Magnetized Outflows from Neutron Star Mergers and Collapsars

2017年，通过引力波和电磁光首次观察到了两个密度极高的恒星残骸或中子星的合并。尽管这场名为GW 170817的爆炸已经得到了广泛的研究，但人们仍然不太清楚爆炸的某些方面，也就是所谓的“基诺瓦”。使用详细的计算机模型，哥伦比亚大学的一个研究小组将详细调查合并后的恒星可能如何产生新的中子星残留物(最终屈服于重力并坍塌到黑洞)。他们将探索爆炸性物质如何从合并地点流出，产生重原子核，其放射性衰变为基诺瓦提供动力。同时，他们将研究一颗大质量恒星在其核燃烧寿命结束时发生坍塌后发生的类似物理条件，当时该恒星正在非常迅速地旋转(所谓的“崩塌星”)。这项工作将探索极端的物理过程，如黑洞周围的相对论时空和在大质量中子星核心中发现的极端密度。该项目对宇宙中最重元素的起源也有影响。灾难性的事件，如中子星合并，激发了学生、研究人员和公众的想象力。研究和教育目标将通过三种方式结合起来：本科生研究，为期一周的关于时间域引力波天体物理学的国际学校，以及一个向当地教师传播的网站，用作课堂教育工具。双中子星合并GW170817伴随着热辐射(“千诺瓦”)，由通过快速中子俘获过程(r过程)合成的重核的放射性衰变提供动力。然而，需要与观测结果相匹配的喷射物的数量、组成和速度与动力合并阶段的数值模拟所预测的结果不一致。相反，最好的解释是，大部分抛射物来自合并后阶段较长时间尺度上的流出，来自强磁化的中子星残余物(“毫秒磁星”)或围绕着致密天体的中微子冷却的吸积环。研究人员将使用三维广义相对论磁流体动力学(GRMHD)模拟，探索中子星合并和崩塌的毫秒磁星遗迹中中微子加热的磁化流出的特性和千诺瓦特征，无论是孤立的还是被磁化吸积盘包围的。数值代码将进行升级，以包括一个现实的状态方程(将中子星内部连接到低密度风区)和一个中微子传输方案，以捕捉中微子驱动的风的质量负载及其电子部分的演化。针对不同的中微子光度和质量吸积率进行的一系列模拟，将涵盖合并后演化的不同时期。这些模拟将解决从恒星和圆盘流出的流之间的相对重要性和相互作用，以及它们各自的重子加载。这一裁决反映了NSF的法定任务，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

The merger of two extremely dense stellar remnants, or neutron stars, was observed for the first time through gravitational waves and electromagnetic light in 2017. Although this event, named GW 170817, has been studied extensively, there are aspects of the explosion, called a “kilonova,” which are still not well understood. Using detailed computer modeling, a research team at Columbia University will investigate in detail how the merged stars may have produced a new neutron star remnant (before ultimately succumbing to gravity and collapsing to a black hole). They will explore how explosive ejecta flowed out from the merger site, producing heavy atomic nuclei, whose radioactive decay powered the kilonova. In parallel they will study similar physical conditions that occur following the collapse of a massive star at the end of its nuclear burning life when the star is rotating very rapidly (a so-called "collapsar"). This work will probe extreme physical processes such as relativistic spacetime around black holes and the extreme densities found in the cores of massive neutron stars. The project also has implications for the origin of the heaviest elements in the Universe. Cataclysmic events, such as neutron star mergers, capture the imagination of students, researchers, and the public alike. Research and educational goals will be integrated in three ways: undergraduate student research, a week-long international school on time-domain gravitational wave astrophysics, and a website disseminated to local teachers for use as classroom educational tools. The binary neutron star merger GW170817 was accompanied by thermal emission ("kilonova"), powered by the radioactive decay of heavy nuclei synthesized via the rapid neutron capture process (r-process). However, the quantity, composition, and velocity of the ejecta needed to match the observations disagree with those predicted by numerical simulations of the dynamical merger phase. Instead, the bulk of the ejecta is best explained as originating in outflows over longer timescales during the post-merger phase, from either a strongly-magnetized neutron star remnant ("millisecond magnetar") or the neutrino-cooled accretion torus surrounding the compact object. The researchers will explore the properties and kilonova signatures of neutrino-heated magnetized outflows from millisecond magnetar remnants of neutron star mergers and collapsars, both in isolation as well as surrounded by magnetized accretion disks, using three-dimensional general-relativistic magnetohydrodynamical (GRMHD) simulations. The numerical code will be upgraded to include a realistic equation of state (bridging the interior of the neutron star to the lower density wind region) and a neutrino transport scheme to capture the neutrino-driven mass-loading of the winds and the evolution of its electron fraction. A suite of simulations, performed for different neutrino luminosities and mass accretion rates, will cover different epochs in the post-merger evolution. The simulations will address the relative importance and interplay between outflows from the star versus the disk, and their respective baryon-loading.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.