混合世界是JWST表征的M矮星亚海王星中最常见的结果
Beyond gas dwarfs and water worlds: hybrid envelopes among the M-dwarf sub-Neptunes characterised with JWST
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中文总结 AI 辅助
研究通过JWST光谱反演大气平均分子量，发现M矮星亚海王星中混合世界（既非原始氢包层也非富冰内部）是最常见结果，挑战了传统二元分类。
中文摘要 AI 辅助
围绕M矮星运行的亚海王星通常被归为两类：具有原始氢包层的岩石核，或在冰线之外形成的富挥发分天体。体密度无法区分这两类，因为一旦自洽地模拟包层演化，它们在质量和半径上存在简并（Rogers et al. 2023）；大气平均分子量{\mu}是突破这一困境的途径。我们汇总了八颗具有已发表JWST光谱且{\mu}受约束的M矮星亚海王星，并将其（按证据质量分级）与受限的Jeans参数以及在整个系统寿命内积分的能量限制逃逸相结合。XUV历史取自各宿主恒星光谱型对应的M矮星活动-年龄关系，而非参数化形式，并明确转换到1-912埃波段。八颗中有四颗不满足任一类别，其包层{\mu}值在3.8到18 amu之间，既非原始氢，也非50%冰内部的标志。其中一颗是严格意义上的水世界。没有一颗的{\mu}是从探测到的分子特征中反演得出并归入气态矮星类的。我们将这些中间天体称为混合世界；它们是样本中最常见的结果，而选择函数不利于这一结果，因为为透射光谱有利性而选择的目标应过度代表延展的、低{\mu}的包层。证据在同一方向上不对称：混合世界的归类基于正面的分子探测，而两个端元类别则由暂定探测填充。这场争论并非二元的，其两个模型并不能描述它们被提出以解释的大部分种群。
英文摘要
Sub-Neptunes orbiting M dwarfs are commonly interpreted either as rocky cores beneath primordial H/He envelopes (gas dwarfs) or as volatile-rich bodies formed beyond the water-ice line (water worlds). Because the two are degenerate in mass and radius once envelope evolution is treated self-consistently, the atmospheric mean molecular weight $μ$ is the more direct discriminant. We compile the eight M-dwarf sub-Neptunes with published JWST transmission spectra and a constraint on $μ$, grade each constraint by the quality of the underlying evidence, and combine it with the restricted Jeans parameter and with energy-limited escape integrated over the system lifetime, using XUV histories from M-dwarf activity-age relations. Planets are classified by what their present composition implies about their origin. At the $μ= 3.8$ amu threshold adopted in the gas-dwarf literature, four planets fit neither end-member: their envelopes ($3.8 < μ\le 18$ amu) are too heavy for primordial H/He, yet neither their composition nor their escape history requires a volatile-rich origin. Two of these assignments rest on molecular detections (LP 791-18 c and TOI-270 d) and two on bounded or conditional metallicities. One planet, LHS 1140 b, requires a volatile-rich origin; one, TOI-776 c, is tentatively a gas dwarf; two remain ambiguous. We propose the term hybrid worlds for the intermediate class, whose envelopes can be produced by magma-ocean equilibration, supercritical miscibility, preferential hydrogen loss or accretion interior to the ice line. The count depends on the adopted threshold (three planets at 5 amu, one at 10 amu), but no threshold leaves both end-member classes physically consistent. The sample is set by JWST target selection, so these fractions are not occurrence rates; a homogeneous survey testing whether $μ$ is bimodal would test the proposed class directly.
