[论文解读] Very Massive Star Models: I. Impact of Rotation and Metallicity and Comparisons with Observations

S. Martinet, G. Meynet+3 more|arXiv (Cornell University)|Aug 31, 2023

Stellar, planetary, and galactic studies参考文献 3被引用 4

一句话总结

本研究利用GENEC代码中更新的方程态，结合旋转与金属丰度效应，构建了从第三星族到太阳金属丰度的非常大质量恒星（VMS）模型网格。研究发现，低金属丰度VMS对旋转极为敏感，而高金属丰度模型则主要受质量损失主导；只有在R136类金属丰度（Z ≈ 0.006）下非旋转或慢速旋转的VMS才能产生对不稳定超新星，从而将黑洞质量限制在<60 M⊙以内。

ABSTRACT

In addition to being spectacular objects, Very Massive Stars (VMS) are suspected to have a tremendous impact on their environment and on the whole cosmic evolution. The nucleosynthesis both during their advanced stages and their final explosion may contribute greatly to the overall enrichment of the Universe. Their resulting supernovae are candidates for the most superluminous events and their extreme conditions also lead to very important radiative and mechanical feedback effects, from local to cosmic scale. We explore the impact of rotation and metallicity on the evolution of very massive stars across cosmic times. With the recent implementation of an equation of state in the GENEC stellar evolution code, appropriate for describing the conditions in the central regions of very massive stars in the advanced phases, we present new results on VMS evolution from Population III to solar metallicity. Low metallicity VMS models are highly sensitive to rotation, while the evolution of higher metallicity models is dominated by mass loss effects. The mass loss affects strongly their surface velocity evolution, breaking quickly at high metallicity while reaching the critical velocity for low metallicity models. The comparison to observed VMS in the LMC shows that the mass loss prescriptions used for these models are compatible with observed mass loss rates. In our framework for modelling rotation, our models of VMS need a high initial velocity to reproduce the observed surface velocities. The surface enrichment of these VMS is difficult to explain with only one initial composition, and could suggest multiple populations in the R136 cluster. At a metallicity typical of R136, only our non- or slowly rotating VMS models may produce Pair Instability supernovae. The most massive black holes that can be formed are less massive than about 60 M$_\odot$.

研究动机与目标

• 研究从第三星族到太阳金属丰度的非常大质量恒星（VMS）演化过程中，旋转与金属丰度的影响。
• 评估质量损失与旋转混合对VMS表面速度演化及表面富集的影响。
• 将模型预测与大麦哲伦云中R136星团内观测到的VMS进行比较。
• 确定VMS产生对不稳定超新星的条件，以及所形成的最大黑洞质量。
• 评估当前质量损失律在不同金属丰度下重现观测VMS特性的可靠性。

提出的方法

• 采用更新的方程态实现的GENEC恒星演化代码，模拟VMS在先进演化阶段的演化。
• 构建了初始质量从100至500 M⊙、金属丰度从Z=0（第三星族）到Z=0.02（太阳金属丰度）的VMS模型网格。
• 通过旋转混合与角动量输运引入旋转效应，初始旋转速度最高达400 km/s。
• 针对不同金属丰度应用更新的质量损失律，包括线驱动风及接近爱丁顿极限时的影响。
• 追踪核心质量演化，特别是氦燃烧末期的碳氧核心质量，以评估最终命运（如对不稳定超新星）。
• 利用方程态模拟恒星核心内部的对产生与不稳定性条件。

Figure 1: HRD of the non-rotating (solid lines) and rotating at V/V c =0.4 (dashed lines) models at Z=0.014 (solar) and Z=0.006 (LMC). The top/middle/bottom row shows model with initial masses of 180/250/300M ☉ . The tracks are color-coded according to their mass loss rates. The beginning and the en

实验结果

研究问题

• RQ1 在低金属丰度下，旋转如何影响非常大质量恒星的演化与表面特性？
• RQ2 在高金属丰度VMS中，质量损失率在多大程度上主导于旋转效应？
• RQ3 当前的质量损失律能否重现R136星团中VMS的观测质量损失率？
• RQ4 为使模型匹配R136 VMS的观测表面速度，所需初始旋转速度是多少？
• RQ5 在何种条件下VMS能产生对不稳定超新星，且形成的黑洞质量最大为多少？

主要发现

• 低金属丰度VMS模型对旋转极为敏感，由于质量损失微弱，表面速度可达到临界值。
• 高金属丰度VMS的演化主要受质量损失主导，其快速减少表面速度并抑制旋转效应。
• R136星团中观测到的VMS需要高达400 km/s的初始旋转速度，才能在模型中匹配其观测表面速度。
• R136 VMS的表面富集难以用单一初始组成解释，暗示存在多个星族或复杂相互作用。
• 仅在R136类金属丰度（Z ≈ 0.006）下非旋转或慢速旋转的VMS才能产生对不稳定超新星。
• 由VMS形成的最重黑洞质量被限制在小于60 M⊙，主要由于对不稳定与质量损失效应。

Figure 2: HRD for 180M ☉ non-rotating models at Z=0.014, Z=0.006, Z=10 -5 and Z=0. The tracks are color-coded according to their current total mass. The MS of the models at Z=0.014, Z=0.006, Z=10 -5 are plotted in dashed grey in the lower right panel for comparison.