arXiv 2009.018492020-09-03astro-ph.SRastro-ph.GAastro-ph.HE

大质量氦星风的性质和Wolf-Rayet型质量损失

On the nature of massive helium star winds and Wolf-Rayet-type mass loss

Andreas A. C. Sander, Jorick S. Vink

备注：: 20 pages, 22+9 figures, accepted for publication in MNRAS; a python routine implementing the resulting mass-loss recipes can be found at https://armagh.space/asander
DOI：: 10.1093/mnras/staa2712

摘要：

大质量氦星的质量损失率是现代天体物理学中的主要不确定性之一。不管它们是被一个双星伴星剥离，还是设法自己剥离了它们的外层，氦星的影响和最终命运--特别是由此产生的黑洞质量--在很大程度上取决于它们作为剥离包络体的风质量损失。虽然大质量氦星的经验质量损失限制在过去几十年中有所改善，但由此产生的配方仅限于具有足够分辨单个恒星的观测能力的金属丰度。然而，理论上的努力受到了来自更大质量氦星的沃尔夫-拉叶（WR）风的复杂性的阻碍。在一个前所未有的努力，我们计算下一代恒星大气模型类似于大质量氦主序星与铁凸驱动风高达$500\,M_\odot$在$2.0$和$0.02\,Z_\odot$之间的广泛的金属丰度范围。我们发现了一个复杂的$Γ_\text{e}$依赖WR型风和它们的金属性依赖的故障。后者可能与多重散射的发生有关，需要在较低金属丰度下具有较高的$L/M$ -比率。基于我们的发现，我们推导出了有史以来第一个理论上的大质量氦星的质量损失配方。我们还提供了LyC和He II电离通量的估计，发现剥离氦星在低金属丰度的贡献相当大。与OB星风形成鲜明对比的是，氦星的质量损失与终极速度成比例。虽然仅限于氦主序，但我们的研究标志着朝着更好地理解氦星演化的理论迈出了重要一步。

英文摘要：

The mass-loss rates of massive helium stars are one of the major uncertainties in modern astrophysics. Regardless of whether they were stripped by a binary companion or managed to peel off their outer layers by themselves, the influence and final fate of helium stars -- in particular the resulting black hole mass -- highly depends on their wind mass loss as stripped-envelope objects. While empirical mass-loss constraints for massive helium stars have improved over the last decades, the resulting recipes are limited to metallicities with the observational ability to sufficiently resolve individual stars. Yet, theoretical efforts have been hampered by the complexity of Wolf-Rayet (WR) winds arising from the more massive helium stars. In an unprecedented effort, we calculate next-generation stellar atmosphere models resembling massive helium main sequence stars with Fe-bump driven winds up to $500\,M_\odot$ over a wide metallicity range between $2.0$ and $0.02\,Z_\odot$. We uncover a complex $Γ_\text{e}$-dependency of WR-type winds and their metallicity-dependent breakdown. The latter can be related to the onset of multiple scattering, requiring higher $L/M$-ratios at lower metallicity. Based on our findings, we derive the first ever theoretically-motivated mass-loss recipe for massive helium stars. We also provide estimates for LyC and He II ionizing fluxes, finding stripped helium stars to contribute considerably at low metallicity. In sharp contrast to OB-star winds, the mass loss for helium stars scales with the terminal velocity. While limited to the helium main sequence, our study marks a major step towards a better theoretical understanding of helium star evolution.

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