重双中子星的形成 II：重的初生中子星与低金属丰度的作用
Formation of heavy double neutron stars II: the role of heavy first-born neutron stars and low metallicity
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中文总结 AI 辅助
该研究通过MESA工具在不同金属丰度下模拟中子星-氦星双星演化，发现重双中子星占比仅约0.5%，主要来自标准形成通道，低金属丰度下多经不稳定质量转移形成，且无独立子种群，为标准DNS的高质量尾部。
中文摘要 AI 辅助
GW190425的总质量极高，挑战了我们对双中子星（DNS）形成的理解，因为在银河系盘内尚未观测到此类质量≥3倍太阳质量（M☉）的重DNS系统。已有众多形成场景被提出以解释其形成机制，我们在经银河系DNS种群校准的自洽双星演化框架内对这些场景进行了检验。在本系列论文的第一篇（Paper I）中，我们研究了太阳金属丰度（Z = Z☉）下，1.4倍太阳质量（M☉）中子星（NS）与2.5至10倍太阳质量氦星组成的双星的演化，假设中子星的吸积遵循爱丁顿极限，使用的工具为MESA。在本文中，我们考虑了金属丰度为Z = Z☉、0.1Z☉和0.01Z☉的情况下，中子星-氦星双星的演化，其中中子星质量范围为1.1至1.9倍太阳质量。我们发现重DNS的形成十分罕见，仅占DNS的约0.5%，其中大部分通过“标准形成”通道形成，该通道被广泛认为形成了GW170817以及已观测到的银河系DNS。我们发现，在太阳金属丰度下，“快并合”通道无贡献，但低金属丰度下的系统主要通过不稳定质量转移形成。此外，在本文所考虑的金属丰度范围内，重DNS的形成比例无显著的金属丰度依赖性。我们得出结论，重DNS并非独立的子种群，而仅代表标准DNS种群的高质量尾部。
英文摘要
The high total mass of GW190425 challenges our understanding of double neutron star (DNS) formation, as no such heavy ($\ge 3$\, M$_\odot$) DNS system has been observed in the Milky Way disk. Numerous formation scenarios have been proposed to explain its formation. We test these within a self-consistent binary evolution framework calibrated to the Galactic DNS population. In Paper~I of this series, we studied the evolution of a $1.4$\,M$_\odot$ neutron star (NS) in a binary with a $2.5$--$10$\,M$_\odot$ helium star at solar metallicity ($Z = Z_\odot$), assuming Eddington-limited accretion onto the NS using \texttt{MESA}. In this paper, we consider the evolution of NS-He star binaries with a broad range of NS masses from $1.1$--$1.9$\,M$_\odot$ at $Z=Z_\odot$, 0.1\,$Z_\odot$ and 0.01\,$Z_\odot$. We find that the formation of heavy DNSs is rare, accounting for only $\sim 0.5$ per cent of DNSs. The majority of these are formed through the `standard formation' channel, widely believed to form GW170817 and observed Galactic DNSs. We find no contribution through the `fast-merger' channel at solar metallicity, but systems at low metallicity predominantly form through unstable mass transfer. Furthermore, we find no significant dependence of the heavy DNS formation fraction on metallicity over the range considered here. We conclude that heavy DNSs do not form a separate subpopulation and merely represent the high-mass tail of the standard DNS population.
