Exomoon Phase Curves: Toroidal Exosphere Simulations of Exo-Ios Orbiting 8 Exoplanets in Alkali Spectroscopy

Toroidal atmospheres and exospheres characterized at exoplanets may be fueled by volcanically- active exomoons, often referred to as exo-Ios. We study the neutral outgassing and volatile evolution of a close-orbiting, evaporating satellite at eight candidate exoplanet-exomoon systems WASP-49,-96,-69,-17 b, XO-2N b, HAT-P-1 b, HD-189733 b, and HD-209458 b by developing a 3-D test-particle Monte-Carlo simulation, SERPENS. The module is cou- pled to dishoom, approximating the minimum mass-flux needed to reproduce observa- tions of alkali line profiles identified in dozens of transmission spectra. We focus on sput- tered neutral sodium, limited by photoionization and radiative effects. By considering Earth-, Io-, and Enceladus-like masses, we systematically simulate the imprint of a non- hydrostatic medium (characteristic of volcanic exospheres) in density and velocity space using a novel Delaunay tesselation field estimator algorithm. Our results demonstrate how exomoons can considerably modulate gas density observations probed near exoplanet transit, depending on the orbital phase of the putative satellite at the time of observa- tion. The density evolution, therefore, manifests on orbital timescales as ‘exomoon phase curves’ from shadow to occultation. We find two regimes of density evolution, charac- teristic of a: (A) localized cloud and (B) an azimuthally symmetric exoring/torus, de- generate with an exoplanet atmosphere, ranging from ∼109.5±0.5 cm−2 to ∼1015±0.25 cm−2 at our leading candidate WASP-69b I. In certain orbital architectures, the smallest evap- orating satellite mass surprisingly generates the brightest sodium signal, fueling optimism for discovering photometrically-indiscernible rocky exomoons. We suggest long baseline monitoring of alkali and SO2 systems in spectroscopy to search for the temporal and spa- tial variability predicted here.

系外行星所拥有的环状大气与外逸层（exospheres），其物质供给可能来自火山活动活跃的系外卫星——这类卫星常被称为木卫一型系外卫星（exo-Ios）。本研究针对8个候选系外行星-系外卫星系统（包含WASP-49 b、WASP-96 b、WASP-69 b、WASP-17 b、XO-2N b、HAT-P-1 b、HD 189733 b与HD 209458 b），通过构建名为SERPENS的三维测试粒子蒙特卡洛模拟程序，探究近轨道蒸发卫星的中性脱气过程与挥发性组分演化规律。该模拟模块耦合至dishoom程序，以近似估算重现数十条透射光谱中观测到的碱金属谱线轮廓所需的最小质量通量。本研究聚焦于受光致电离与辐射效应约束的溅射中性钠原子。通过参考类地球、类木卫一以及类土卫二的卫星质量，我们采用一种新型德劳内三角剖分场估计器（Delaunay tesselation field estimator）算法，系统模拟了火山活动外逸层典型的非静力学介质在密度与速度空间中的分布特征。研究结果表明，系外卫星可显著调制系外行星凌星阶段探测到的气体密度分布，其调制效果取决于观测时刻假定卫星的轨道相位。因此，气体密度演化在轨道时间尺度上表现为从阴影区到掩星阶段的‘系外卫星相位曲线’。我们发现两类密度演化模式：分别对应（A）局域云团与（B）方位对称的系外环/环面结构，后者与系外行星大气存在简并效应；在首选候选系统WASP-69 b I中，这类信号的柱密度范围约为~10^9.5±0.5 cm⁻²至~10^15±0.25 cm⁻²。在特定轨道构型下，质量最小的蒸发型卫星反而能产生最强的钠原子信号，这为发现测光不可分辨的岩质系外卫星带来了研究信心。我们建议通过光谱学手段对碱金属与二氧化硫（SO₂）体系开展长基线监测，以搜寻本文预测的时空变异性信号。