亚海王星的成分梯度：以K2-18 b和TOI-270 d为案例研究
Composition gradients in sub-Neptunes: K2-18 b and TOI-270 d as case studies
- University of Zurich（苏黎世大学）
机构由 AI 辅助整理，请以论文原文为准。
AI总结：
本研究以K2-18 b和TOI-270 d为案例，对比纯绝热分层模型与含成分梯度的模型，发现成分梯度可扩大亚海王星可行内部结构范围、减弱相关丰度相关性，建议采用更复杂内部模型研究行星。
AI中文摘要：
亚海王星的结构模型通常假设内部为纯绝热结构，由成分均匀的不同层组成。本研究评估了允许内部包含更复杂的成分梯度结构，会如何影响对亚海王星K2-18 b和TOI-270 d的内部结构及整体成分的推断。我们将纯绝热分层模型与包含成分梯度和稳定非对流区域的模型进行对比，给出了与观测质量、半径及大气边界条件一致的解。研究发现，成分梯度显著扩大了可行内部结构的范围：例如，即使质量和半径固定，内部简并性仍然显著，K2-18 b的氢-氦（H-He）质量分数最大值可增加多达5倍。我们进一步表明，引入成分梯度后，氢-氦丰度与冰岩比或岩铁比等相关性会减弱，例如TOI-270 d的氢-氦丰度与铁丰度之间的斯皮尔曼秩相关系数，在本研究模型中从约0.7降至约0.4。纯绝热模型低估了合理成分的范围，且高估了更精确测量、大气模型及主恒星约束对更准确表征行星的影响。我们建议，在利用数据解释行星结构、形成与演化时，亚海王星的内部模型应默认包含成分梯度和非对流区域等更复杂的内部结构。
英文摘要：
Structure models of sub-Neptunes commonly assume purely adiabatic interiors with distinct layers of homogeneous composition. We assess how allowing for more complex interiors with composition gradients affects the inferred internal structure and bulk compositions of the sub-Neptunes K2-18 b and TOI-270 d. We compare purely adiabatic models with distinct layers against models that include composition gradients and stable non-convective regions. We present solutions that are consistent with the observed masses, radii, and atmospheric boundary conditions. We find that composition gradients significantly increase the range of viable interior structures: For example, the interior degeneracy remains substantial even for fixed mass and radius and the maximum hydrogen-helium (H-He) mass fraction can increase by up to a factor of five for K2-18 b. We further show that correlations, such as those between the H-He abundance and the ice-to-rock or rock-to-iron ratios, can weaken when composition gradients are introduced. For example, the Spearman rank correlation coefficient between the H-He and iron abundances decreases from $\sim0.7$ to $\sim0.4$ for TOI-270 d in our models. Purely adiabatic models underestimate the range of plausible compositions and overestimate the impact of more precise measurements, atmosphere models, and host star constraints on a more accurate characterization. We suggest that interior models of sub-Neptunes should by default include more complex interiors, such as composition gradients and non-convective regions, when using data for interpreting the planetary structure, formation, and evolution.
