[论文解读] Two-Dimensional Simulations of the Thermonuclear Runaway in an Accreted Atmosphere of a C+O White Dwarf

A. Kercek, W. Hillebrandt+1 more|ArXiv.org|Jan 7, 1998

Astro and Planetary Science参考文献 1被引用 4

一句话总结

本文使用基于PPM的显式代码，对质量为1 M☉的C+O白矮星上吸积的富氢包层中的热核爆发进行了二维流体动力学模拟。结果表明，对流超射并不会导致核心碳和氧快速混合进入包层；相反，在分辨率相关的尺度上形成了持久的高涡度结构，导致自增丰化过程比以往认为的要缓慢得多。

ABSTRACT

We present the results of two-dimensional calculations of turbulent nuclear burning of hydrogen-rich material accreted onto a white dwarf of 1 solar mass. The main aim of the present paper is to investigate the question as to whether and how the general properties of the burning are affected by numerical resolution effects. In particular, we want to see whether or not convective overshooting into the surface layers of the C+O white dwarf can lead to self-enrichment of the initially solar composition of the hydrogen-rich envelope with carbon and oxygen from the underlying white dwarf core. Our explicit hydrodynamic code is based on the PPM-method and computes the onset of the thermonuclear runaway on a Cartesian grid. In contrast to previous works we do not observe fast mixing of carbon and oxygen from the white dwarf's surface into the envelope by violent overshooting of large eddies. The main features of the flow fields in our simulations are the appearance of small persistent coherent structures of very high vorticity (and velocity) compared to the background flow. Their typical linear scales are about 10 to 20 grid zones and thus their physical size depends on the numerical resolution, i.e, their size decreases with increasing resolution. The two simulations (low and high resolution) which are presented here show only moderate differences in spatially integrated quantities such as laterally averaged temperature, energy generation rate, and chemical composition. We have not expanded both simulations equally long, but for the physical time under consideration the major difference seems to be that the highly resolved simulation is a bit less violent. In conclusion, we do find some self-enrichment, but on time-scales much longer than in previous calculations.

研究动机与目标

• 研究数值分辨率如何影响在C+O白矮星上富氢吸积包层中热核爆发的动力学与组分演化。
• 确定对流超射是否会导致白矮星核心中的碳和氧显著混合进入吸积包层。
• 评估相干涡旋结构在影响燃烧动力学和化学混合中的作用。
• 通过比较低分辨率与高分辨率模拟，评估分辨率对温度、能量生成率和组分等关键物理量的影响。

提出的方法

• 本研究采用基于分段抛物法（PPM）的二维显式流体动力学代码，在笛卡尔网格上求解流体动力学方程。
• 模拟对象为质量为1 M☉的C+O白矮星，其上具有太阳组成、富含氢的吸积包层，模拟了热核燃烧的起始与传播过程。
• 代码追踪温度、密度、速度及核能生成率的演化，包括详细的核反应网络。
• 通过改变数值分辨率，研究其对流动场中相干涡旋结构形成与行为的影响。
• 计算并比较低分辨率与高分辨率模拟中温度、能量生成率和化学组分等空间平均量。

实验结果

研究问题

• RQ1 对流超射是否会导致白矮星核心中的碳和氧通过大尺度超射涡旋快速混合进入吸积包层？
• RQ2 数值分辨率效应如何影响相干涡旋结构的发展及其对混合的影响？
• RQ3 在缺乏剧烈混合的情况下，包层自增丰化的核心元素所需的时间尺度是多少？
• RQ4 低分辨率与高分辨率模拟中，能量生成率和温度等积分量的演化有何不同？

主要发现

• 未观察到通过大尺度超射涡旋导致白矮星核心中的碳和氧快速混合进入包层的证据。
• 模拟揭示了尺寸为10–20个网格区域、持久存在的高涡度相干结构，其大小与分辨率相关，并随分辨率提高而减小物理尺寸。
• 尽管分辨率不同，低分辨率与高分辨率模拟在空间平均温度、能量生成率和化学组分方面仅表现出中等差异。
• 高分辨率模拟表现出稍弱的剧烈动力学行为，表明分辨率效应虽存在，但在积分量中并非主导因素。
• 包层中碳和氧的自增丰化确实发生，但其时间尺度远长于早期模型所估计的。