年轻系外行星缺失甲烷的可能原因是恒星耀斑而非高内部温度：以V1298 Tau b为例
Stellar Flares Rather than High $T_{int}$ As Possible Cause for the Missing Methane in Young Exoplanets: The Case Study of V1298~Tau\,b
AI总结：
该研究以V1298 Tau b为案例，提出年轻系外行星甲烷缺失的原因是恒星耀斑而非高内部温度，通过模拟耀斑诱导的大气稳态，复现了该行星的JWST观测特征。
AI中文摘要：
近期对V1298 Tau b的观测显示，该行星是一颗围绕年轻（10-30百万年）K型恒星运行的温暖亚海王星（半径R=9.85±0.3R⊕，质量M=13.1±5.3M⊕，平衡温度T_eq=670K），其大气中甲烷匮乏且二氧化碳丰度惊人，这使其成为研究“甲烷缺失问题”的有趣目标——哈勃空间望远镜（HST）、斯皮策空间望远镜（Spitzer）和詹姆斯·韦布空间望远镜（JWST）已发现，许多有效温度低于800K的亚木星行星，其甲烷含量与平衡化学预测值相比存在匮乏。若认为温暖内部的强混合是甲烷缺失的原因，V1298 Tau b需要500K的内部温度（T_int）来解释甲烷匮乏，但根据标准内部结构演化理论，该温度对于其年龄和大小的行星而言过高。本文表明，恒星耀斑可使温暖行星大气在内部温度（T_int）约为100K的标称值下出现甲烷匮乏：恒星耀斑期间，甲烷的快速光解速率无法被从下方扩散到观测探测的上层大气所平衡；年轻活跃恒星频繁发生耀斑，研究显示甲烷从耀斑诱导的匮乏中恢复的扩散时间尺度相对于耀斑频率而言较慢；随着时间推移，大气达到由平均耀斑频率和能量决定的新稳态，该稳态在2毫巴区域呈现甲烷匮乏，且一氧化碳、二氧化碳和氰化氢丰度充足，其合成透射光谱可复现JWST近期对V1298 Tau b观测的主要特征。
英文摘要：
Recent observations of V1298~Tau\,b, a warm sub-Neptune (R$=9.85 \pm 0.3\,R_\oplus$, M$=13.1 \pm 5.3, M_\oplus$, T$_\mathrm{eq} = 670\,$K) orbiting a young (10-30\,Myr) K star, show depleted methane and surprising abundances of CO$_2$ in its atmosphere. This makes the planet an interesting target to study the "missing methane problem", where HST, Spitzer, and JWST have revealed many sub-Jovian planets with effective temperatures cooler than 800\,K to be depleted in methane compared to equilibrium chemistry predictions. On V1298~Tau\,b, an intrinsic temperature (T$_\mathrm{int}$) of 500\,K is required to explain the methane depletion if strong mixing from a warm interior is the reason for the missing methane, but this T$_\mathrm{int}$ is too large for a planet of its age and size according to standard interior structure evolution theories. Here, we show that stellar flares can result in methane depletion in a warm planet's atmosphere with nominal T$_\mathrm{int} \sim 100$\,K. During a stellar flare, fast methane photolysis rates cannot be balanced by diffusion from below into the upper atmospheric layers probed by observations. Young active stars flare frequently, and we show that the timescale of diffusion for the methane to recover from a flare-induced depletion can be slow compared to the flare frequency. Over time, the atmosphere reaches a new steady state set by the average flare frequency and energy. This new steady state shows depleted methane in the 2\,mbar region, and abundant CO, CO${_2}$, and HCN, which results in synthetic transmission spectra that reproduce the main features of the recent JWST observations of V1298~Tau\,b.
