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arXiv 2006.11286astro-ph.SRastro-ph.HE
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它必须很酷：来自共同包络演化的二元黑洞合并的超巨星祖先

It has to be cool: on supergiant progenitors of binary black hole mergers from common-envelope evolution

J. Klencki, G. Nelemans, A. G. Istrate, M. Chruslinska

备注：
20 pages + App., accepted for publication in A&A. For $λ_{\rm CE}$ fits, see: https://ftp.science.ru.nl/astro/jklencki/
发表信息：
A&A 645, A54 (2021)
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摘要：

大质量双星系统的共包络演化被认为是形成致密双星合并的最有希望的途径之一。在合并双黑洞（BBH）的情况下，基本CE阶段发生在第一个BH已经形成的阶段，伴星膨胀为超巨星。我们研究的BH双星与超巨星的同伴将通过进化和潜在的生存CE阶段。为此，我们计算信封结合能从详细的大质量恒星模型在不同的演化阶段和金属丰度。我们做了多个物理极端的假设，有利于更容易CE弹射，以及占传质稳定性标准的最新进展的选择。我们发现，即使是最乐观的假设，一个成功的CE弹射BH（也NS）双星是唯一可能的，如果捐助者是一个巨大的对流信封巨人，红超巨星（RSG）。换句话说，前CE祖先的BBH合并是BH双星与RSG同伴。我们发现，由于金属丰度对大质量巨星径向膨胀的影响，它对RSG的包络结构和结合能有间接但非常强烈的影响。我们的研究结果表明，人口合成模型的合并率可能被严重高估，特别是在低金属丰度。此外，观测到的亮度高于log（$L/L_{\odot}$）= 5.6-5.8的RSG的缺乏，对应于具有$M > 40 M_{\odot}$的恒星，使CE通道形成最大质量BBH合并的可行性受到质疑。这样的RSG要么由于非常短的寿命而逃避检测，要么它们不存在并且CE通道只能产生具有总质量$< 50 M_{\odot}$的BBH系统。我们讨论了另一种CE方案，其中一个部分的信封喷射之后，可能长期和稳定的质量转移的阶段。

英文摘要：

Common-envelope (CE) evolution in massive binary systems is thought to be one of the most promising channels for the formation of compact binary mergers. In the case of merging binary black holes (BBHs), the essential CE phase takes place at a stage when the first BH is already formed and the companion star expands as a supergiant. We study which BH binaries with supergiant companions will evolve through and potentially survive a CE phase. To this end, we compute envelope binding energies from detailed massive stellar models at different evolutionary stages and metallicities. We make multiple physically extreme choices of assumptions that favor easier CE ejection as well as account for recent advancements in mass transfer stability criteria. We find that even with the most optimistic assumptions, a successful CE ejection in BH (and also NS) binaries is only possible if the donor is a massive convective-envelope giant, a red supergiant (RSG). In other words, pre-CE progenitors of BBH mergers are BH binaries with RSG companions. We find that due to its influence on the radial expansion of massive giants, metallicity has an indirect but a very strong effect on the envelope structure and binding energies of RSGs. Our results suggest that merger rates from population synthesis models could be severely overestimated, especially at low metallicity. Additionally, the lack of observed RSGs with luminosities above log($L/L_{\odot}$) = 5.6-5.8, corresponding to stars with $M > 40 M_{\odot}$, puts into question the viability of the CE channel for the formation of the most massive BBH mergers. Either such RSGs elude detection due to very short lifetimes, or they do not exist and the CE channel can only produce BBH systems with total mass $< 50 M_{\odot}$. We discuss an alternative CE scenario, in which a partial envelope ejection is followed by a phase of possibly long and stable mass transfer.

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