恒星周围成像的行星与褐矮星的元素丰度（ELPIS）：I. 系外行星 AF Lep b 的潜在金属增丰及一种针对多云自发光大气的新型反演方法
ELemental abundances of Planets and brown dwarfs Imaged around Stars (ELPIS): I. Potential Metal Enrichment of the Exoplanet AF Lep b and a Novel Retrieval Approach for Cloudy Self-luminous Atmospheres
AI总结：
该研究通过新型温度梯度约束反演方法，分析直接成像系外行星 AF Lep b 的大气，发现其金属增丰（[Fe/H] > 1.0 dex）并存在硅酸盐云，为行星形成与演化提供新见解。
摘要：
AF Lep A+b 是一个引人注目的行星系统，其拥有一颗巨行星，该行星在直接成像的系外行星中具有最低的动力学质量。我们对该恒星和行星的大气成分进行了深入分析，以探究行星的形成路径。基于 AF Lep A 的新高分辨率光谱，我们测量了一组均匀的恒星参数和元素丰度（例如，[Fe/H] = $-0.27 \pm 0.31$ dex）。利用已发表的径向速度、相对天体测量和绝对天体测量，我们还对行星的动力学质量（$2.8^{+0.6}_{-0.5}$ M$_{\rm Jup}$）和轨道进行了修正。我们使用 petitRADTRANS 对 AF Lep b 进行化学自洽的大气反演。辐射对流平衡温度剖面作为参数化先验被纳入行星热结构，从而实现对多云自发光大气的稳健表征。这一新颖方法通过温度梯度 $(d\ln{T}/d\ln{P})$ 约束温度-压力剖面而实现，这不同于以往仅对温度进行建模的研究。通过在 $0.9-4.2$ $\mu$m 分光光度数据的不同部分以及行星质量和半径的不同先验下进行多次反演，我们推断 AF Lep b 可能拥有金属增丰的大气（[Fe/H] $> 1.0$ dex）。AF Lep b 潜在的金属增丰可能源于星子吸积、巨大撞击和/或核心侵蚀。第一种过程与该系统中的碎片盘相吻合，该碎片盘可能被 AF Lep b 动态激发并导致星子轰击。我们的分析还确定了 $T_{\rm eff} \approx 800$ K、$\log{(g)} \approx 3.7$ dex，以及大气中存在硅酸盐云和非平衡化学。跨越 L/T 转变，AF Lep b 是迄今提出存在硅酸盐云证据的最冷系外行星。
英文摘要：
AF Lep A+b is a remarkable planetary system hosting a gas-giant planet that has the lowest dynamical mass among directly imaged exoplanets. We present an in-depth analysis of the atmospheric composition of the star and planet to probe the planet's formation pathway. Based on new high-resolution spectroscopy of AF Lep A, we measure a uniform set of stellar parameters and elemental abundances (e.g., [Fe/H] = $-0.27 \pm 0.31$ dex). The planet's dynamical mass ($2.8^{+0.6}_{-0.5}$ M$_{\rm Jup}$) and orbit are also refined using published radial velocities, relative astrometry, and absolute astrometry. We use petitRADTRANS to perform chemically-consistent atmospheric retrievals for AF Lep b. The radiative-convective equilibrium temperature profiles are incorporated as parameterized priors on the planet's thermal structure, leading to a robust characterization for cloudy self-luminous atmospheres. This novel approach is enabled by constraining the temperature-pressure profiles via the temperature gradient $(d\ln{T}/d\ln{P})$, a departure from previous studies that solely modeled the temperature. Through multiple retrievals performed on different portions of the $0.9-4.2$ $μ$m spectrophotometry, along with different priors on the planet's mass and radius, we infer that AF Lep b likely possesses a metal-enriched atmosphere ([Fe/H] $> 1.0$ dex). AF Lep b's potential metal enrichment may be due to planetesimal accretion, giant impacts, and/or core erosion. The first process coincides with the debris disk in the system, which could be dynamically excited by AF Lep b and lead to planetesimal bombardment. Our analysis also determines $T_{\rm eff} \approx 800$ K, $\log{(g)} \approx 3.7$ dex, and the presence of silicate clouds and dis-equilibrium chemistry in the atmosphere. Straddling the L/T transition, AF Lep b is thus far the coldest exoplanet with suggested evidence of silicate clouds.
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