[论文解读] New evolutionary models for massive ZZ Ceti stars. II. The effects of crystallization on their pulsational properties

A. H. Córsico, L. G. Althaus+3 more|Americanae (AECID Library)|Aug 12, 2004

Stellar, planetary, and galactic studies参考文献 62被引用 20

一句话总结

本研究提出了大质量ZZ Ceti星的新演化与脉动模型，纳入了结晶化和核心过冲效应。研究发现，结晶化强烈改变了脉动周期谱，尤其通过模态捕获效应；若BPM 37093的质量约为1.00 M☉，其观测到的周期谱可能反映了核心过冲效应。

ABSTRACT

We present in this work new pulsational calculations for improved carbon-oxygen DA white dwarf models suitable for the study of massive ZZ Ceti stars. The background models employed in this study, presented in detail in a recent paper by Althaus et al. (2003), are the result of the complete evolution of massive white dwarf progenitors from the zero-age main sequence through the Asymptotic Giant Branch (AGB) and mass loss phases to the white dwarf regime. Abundance changes are accounted for by means of a full coupling between nuclear evolution and time-dependent mixing due to convection, salt fingers, and diffusive overshoot. In addition, time-dependent element diffusion for multicomponent gases has been considered during the white dwarf evolution. Crystallization and chemical rehomogenization due to phase separation upon crystallization in the core of our models have been fully considered. The effects of crystallization on the period spectrum of these massive white dwarf models are assessed by means of a detailed pulsational analysis. We find that the theoretical pulsation spectrum is strongly modified when crystallization is considered, in particular concerning the mode trapping properties of the equilibrium models. We also discuss at some length the implications of our study for BPM 37093, the most massive ZZ Ceti star presently known. We find that if BPM 37093 has a stellar mass of $\approx 1.00$ \msun its observed spectrum could bear the signature of overshoot episodes during the helium core burning.

研究动机与目标

• 评估结晶化对大质量ZZ Ceti星脉动特性的影响。
• 确定星震学是否能约束大质量白矮星前体在氦燃烧阶段的核心过冲。
• 评估结晶过程中的化学再均质化与相分离在塑造脉动谱中的作用。
• 检验BPM 37093观测到的周期谱是否反映了核心过冲或结晶化效应。
• 基于其脉动与演化特性，为BPM 37093的物理历史提供约束。

提出的方法

• 从零龄主序开始构建详细的演化模型，涵盖AGB阶段与质量损失过程，完整耦合核燃烧、对流、盐指效应与扩散性过冲。
• 在白矮星演化过程中，对多组分气体实施时间依赖的元素扩散建模。
• 利用相分离物理原理与振荡本征函数中的固液界面条件，对核心的结晶化与化学再均质化进行建模。
• 对模型中的重力模（g模）进行线性、非球对称、绝热脉动计算。
• 在固液界面应用特殊边界条件，以准确捕捉固相对振荡本征函数的影响。
• 采用简单解析模型重现过冲模型中观测到的强烈模态捕获现象。

实验结果

研究问题

• RQ1 结晶化如何影响大质量ZZ Ceti星的脉动周期谱？
• RQ2 BPM 37093中观测到的周期间距是否可由其前体演化过程中的核心过冲解释？
• RQ3 结晶过程中化学再均质化在改变脉动特性方面起什么作用？
• RQ4 结晶化在多大程度上抹去了核心过冲在脉动谱中的痕迹？
• RQ5 BPM 37093中观测到的模态捕获可归因于先前的核心过冲还是结晶化效应？

主要发现

• 结晶化显著改变了脉动周期谱，尤其在具有核心过冲的模型中通过增强模态捕获效应体现。
• 具有过冲的理论模型表现出强烈的模态捕获，该现象可被一个简单解析模型重现。
• 对于质量为1.00 M☉的白矮星，当T_eff ≈ 11,700 K时结晶度达到37%，表明观测谱可能保留了先前核心过冲的痕迹。
• 若BPM 37093的质量约为1.10 M☉，则预期无显著模态捕获，因此无法通过脉动推断核心过冲。
• BPM 37093中观测到的周期间距为17.125 s，偏差±8 s，可能由ℓ=1与ℓ=2模态的混合而非纯ℓ=2模态解释。
• 质量M ≳ 1.0 M☉的白矮星最适合研究结晶化效应，而质量接近0.90 M☉的白矮星则更适合分离核心过冲的痕迹。