AI 中文总结
本研究构建亚海王星内部条件下超临界MgSiO₃H₄的物态方程，发现其热力学与结构特性异于纯MgSiO₃液体，将其应用于亚海王星结构模型可更准确分析氢分配对行星绝热曲线的影响。
AI 中文摘要
许多亚海王星被认为在致密富氢包层下方存在长寿命的熔融硅酸盐内部。在大气-内部边界附近的压力和温度条件下，MgSiO₃与H₂可能完全互溶，形成超临界硅酸盐-氢流体。本研究采用密度泛函理论分子动力学模拟，构建了超临界MgSiO₃H₄的自洽物态方程，该物质对应以等效H₂表示的3.86 wt%氢含量。研究人员用亥姆霍兹自由能公式拟合模拟结果，可从单一热力学表面得到密度、熵、热容、热膨胀率、体积模量和格吕乃森参数。结果发现，含氢流体的密度低于无水MgSiO₃液体，且与MgSiO₃和H₂之间的理想比体积混合存在显著偏差；产生的超额体积随压力变化，表明氢与硅酸盐骨架的相互作用随压缩而演变。结构分析显示，高压下Si-H配位数增加，持续的H-H键合减少，这与从类分子H₂键合向更强耦合的硅酸盐-氢流体转变一致。研究人员将MgSiO₃H₄物态方程纳入成分依赖的MgSiO₃-H查找表，并应用于代表性亚海王星结构模型。模型表明，大气与凝聚内部之间的氢分配会相对于MgSiO₃液相线影响行星绝热曲线。这些结果证明，超临界硅酸盐-氢流体具有独特的热力学和结构性质，在模拟亚海王星内部时必须加以考虑。
英文摘要
Many sub-Neptunes are expected to contain long-lived molten silicate interiors beneath dense H2-rich envelopes. At pressures and temperatures near the atmosphere-interior boundary, MgSiO3 and H2 may become fully miscible, forming a supercritical silicate-hydrogen fluid. Here we use density functional theory molecular dynamics simulations to construct a self-consistent equation of state for supercritical MgSiO3H4, corresponding to 3.86 wt% hydrogen expressed as equivalent H2. We fit the simulation results with a Helmholtz free-energy formulation that yields density, entropy, heat capacity, thermal expansivity, bulk modulus, and Gruneisen parameter from a single thermodynamic surface. We find that the hydrogen-bearing fluid is lower in density than dry MgSiO3 liquid and deviates significantly from ideal specific-volume mixing between MgSiO3 and H2. The resulting excess volume varies with pressure, indicating that the interaction between hydrogen and the silicate framework evolves with compression. Structural analysis shows increasing Si-H coordination and decreasing persistent H-H bonding at high pressure, consistent with a transition away from molecular H2-like bonding toward a more strongly coupled silicate-hydrogen fluid. We incorporate the MgSiO3H4 equation of state into a composition-dependent MgSiO3-H lookup table and apply it to representative sub-Neptune structure models. The models show that hydrogen partitioning between the atmosphere and condensed interior affects the planetary adiabat relative to the MgSiO3 liquidus. These results demonstrate that supercritical silicate-hydrogen fluids have distinct thermodynamic and structural properties that must be accounted for when modeling sub-Neptune interiors.
Comments13 pages, 7 figures, submitted to ApJ
