引力不稳定坍缩星盘中的碎裂现象与亚太阳质量中子星并合
Fragmentation in Gravitationally-Unstable Collapsar Disks and Sub-Solar Neutron Star Mergers
AI总结：
本文提出在引力不稳定的大质量坍缩星盘外部区域可形成亚太阳质量中子星，其后续并合可解释亚太阳质量引力波事件，并预言多普勒调制、伴随黑洞并合及超新星内千新星等观测特征。
摘要：
尽管稳定的中子星（NS）原则上可以存在低至约0.1倍太阳质量（Mns ~ 0.1Msun）的质量，但恒星核心坍缩的标准模型预测出一个稳健的下限 Mns >~ 1.2Msun，大致与前身星铁核的钱德拉塞卡质量 Mch 相当（电子丰度 Ye ~ 0.5）。然而，在足够致密的富中子环境中（Ye << 0.5），由于 Mch ~ Ye^2 降低至小于一个太阳质量，这一下限可能被规避。此类物理条件可能出现在由快速旋转恒星坍缩形成的黑洞吸积盘（“坍缩星”）中，其成因是引力不稳定性和冷却诱导的碎裂，类似于原恒星盘中行星形成的模型。我们确认，在大质量中微子冷却坍缩星盘的外部区域，形成亚太阳质量中子星（ssNS）的条件可能被勉强满足。如果该盘碎裂成多个 ssNS，它们随后的并合提供了一条触发亚太阳质量 LIGO/Virgo 引力波并合事件的途径，且不涉及原初黑洞。该模型还给出了若干额外预言：（1）由于双星在盘中的轨道运动，ssNS 并合引力波信号会受到约 Hz 频率的多普勒调制；（2）至少还有一次额外的引力波事件（在约数小时以内重合），来自 ssNS 并合残骸与中心黑洞的并合；（3）伴随的伽马射线暴和超新星对应体，后者因嵌入爆发恒星包层内部的中子星并合而获得能量增强并富集 r-过程元素（“超新星内部的千新星”）。
英文摘要：
Although stable neutron stars (NS) can in principle exist down to masses Mns ~ 0.1Msun, standard models of stellar core-collapse predict a robust lower limit Mns >~ 1.2Msun, roughly commensurate with the Chandrasekhar mass Mch of the progenitor's iron core (electron fraction Ye ~ 0.5). However, this limit may be circumvented in sufficiently dense neutron-rich environments (Ye << 0.5) for which Mch ~ Ye^2 is reduced to < Msun. Such physical conditions could arise in the black hole accretion disks formed from the collapse of rapidly-rotating stars ("collapsars"), as a result of gravitational instabilities and cooling-induced fragmentation, similar to models for planet formation in protostellar disks. We confirm that the conditions to form sub-solar mass NS (ssNS) may be marginally satisfied in the outer regions of massive neutrino-cooled collapsar disks. If the disk fragments into multiple ssNS, their subsequent coalescence offers a channel for precipitating sub-solar mass LIGO/Virgo gravitational-wave mergers that does not implicate primordial black holes. The model makes several additional predictions: (1) ~Hz frequency Doppler modulation of the ssNS-merger gravitational wave signals due to the binary's orbital motion in the disk; (2) at least one additional gravitational wave event (coincident within <~ hours), from the coalescence of the ssNS-merger remnant(s) with the central black hole; (3) an associated gamma-ray burst and supernova counterpart, the latter boosted in energy and enriched with r-process elements from the NS merger(s) embedded within the exploding stellar envelope ("kilonovae inside a supernova").
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