Lawrence Livermore National Laboratory, Livermore, CA, USA.
Department of Mechanical Engineering, University of Rochester, Rochester, NY, USA.
Nature. 2020 Aug;584(7819):51-54. doi: 10.1038/s41586-020-2535-y. Epub 2020 Aug 5.
White dwarfs represent the final state of evolution for most stars. Certain classes of white dwarfs pulsate, leading to observable brightness variations, and analysis of these variations with theoretical stellar models probes their internal structure. Modelling of these pulsating stars provides stringent tests of white dwarf models and a detailed picture of the outcome of the late stages of stellar evolution. However, the high-energy-density states that exist in white dwarfs are extremely difficult to reach and to measure in the laboratory, so theoretical predictions are largely untested at these conditions. Here we report measurements of the relationship between pressure and density along the principal shock Hugoniot (equations describing the state of the sample material before and after the passage of the shock derived from conservation laws) of hydrocarbon to within five per cent. The observed maximum compressibility is consistent with theoretical models that include detailed electronic structure. This is relevant for the equation of state of matter at pressures ranging from 100 million to 450 million atmospheres, where the understanding of white dwarf physics is sensitive to the equation of state and where models differ considerably. The measurements test these equation-of-state relations that are used in the modelling of white dwarfs and inertial confinement fusion experiments, and we predict an increase in compressibility due to ionization of the inner-core orbitals of carbon. We also find that a detailed treatment of the electronic structure and the electron degeneracy pressure is required to capture the measured shape of the pressure-density evolution for hydrocarbon before peak compression. Our results illuminate the equation of state of the white dwarf envelope (the region surrounding the stellar core that contains partially ionized and partially degenerate non-ideal plasmas), which is a weak link in the constitutive physics informing the structure and evolution of white dwarf stars.
白矮星代表了大多数恒星演化的最终状态。某些类别的白矮星脉动，导致可观测的亮度变化，对这些变化的分析与恒星理论模型相结合，可以探测它们的内部结构。这些脉动恒星的建模为白矮星模型提供了严格的检验，并提供了恒星演化晚期结果的详细图片。然而，白矮星中存在的高能量密度状态极难在实验室中达到和测量，因此这些条件下的理论预测在很大程度上未经检验。在这里，我们报告了在主激波 Hugoniot 沿线（从物质守恒定律推导出的样品材料在激波通过前后的状态方程）测量碳氢化合物的压力和密度关系的结果，误差在 5%以内。观察到的最大可压缩性与包括详细电子结构的理论模型一致。这对于物质的状态方程在 1 亿到 4.5 亿大气压的范围内是相关的，在这个范围内，白矮星物理学对白矮星物理学和惯性约束聚变实验中的状态方程非常敏感，而且模型差异很大。这些测量检验了在白矮星建模和惯性约束聚变实验中使用的状态方程关系，我们预测由于碳的内芯轨道的电离，可压缩性会增加。我们还发现，为了捕捉碳氢化合物在达到峰值压缩之前的压力-密度演化的测量形状，需要对电子结构和电子简并压力进行详细处理。我们的结果阐明了白矮星包层的状态方程（包含部分电离和部分简并的非理想等离子体的恒星核心周围区域），这是构成物理中的一个薄弱环节，它影响着白矮星的结构和演化。
