Recent detections of gravitational waves from mergers of neutron stars (NSs) and black holes (BHs) in the low- and high-end mass gap regimes pose a puzzle to standard stellar and binary evolution theory. Mass-gap mergers may originate from successive mergers in hierarchical systems such as quadruples. Here, we consider repeated mergers of NSs and BHs in stellar 2+2 quadruple systems, in which secular evolution can accelerate the merger of one of the inner binaries. Subsequently, the merger remnant may interact with the companion binary, yielding a second-generation merger. We model the initial stellar and binary evolution of the inner binaries as isolated systems. In the case of successful compact object formation, we subsequently follow the secular dynamical evolution of the quadruple system. When a merger occurs, we take into account merger recoil, and model subsequent evolution using direct N-body integration. With different assumptions on the initial properties, we find that the majority of first-generation mergers are not much affected by secular evolution, with their observational properties mostly consistent with isolated binaries. A small subset shows imprints of secular evolution through residual eccentricity in the LIGO band, and retrograde spin-orbit orientations. Second-generation mergers are ∼107 times less common than first-generation mergers, and can be strongly affected by scattering (i.e. three-body interactions) induced by the first-generation merger. In particular, scattering can account for mergers within the low-end mass gap, although not the high-end mass gap. Also, in a few cases, scattering could explain highly eccentric LIGO sources and negative effective spin parameters.

中文翻译：

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第一代和第二代黑洞和中子星以 2+2 四倍合并：人口统计

最近检测到的中子星 (NSs) 和黑洞 (BHs) 在低端和高端质量间隙区域合并产生的引力波对标准的恒星和双星演化理论构成了难题。质量差距合并可能源于分层系统中的连续合并，例如四重系统。在这里，我们考虑了恒星 2+2 四重系统中 NS 和 BH 的重复合并，其中长期演化可以加速内部双星之一的合并。随后，合并残余可能与伴星相互作用，产生第二代合并。我们将内部双星的初始恒星和双星演化建模为孤立系统。在成功形成致密物体的情况下，我们随后遵循四重系统的长期动力学演化。当合并发生时，我们考虑了合并后坐力，并使用直接 N 体集成对后续进化进行建模。通过对初始属性的不同假设，我们发现大多数第一代合并受长期演化的影响不大，其观测属性大多与孤立的双星相一致。一小部分通过 LIGO 带中的残余偏心率和逆行自旋轨道方向显示了长期演化的印记。第二代合并比第一代合并少 107 倍，并且会受到第一代合并引起的散射（即三体相互作用）的强烈影响。特别是，分散可以解释低端质量差距内的合并，尽管不能解释高端质量差距。此外，在少数情况下，