Hughto J, Horowitz C J, Schneider A S, Medin Zach, Cumming Andrew, Berry D K
Department of Physics and Nuclear Theory Center, Indiana University, Blooomington, Indiana 47405, USA.
Phys Rev E Stat Nonlin Soft Matter Phys. 2012 Dec;86(6 Pt 2):066413. doi: 10.1103/PhysRevE.86.066413. Epub 2012 Dec 28.
The neutron-rich isotope ²²Ne may be a significant impurity in carbon and oxygen white dwarfs and could impact how the stars freeze. We perform molecular dynamics simulations to determine the influence of ²²Ne in carbon-oxygen-neon systems on liquid-solid phase equilibria. Both liquid and solid phases are present simultaneously in our simulation volumes. We identify liquid, solid, and interface regions in our simulations using a bond angle metric. In general we find good agreement for the composition of liquid and solid phases between our MD simulations and the semianalytic model of Medin and Cumming. The trace presence of a third component, neon, does not appear to strongly impact the chemical separation found previously for two-component carbon and oxygen systems. This suggests that small amounts of ²²Ne may not qualitatively change how the material in white dwarf stars freezes. However, we do find systematically lower melting temperatures (higher Γ) in our MD simulations compared to the semianalytic model. This difference seems to grow with impurity parameter Q_{imp} and suggests a problem with simple corrections to the linear mixing rule for the free energy of multicomponent solid mixtures that is used in the semianalytic model.
富中子同位素²²Ne可能是碳氧白矮星中的一种重要杂质，并且会影响恒星的冻结方式。我们进行分子动力学模拟，以确定²²Ne在碳 - 氧 - 氖系统中对液 - 固相变平衡的影响。在我们的模拟体积中，液相和固相同时存在。我们使用键角度量在模拟中识别液相、固相和界面区域。总体而言，我们发现分子动力学模拟得到的液相和固相组成与梅丁和卡明的半解析模型之间具有良好的一致性。第三种成分氖的微量存在似乎并未强烈影响先前在二元碳氧系统中发现的化学分离。这表明少量的²²Ne可能不会在定性上改变白矮星中物质的冻结方式。然而，与半解析模型相比，我们确实在分子动力学模拟中系统地发现了更低的熔化温度（更高的Γ）。这种差异似乎随着杂质参数Q_{imp}的增加而增大，这表明半解析模型中用于多组分固体混合物自由能的线性混合规则的简单修正存在问题。
