A Possible Mechanism for the “Late Phase” in Stellar White-light Flares
A Possible Mechanism for the “Late Phase” in Stellar White-light Flares
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DOI：
10.3847/1538-4357/ad077d
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发表时间：
2023-12
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影响因子：
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通讯作者：
Kai E. 凯 Yang 杨;X. Sun 孙;Graham S. Kerr;Hugh S. Hudson
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作者：
Kai E. 凯 Yang 杨;X. Sun 孙;Graham S. Kerr;Hugh S. Hudson
凌日系外行星探测卫星(TESS)观测到的M矮星耀斑有时表现出峰凸不平的光曲线形态，其特征是在主脉冲峰值之后出现一个次要的渐变峰值。类似的后期阶段经常在太阳耀斑中被观测到，这些耀斑是从更长的热冕环中观察到的，与冲动性耀斑结构不同。在非边缘太阳耀斑环中也观察到了白光发射。在这里，我们对受到这些太阳例子启发的M矮星耀斑进行了一组一维流体力学回路模拟。我们的结果表明，脉冲耀斑加热后的日冕等离子体凝聚可以在回路中产生高电子数密度，使其通过自由束缚和自由自由发射机制对光学曲线做出显著贡献。我们的模拟结果与TESS观测结果定性一致：日冕环的演化时间尺度越长，就会产生一个明显的次级发射峰；其强度随着注入耀斑能量的增加而增加。我们认为日冕等离子体凝聚是TESS后期耀斑的一种可能机制。
M dwarf flares observed by the Transiting Exoplanet Survey Satellite (TESS) sometimes exhibit a peak-bump light-curve morphology, characterized by a secondary, gradual peak well after the main, impulsive peak. A similar late phase is frequently detected in solar flares observed in the extreme ultraviolet from longer hot coronal loops distinct from the impulsive flare structures. White-light emission has also been observed in off-limb solar flare loops. Here, we perform a suite of one-dimensional hydrodynamic loop simulations for M dwarf flares inspired by these solar examples. Our results suggest that coronal plasma condensation following impulsive flare heating can yield high electron number density in the loop, allowing it to contribute significantly to the optical light curves via free-bound and free–free emission mechanisms. Our simulation results qualitatively agree with TESS observations: the longer evolutionary timescale of coronal loops produces a distinct, secondary emission peak; its intensity increases with the injected flare energy. We argue that coronal plasma condensation is a possible mechanism for the TESS late-phase flares.