Magnetized Outflows from Neutron Star Mergers and Collapsars

2017年，人们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。 虽然这一事件，命名为GW 170817，已被广泛研究，有爆炸的方面，称为“千诺瓦”，这仍然没有得到很好的理解。 利用详细的计算机建模，哥伦比亚大学的一个研究小组将详细调查合并的恒星如何产生新的中子星星残骸（在最终屈服于引力并坍缩成黑洞之前）。 他们将探索爆炸性喷出物如何从合并地点流出，产生重原子核，其放射性衰变为千诺瓦提供动力。 与此同时，他们还将研究大质量星星在其核燃烧寿命结束时发生坍缩后的类似物理条件，此时星星旋转非常迅速（所谓的“恒星星”）。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。像中子星星合并这样的灾难性事件，吸引了学生、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，为期一周的时域引力波天体物理学国际学校，以及向当地教师传播的网站，作为课堂教育工具。双星合并的中子星星GW 170817伴随着热辐射（“kilonova”），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，喷出物的数量，成分和速度需要匹配的意见不同意的动态合并阶段的数值模拟预测。相反，大部分喷出物最好解释为在合并后阶段的较长时间尺度内从强磁化中子星星残余物（“毫秒磁星”）或围绕致密物体的中微子冷却吸积环流出。研究人员将利用三维广义相对论磁流体动力学（GRMHD）模拟，探索中子星星合并和双星的毫秒磁星残余物中中微子加热磁化流出物的性质和千诺瓦特征，无论是孤立的还是被磁化吸积盘包围的。将对数字代码进行升级，以包括一个现实的状态方程（将中子星星内部与低密度风区连接起来）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分数的演变。一套模拟，不同的中微子光度和质量吸积率进行，将涵盖不同的时代合并后的演变。模拟将解决相对重要性和流出之间的相互作用从星星与磁盘，和他们各自的重子loading.This奖项反映了NSF的法定使命，并已被认为是值得通过使用基金会的智力价值和更广泛的影响审查标准进行评估的支持。

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.