Magnetized Outflows from Neutron Star Mergers and Collapsars

2017年，科学家首次通过引力波和电磁光观测到两颗密度极高的恒星残骸或中子星的合并。尽管这一被命名为GW 170817的事件已经被广泛研究，但这次爆炸的一些方面，被称为“千新星”，仍然没有得到很好的理解。哥伦比亚大学的一个研究小组将利用详细的计算机模型，详细研究合并的恒星是如何产生新的中子星遗迹的（在最终屈服于引力并坍缩成黑洞之前）。他们将探索爆炸喷出物如何从合并地点流出，产生重原子核，其放射性衰变为千新星提供动力。与此同时，他们将研究一颗大质量恒星在其快速旋转的核燃烧生命结束时（即所谓的“坍缩星”）坍缩后发生的类似物理条件。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。该项目还对宇宙中最重元素的起源产生了影响。诸如中子星合并之类的灾难性事件激发了学生、研究人员和公众的想象力。研究和教育目标将以三种方式整合：本科生研究，为期一周的时域引力波天体物理学国际学校，以及向当地教师传播作为课堂教育工具的网站。GW170817双中子星合并伴随着热辐射（“kilonova”），热辐射是由通过快中子俘获过程（r过程）合成的重核的放射性衰变提供动力的。然而，与观测相匹配的喷出物的数量、组成和速度与动态合并阶段的数值模拟所预测的不一致。相反，大部分喷射物最好的解释是，在合并后阶段，在较长的时间尺度上，来自强磁化的中子星残骸（“毫秒磁星”）或围绕致密物体的中微子冷却吸积环。研究人员将利用三维广义相对论磁流体力学（GRMHD）模拟，探索中子星合并和坍缩的毫秒磁星残余物中中微子加热的磁化流出物的特性和千新星特征，无论是孤立的还是被磁化吸积盘包围的。数值代码将被升级，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，以捕获中微子驱动的风的质量负载及其电子分数的演变。对不同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。模拟将解决相对重要性和相互作用之间的流出的恒星与磁盘，和他们各自的重子负载。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值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.