哈勃PanCET计划：M矮星宿主GJ 3470的长期色球演化和耀斑活动
The Hubble PanCET program: Long-term chromospheric evolution and flaring activity of the M dwarf host GJ 3470
摘要：
海王星大小的系外行星似乎对大气蒸发特别敏感，这使得描述驱动这一机制的恒星高能辐射至关重要。这对于M矮星来说尤其重要，因为它们在紫外线下发出的光度很大且变化不定，并且可以显示出强烈的耀斑行为。温暖的海王星GJ 3470 b，由一颗M2矮星主持，被发现拥有一个巨大的氢散逸层，这要归功于HST/STIS观察到的3次凌日。在这里，我们报告3个额外的过境观测从PanCET计划，获得与HST/COS。这些数据证实了GJ 3470 b的恒星Ly-alpha线外逸层的吸收特征，并证明了其随时间的稳定性。在其他谱线上没有发现行星的特征，因此GJ 3470 b的FUV半径的3 σ极限是其洛希瓣半径的1.3倍。我们探测到GJ 3470的3个耀斑。他们表现出不同的光谱能量分布，但峰值一致的Si III线，跟踪中温层的过渡区。这些层似乎在GJ 3470的活性中起着特殊的作用，因为在比Si III更低或更高的温度下形成的发射谱线在长期内演变不同。基于所测得的发射谱线，我们得到了6个观测到的静止期的合成XUV光谱，涵盖了一年，以及为3耀斑发作。我们的研究结果表明，大部分的GJ 3470的静态高能量发射来自EUV域，与耀斑放大FUV发射更强烈。GJ 3470 b的氢光电离寿命和质量损失在各个时期的变化很小，与外逸层的稳定性一致。我们的XUV光谱提供的模拟需要了解GJ 3470 b的大气结构和演变以及蒸发在热海王星沙漠形成中所起的作用。
英文摘要：
Neptune-size exoplanets seem particularly sensitive to atmospheric evaporation, making it essential to characterize the stellar high-energy radiation that drives this mechanism. This is particularly important with M dwarfs, which emit a large and variable fraction of their luminosity in the UV and can display strong flaring behavior. The warm Neptune GJ3470b, hosted by an M2 dwarf, was found to harbor a giant hydrogen exosphere thanks to 3 transits observed with the HST/STIS. Here we report on 3 additional transit observations from the PanCET program, obtained with the HST/COS. These data confirm the absorption signature from GJ3470b's exosphere in the stellar Ly-alpha line and demonstrate its stability over time. No planetary signatures are detected in other lines, setting a 3sigma limit on GJ3470b's FUV radius at 1.3x its Roche lobe radius. We detect 3 flares from GJ3470. They show different spectral energy distributions but peak consistently in the Si III line, which traces intermediate-temperature layers in the transition region. These layers appear to play a particular role in GJ3470's activity as emission lines that form at lower or higher temperatures than Si III evolved differently over the long term. Based on the measured emission lines, we derive synthetic XUV spectra for the 6 observed quiescent phases, covering one year, as well as for the 3 flaring episodes. Our results suggest that most of GJ3470's quiescent high-energy emission comes from the EUV domain, with flares amplifying the FUV emission more strongly. The hydrogen photoionization lifetimes and mass loss derived for GJ3470b show little variation over the epochs, in agreement with the stability of the exosphere. Simulations informed by our XUV spectra are required to understand the atmospheric structure and evolution of GJ3470b and the role played by evaporation in the formation of the hot-Neptune desert.
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