Evryscope和K2对TRAPPIST-1超耀斑发生和行星可居住性的约束
Evryscope and K2 Constraints on TRAPPIST-1 Superflare Occurrence and Planetary Habitability
摘要：
附近的超冷矮行星TRAPPIST-1拥有几颗地球大小的类地行星，其中三颗行星的平衡温度可能支持液态表面水，使其成为系外行星表征的引人注目的目标。TRAPPIST-1是一颗活跃的恒星，经常发生耀斑，这对它的行星的可居住性有影响。超级耀斑（能量超过10^33尔格的恒星耀斑）可以完全摧毁冷恒星行星的大气层，使紫外线辐射和高能粒子轰击它们的表面。然而，超冷矮星在静止时几乎不发出紫外线通量，这增加了需要紫外线的生命前化学所必需的频繁耀斑的可能性。我们结合Evryscope和开普勒观测来描述TRAPPIST-1的高能耀斑速率。Evryscope是一个由22个小型望远镜组成的阵列，每两分钟对整个南部天空进行一次成像。Evryscope观测，跨越170个夜晚超过2年，补充了80天的连续短节奏K2观测采样TRAPPIST-1的长期耀斑活动。我们更新了TRAPPIST-1的超级耀斑率，发现每年4.2（+1.9 -0.2）次超级耀斑的累积率。我们计算了消耗可居住区行星大气中臭氧所需的耀斑速率，发现TRAPPIST-1的耀斑速率不足以消耗其行星上的臭氧。此外，我们还计算了提供足够的紫外线通量来为生命前化学提供动力所需的耀斑速率。我们发现TRAPPIST-1的耀斑速率可能不足以催化一些被认为导致RNA合成的地球化学途径，并且由于生物相关UV-B波段的耀斑导致的通量对于任何TRAPPIST-1行星来说都比地球历史上任何时候经历的要小几个数量级。
英文摘要：
The nearby ultracool dwarf TRAPPIST-1 possesses several Earth-sized terrestrial planets, three of which have equilibrium temperatures that may support liquid surface water, making it a compelling target for exoplanet characterization. TRAPPIST-1 is an active star with frequent flaring, with implications for the habitability of its planets. Superflares (stellar flares whose energy exceeds 10^33 erg) can completely destroy the atmospheres of a cool star's planets, allowing ultraviolet radiation and high-energy particles to bombard their surfaces. However, ultracool dwarfs emit little ultraviolet flux when quiescent, raising the possibility of frequent flares being necessary for prebiotic chemistry that requires ultraviolet light. We combine Evryscope and Kepler observations to characterize the high-energy flare rate of TRAPPIST-1. The Evryscope is an array of 22 small telescopes imaging the entire Southern sky in g' every two minutes. Evryscope observations, spanning 170 nights over 2 years, complement the 80-day continuous short-cadence K2 observations by sampling TRAPPIST-1's long-term flare activity. We update TRAPPIST-1's superflare rate, finding a cumulative rate of 4.2 (+1.9 -0.2) superflares per year. We calculate the flare rate necessary to deplete ozone in the habitable-zone planets' atmospheres, and find that TRAPPIST-1's flare rate is insufficient to deplete ozone if present on its planets. In addition, we calculate the flare rate needed to provide enough ultraviolet flux to power prebiotic chemistry. We find TRAPPIST-1's flare rate is likely insufficient to catalyze some of the Earthlike chemical pathways thought to lead to RNA synthesis, and flux due to flares in the biologically relevant UV-B band is orders of magnitude less for any TRAPPIST-1 planet than has been experienced by Earth at any time in its history.
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