[论文解读] The core helium flash revisited III. From Pop I to Pop III stars

Miroslav Mocák, S. W. Campbell+2 more|arXiv (Cornell University)|Mar 18, 2010

Stellar, planetary, and galactic studies参考文献 1被引用 23

一句话总结

本研究利用二维和三维水动力学模拟，探究低质量恒星核心氦闪的物理过程，发现湍流卷吸作用导致第一星族恒星（1.25 M⊙，Z=0.02）中单一直流区迅速增长，约23天后引发氢注入，最终形成双对流区。相比之下，第三星族恒星（0.85 M⊙，Z=0）中的双对流区迅速衰减，主要受内波主导，且三维模型中的对流速度与一维预测值相比更接近，挑战了标准恒星演化模型中的既有假设。

ABSTRACT

Degenerate ignition of helium in low-mass stars at the end of the red giant branch phase leads to dynamic convection in their helium cores. One-dimensional (1D) stellar modeling of this intrinsically multi-dimensional dynamic event is likely to be inadequate. Previous hydrodynamic simulations imply that the single convection zone in the helium core of metal-rich Pop I stars grows during the flash on a dynamic timescale. This may lead to hydrogen injection into the core, and a double convection zone structure as known from one-dimensional core helium flash simulations of low-mass Pop III stars. We perform hydrodynamic simulations of the core helium flash in two and three dimensions to better constrain the nature of these events. To this end we study the hydrodynamics of convection within the helium cores of a 1.25 \Msun metal-rich Pop I star (Z=0.02), and a 0.85 \Msun metal-free Pop III star (Z=0) near the peak of the flash. These models possess single and double convection zones, respectively. We use 1D stellar models of the core helium flash computed with state-of-the-art stellar evolution codes as initial models for our multidimensional hydrodynamic study, and simulate the evolution of these models with the Riemann solver based hydrodynamics code Herakles which integrates the Euler equations coupled with source terms corresponding to gravity and nuclear burning. The hydrodynamic simulation of the Pop I model involving a single convection zone covers 27 hours of stellar evolution, while the first hydrodynamic simulations of a double convection zone, in the Pop III model, span 1.8 hours of stellar life. We find differences between the predictions of mixing length theory and our hydrodynamic simulations. The simulation of the single convection zone in the Pop I model shows a strong growth of the size of the convection zone due to turbulent entrainment. Hence we predict that for the Pop I model a hydrogen injection phase (i.e. hydrogen injection into the helium core) will commence after about 23 days, which should eventually lead to a double convection zone structure known from 1D stellar modeling of low-mass Pop III stars. Our two and three-dimensional hydrodynamic simulations of the double (Pop III) convection zone model show that the velocity field in the convection zones is different from that predicted by stellar evolutionary calculations. The simulations suggest that the double convection zone decays quickly, the flow eventually being dominated by internal gravity waves.

研究动机与目标

• 利用多维模拟研究低质量恒星核心氦闪的流体动力学过程，弥补一维恒星演化模型的局限性。
• 确定湍流卷吸是否导致金属丰度较高的第一星族恒星中氢注入至氦核心，如一维模型所预测。
• 研究金属贫乏第三星族恒星中双对流区的稳定性与动力学特性，特别关注CNO燃烧与内波的作用。
• 通过与高分辨率二维和三维水动力学模拟对比，评估混合长度理论与一维恒星演化预测的准确性。
• 评估数值分辨率与核燃烧对核心氦闪期间对流流动发展与速度分布的影响。

提出的方法

• 采用Herakles代码进行水动力学模拟，求解含重力与核燃烧源项的欧拉方程。
• 初始模型基于最先进的单维恒星演化代码，针对1.25 M⊙第一星族恒星（Z=0.02）与0.85 M⊙第三星族恒星（Z=0）。
• 采用基于黎曼求解器的方法，在二维与三维中进行模拟，以解析湍流对流与卷吸过程。
• 模拟追踪了对流区演化过程，持续27小时（第一星族）与1.8小时（第三星族），重点关注流动动力学与能量产生。
• 分析速度场与动能，以与一维预测对比，并对分辨率与核燃烧激活进行敏感性测试。
• 将高分辨率三维模拟（heflpopIII.3d）与低分辨率模型对比，评估收敛性与数值伪影。

实验结果

研究问题

• RQ1 湍流卷吸是否导致金属丰度较高的第一星族恒星中对流区动态增长，从而引发氢注入至氦核心？
• RQ2 二维与三维水动力学模拟在预测核心氦闪期间对流区演化与速度结构方面有何差异？
• RQ3 为何一维恒星模型预测第三星族恒星中存在双对流区？该结构在多维水动力学模拟中是否持续存在？
• RQ4 水动力学模拟在多大程度上重现了一维恒星演化模型所预测的对流速度？
• RQ5 核燃烧在维持对流中起何作用？若无核燃烧，仅靠温度梯度是否足以触发对流？

主要发现

• 在1.25 M⊙第一星族恒星中，由于湍流卷吸作用，单一对流区迅速增长，约23天后引发氢注入至氦核心。
• 第一星族模型的三维水动力学模拟显示对流区具有超绝热温度梯度，与高分辨率动力学一致，且优于早期低分辨率模型的结果。
• 在0.85 M⊙第三星族恒星中，双对流区结构迅速衰减，对流启动后流动迅速被内波主导。
• 第三星族模型的三维模拟显示，内层氦燃烧区的对流速度约为一维预测值的两倍，但仍与理论预期基本一致。
• 即使在无核燃烧条件下，模拟中仍自发触发对流，表明初始温度梯度本身已足以驱动对流，而核燃烧仅起次要维持作用。
• 双对流区的二维与三维模拟表明，对流流动迅速衰减，暗示一维模型预测的双对流区结构在多维流体动力学中可能并不稳定。