmosphere at very high signal-to-noise in a number of UV mass loss indicators. Furthermore comma information from the UV metal lines is complementary to absorption in the hydrogen Lyman-alpha line comma which is heavily attenuated by the ISM. The metal lines trace material from the base of the thermosphere comma whereas Lyman-alpha samples the highly extended unbound exosphere. Measurements of young planet evaporation rates are vital benchmarks for atmospheric evolution models and AU Mic b is currently the best known exoplanet to perform such an experiment.

已知轨道周期小于100天的超级地球（super-Earth）和亚海王星（sub-Neptune）群体的形成受到大气蒸发的影响，这导致了观测到的半径谷（Radius Valley）出现在约1.7倍地球半径处。然而，目前尚不清楚这些行星的主要质量损失机制是什么。两种可能的原因是宿主恒星高能辐射引发的光蒸发（photoevaporation）和核心驱动蒸发（core-powered evaporation）——行星核心形成过程中的潜热导致大气膨胀并逃逸。区分这两种质量损失机制需要更精确的系外行星半径统计数据，或者对不同年龄和入射高能通量的行星进行质量损失测量。我们提议通过使用宇宙起源光谱仪（COS）观测年龄约为2000万年的行星AU Mic b的凌日现象，并利用碳、硅和氧的紫外金属线测量其质量损失率。AU Mic是已知拥有凌日行星的最亮前主序星（pre-main sequence star），它为在多个紫外质量损失指标中以极高信噪比检测行星大气提供了独特机会。此外，紫外金属线的信息与氢莱曼α线（hydrogen Lyman-alpha line）的吸收互补，后者受到星际介质（ISM）的严重衰减。金属线追踪来自热层底部的物质，而莱曼α线则取样高度扩展的未束缚外逸层。年轻行星蒸发率的测量是大气演化模型的关键基准，而AU Mic b是目前已知最适合开展此类实验的系外行星。