The detection of gravitational waves (GWs) produced by merging black holes has firmly established the reality of GW astrophysics. Upcoming improvements to GW detectors are expected to lead to many more detections of merging compact objects. These will include black hole binaries, but also neutron star-black hole (NSBH) and neutron star-neutron star (NSNS) binaries. The presence of neutron stars will make these mergers even more exciting. Nucleosynthesis in the ejected neutron-rich material can power bright optical/infrared transients (kilonovae), and may be the main source of many heavy elements (e.g. gold, platinum). Accretion disks formed after mergers are the most likely candidates to power short gamma-ray bursts (SGRBs). And finally, the size of neutron stars, which can be constrained through observations of mergers, informs us about the fundamental properties of high-density matter. We propose to use state-of-the art numerical simulations of NSBH and NSNS mergers and of their post-merger remnants to inform upcoming observations of these systems. We will focus on predicting the properties of the ejected material (mass, composition, temperature, velocity, geometry). This constitutes a critical step in the modeling of kilonovae and in the determination of the impact of the mergers on enriching the Universe with heavy elements. A big challenge in modeling these outflows is that they are produced over a wide range of time scales. Material is first ejected dynamically during the merger (over a few milliseconds), then through powerful disk winds (over ~100 ms), and finally through the viscous evolution of the remnant accretion disk (over ~10 s). We propose to perform, for the first time, 3D numerical simulations capable of capturing all three components of the ejecta, with self-consistent evolution from before the merger till the time at which mass ejection becomes negligible. We will simulate NSBH and NSNS mergers using the fully general relativistic radiation-hydrodynamics code SpEC (co-developed by PI Foucart), and then use the outcome of these simulations as initial conditions for long-term general relativistic magnetohydrodynamic simulations of the remnant disk with the GPU code H-AMR (developed by Co-I Tchekhovskoy). H-AMR has recently produced the first 3D simulations of post-merger accretion disks capable of following the system up to the end of the mass ejection. We will study the dependence of the ejecta on the parameters of the compact objects. We will also work with collaborators using the SkyNet nuclear reaction network code and the Sedona radiation transport code in order to predict the yields of r-process nucleosynthesis in mergers, and to generate synthetic kilonova lightcurves. Finally, we will implement improved methods for neutrino transport in the H-AMR code. Neutrino-matter interactions have a significant impact on the ejecta composition, which greatly affects the observable properties of kilonovae and the outcome of r-process nucleosynthesis. A reliable treatment of the neutrinos in our simulations is thus critical to the success of this project. We plan to implement into H-AMR the state-of-the art neutrino transport algorithm used in SpEC (grey, two-moments method). The proposed work matches the objectives of the ATP program by providing theoretical models for likely observations of neutron star mergers by optical and infrared telescopes, including HST, JWST, and WFIRST. It can also help us to better understand SGRBs observed by NASA’s FERMI, Chandra, and XMM-Newton missions.
|Effective start/end date||2/28/18 → 2/27/22|
- University of New Hampshire (180042//80NSSC18K0565)
- National Aeronautics and Space Administration (180042//80NSSC18K0565)
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