Multi-Messenger Research On The Nucleosynthesis Of Heavy Elements In Neutron Star Mergers

The origin of heavy elements in the universe is a fundamental question in astrophysics.The most important process for producing heavy elements is the rapid neutron capture process（r-process）.It has long been believed that the merger of two neutron stars is an ideal site for r-process nucleosynthesis.Recently,the detection of the radioactively powered kilonova AT2017gfo associated with the gravitational wave event GW170817 from a binary neutron star merger provided the first strong evidence that such mergers could be the major sites for r-process nucleosynthesis.The radioactive decay of heavy elements from neutron star mergers may power three different emission signals:kilonova,gamma-rays,and neutrinos.These signals are related to the radioactive decay of heavy element and provide critical clues for understanding the origin of heavy elements.Based on detailed r-process nucleosynthesis simulations,we study the kilonova,gamma-rays,and neutrinos produced by the radioactive decay of heavy elements from binary neutron star mergers,and explore the observational features of heavy elements.We first provide an overview of the kilonova theory and introduce the nuclear reaction network Sky Net model.We have updated the nuclear physics input module in the Sky Net model by using the latest version of the nuclear physics database and a more precise nuclear physics model.Moreover,we have improved the calculation modules for neutron capture reaction cross sections and energy generation rates in the Sky Net model.With these improvements,the improved Sky Net model not only accurately reproduces the heavy element abundance of the solar system but also provides self-consistently energy generation rates for various decay products in nuclear reactions.These improvements lay a solid foundation for further research of the observational features of heavy elements.Kilonovae are powered by the radioactive decay of heavy elements,providing a unique opportunity to study the nuclide composition of the merger ejecta.We analyze the energy generation rates of various isotopes,identifying the dominant nuclides and their characteristic time that contribute to the kilonova light curves.It is found that the late-time kilonova emission（t>20 days）is sensitive to the abundance of ²²⁵Ac,which is the most promising heavy elements to be identified in upcoming kilonova observations.The identification of these specific heavy elements can improve our understanding of the origin of heavy elements beyond iron and the physical conditions for the merger ejecta.The James Webb Space Telescope,with its high sensitivity in the near-infrared band,is a powerful instrument for deep follow-up observations of kilonovae.Radioactive decay of unstable nuclei usually left the daughter nucleus in an excited state.The subsequent decay of the daughter nucleus to a lower-energy state results in gamma-ray emission in specific energy.The detection of these gamma-ray photons can provide conclusive evidence for identifying individual elements and tracking their evolution.We investigate the radioactive gamma-ray emission from binary neutron star mergers and analyze the shape and features of the gamma-ray spectra in the merger ejecta.It is found that the dominant contributors of gamma-ray energy are the nuclides around the second r-process peak（atomic mass number A～130）.The decay chain of¹³²Te→¹³²I→¹³²Xe produces several bright gamma-ray lines at energies of228ke V,668ke V,and 773ke V.We have also calculated gamma-ray emission for ejecta mass and expansion velocity in the ranges of M_ej=0.001-0.05M_⊙and v_ej=0.1-0.4c.It is found that a more massive ejecta tends to power a later and brighter gamma-ray emission,while a faster ejecta tends to power an earlier gamma-ray emission.As weak interaction particles,neutrinos can escape from dense ejecta without any interaction and thus they could be ideal messengers for the dense nuclear matter at early times.The detection of neutrinos generated by the radioactive decay of heavy elements can provide a conclusive evidence for the r-process nucleosynthesis,in principle.We calculate the luminosity and mean energy of neutrinos emitted from beta-decay of r-process elements in neutron star mergers.It is found that about half of beta-decay energy is carried away by neutrinos.The neutrinos energy generation rate remains approximately constant in the early stage（t<1 s）and then decay as a power-law function with an index of-1.3.This powers a short-lived fast neutrino burst with a peak luminosity of～10⁴⁹ erg s^-1 in the early stage.The typical neutrino energy is less than 8Me V,which is within the energy ranges of the water-Cherenkov neutrino detectors such as Super-Kamiokande and future Hyper-Kamiokande.Observations of neutrinos from neutron star mergers will be an important step towards understanding the properties of extremely neutron-rich nuclei and r-process nucleosynthesis.