How Neutron Star Mergers make Heavy Elements

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The incredible density, gravity, and electromagnetic field strengths of neutron stars (NS) make them laboratories for physics under extreme conditions. But probing these exotic objects is difficult. With the 2017 gravitational wave detection of a NS-NS merger, the landscape changed, and we can now get high-quality spectra of the decompressed neutron-rich matter emerging from the collision. This is a new transient astrophysical phenomenon called a 'kilonova'. Kilonovae are a potential treasure trove of information on some of the biggest open questions in physics: understanding the nuclear and astrophysical pathways that created half of all the heavy elements (Z > 30) in the universe, and the physics of very hot and extremely dense matter. For this reason, they are considered a scientific priority and kilonova science is the target of several large new and upgraded facilities. But kilonovae are challenging: the phenomenon is short-lived, requiring rapid follow-up with large telescopes, the outflow is heavy element-dominated making it extremely demanding to model, and the merger itself covers a huge dynamic range and involves complex nuclear physics. To interpret the spectra we require new atomic data, which does not yet exist for most of the heavy elements. To tackle these challenges, HEAVYMETAL assembles experts in astrophysical observations, hydrodynamical merger simulation, numerical radiative transfer, and laboratory heavy element spectroscopy and atomic structure calculation. With this team we will be able to determine the structures and overall geometries of the merger outflow, the elemental abundances, and their stratification within the ejecta. By the full exploitation of kilonovae we will trace the nucleosynthesis pathways in NS mergers, and provide important insights on heavy nuclei, neutrino interactions, and the nature of high-density matter, and we will chart the role of compact object mergers as the cosmic forge of the heaviest elements.