标题： 热电离层光谱学研究最低密度气态巨行星：HAT-P-67b中的金属和潜在的扩展外流 显示英文标题

标题： Transmission spectroscopy of the lowest-density gas giant: metals and a potential extended outflow in HAT-P-67b

摘要： 极端低密度系外行星是大气特征分析的诱人目标，因为它们在透射光谱中显示出有望的强信号。 我们对目前已知最低密度气态巨行星HAT-P-67 b的大气进行了首次分析。 这个膨胀的土星质量系外行星位于热气态巨行星和超热气态巨行星的边界处，此时分子的热离解开始主导大气成分。 我们使用CARMENES以高光谱分辨率观测了HAT-P-67 b的凌日，并通过交叉相关和似然映射搜索原子和分子物种。 此外，我们通过针对H$\alpha$以及亚稳态氦线来探索潜在的大气逃逸。 我们分别以13.2$\sigma$ 和4.6$\sigma$的显著性检测到Ca II和Na I。 与几个超热木星不同，我们没有测量到昼夜风。 Ca II的深线表明，高层大气可能比模型预测的更加电离。 我们在观测期间检测到H$\alpha$ 和氦三重态的强烈变化。 这些信号表明可能存在扩展的行星流出，导致早期进入和晚期退出。 在平均透射光谱中，我们在H$\alpha$ 和He I三重态线中的$\sim 3.8\%$ 和$\sim 4.5\%$水平测量到红移吸收。 从等温帕克风模型中，我们推导出质量损失率为 $\dot{M} \sim 10^{13}~\rm{g/s}$ 和外流温度为 $T \sim 9900~\rm{K}$。 然而，由于我们的数据中缺乏更长的凌星外基线，需要额外的观测来排除恒星变异性作为 H $\alpha$ 和 He 信号的来源。 显示英文摘要

摘要： Extremely low-density exoplanets are tantalizing targets for atmospheric characterization because of their promisingly large signals in transmission spectroscopy. We present the first analysis of the atmosphere of the lowest-density gas giant currently known, HAT-P-67 b. This inflated Saturn-mass exoplanet sits at the boundary between hot and ultrahot gas giants, where thermal dissociation of molecules begins to dominate atmospheric composition. We observed a transit of HAT-P-67 b at high spectral resolution with CARMENES and searched for atomic and molecular species using cross-correlation and likelihood mapping. Furthermore, we explored potential atmospheric escape by targeting H$\alpha$ and the metastable helium line. We detect Ca II and Na I with significances of 13.2$\sigma$ and 4.6$\sigma$, respectively. Unlike in several ultrahot Jupiters, we do not measure a day-to-night wind. The large line depths of Ca II suggest that the upper atmosphere may be more ionized than models predict. We detect strong variability in H$\alpha$ and the helium triplet during the observations. These signals suggest the possible presence of an extended planetary outflow that causes an early ingress and late egress. In the averaged transmission spectrum, we measure redshifted absorption at the $\sim 3.8\%$ and $\sim 4.5\%$ level in the H$\alpha$ and He I triplet lines, respectively. From an isothermal Parker wind model, we derive a mass loss rate of $\dot{M} \sim 10^{13}~\rm{g/s}$ and an outflow temperature of $T \sim 9900~\rm{K}$. However, due to the lack of a longer out-of-transit baseline in our data, additional observations are needed to rule out stellar variability as the source of the H$\alpha$ and He signals.