BIFROST：使用层次化四阶向前辛积分器代码模拟星团中的致密子系统
BIFROST: simulating compact subsystems in star clusters using a hierarchical fourth-order forward symplectic integrator code
AI总结：
BIFROST扩展了FROST代码，支持任意双星比例至100%，高效模拟碰撞恒星系统，包含后牛顿项和并合判据，验证了其精度和扩展性。
摘要：
我们介绍BIFROST，这是GPU加速的层次化四阶向前辛积分器代码FROST的扩展版本。BIFROST（FROST中的双星）能够通过使用长期和正则化积分，在四阶向前积分器框架内，对双星、三重星、多星系统或黑洞周围的小星团进行积分，从而高效演化具有任意双星比例（最高可达f_bin=100%）的碰撞恒星系统。在致密子系统的运动方程中包含了高达PN3.5阶的后牛顿（PN）项，并可选地包含三体和自旋相关项。在整个模拟域中计算与黑洞相互作用的PN1.0项。该代码具有多个并合判据（引力波旋进、潮汐瓦解事件以及恒星和致密天体碰撞），并增加了致密天体并合的相对论反冲。我们表明，对于具有N个粒子的系统，代码的扩展性在N_GPU ~ 40×N/10^6个GPU时仍然良好，并且双星比例增加到100%几乎不增加代码运行时间（小于约1.5倍）。我们还通过展示多个星团模拟来验证BIFROST的数值精度，其中最极端的模拟包括核心坍缩和两个中等质量黑洞的并合，并伴随相对论反冲。
英文摘要：
We present BIFROST, an extended version of the GPU-accelerated hierarchical fourth-order forward symplectic integrator code FROST. BIFROST (BInaries in FROST) can efficiently evolve collisional stellar systems with arbitrary binary fractions up to $f_\mathrm{bin}=100\%$ by using secular and regularised integration for binaries, triples, multiple systems or small clusters around black holes within the fourth-order forward integrator framework. Post-Newtonian (PN) terms up to order PN3.5 are included in the equations of motion of compact subsystems with optional three-body and spin-dependent terms. PN1.0 terms for interactions with black holes are computed everywhere in the simulation domain. The code has several merger criteria (gravitational-wave inspirals, tidal disruption events and stellar and compact object collisions) with the addition of relativistic recoil kicks for compact object mergers. We show that for systems with $N$ particles the scaling of the code remains good up to $N_\mathrm{GPU} \sim 40\times N / 10^6$ GPUs and that the increasing binary fractions up to 100 per cent hardly increase the code running time (less than a factor $\sim 1.5$). We also validate the numerical accuracy of BIFROST by presenting a number of star clusters simulations the most extreme ones including a core collapse and a merger of two intermediate mass black holes with a relativistic recoil kick.
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