Production of Heavy Elements during the Explosion of a Low-Mass Neutron Star in a Close Binary
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uction of Heavy Elements during the Explosion of a Low-Mass Neutron Star in a Close Binary I. V. Panov1, 2* and A. V. Yudin1, 2 1
National Research Center “Kurchatov Institute”—ITEP, ul. Bol’shaya Cheremushkinskaya 25, Moscow, 117218 Russia 2 National Research Center “Kurchatov Institute,” pl. Kurchatova 1, Moscow, 123182 Russia Received May 28, 2020; revised June 20, 2020; accepted June 25, 2020
Abstract—The nucleosynthesis of heavy elements in the scenario for the evolution of a close binary of neutron stars differing greatly in mass is considered. In contrast to the scenario for the merger of two neutron stars of comparable masses considered repeatedly in the literature, the evolution of such a binary at the final stage consists in a rapid mass transfer to the more massive star and an explosive disruption of the low-mass component. We provide the details of the explosion and calculate the abundances of the heavy elements produced in this process for various initial conditions. DOI: 10.1134/S1063773720080034 Keywords: neutron stars, close binaries, nucleosynthesis, nuclear reactions, beta decay.
INTRODUCTION The nucleosynthesis maintained by a rapid neutron capture (the r-process) is responsible for the production of more than half of all elements heavier than iron in nature. It results from the capture of neutrons and the subsequent beta decays of forming shortlived neutron-rich nuclei in an environment with a high neutron number density. The region where it proceeds on the map of nuclei lies near the neutron stability boundary (Burbidge et al. 1957; Cameron et al. 1957; Seeger et al. 1965). A high initial neutron number density, up to 150 per seed nucleus (as a rule, such are the iron-peak nuclei), is needed to create the conditions for the rprocess capable of producing heavy elements up to the heaviest ones. Such conditions are achieved in astrophysical objects in scenarios with a large neutron excess and a high density of matter, for example, during the merger of compact stellar remnants in close binaries or during the explosions of supernovae of a fairly rare type that form jets with a high neutron number density (Thielemann et al. 2017; Cowan et al. 2020) and in a hot wind from young neutron stars (Cameron 2001; Arcones and Thielemann 2013). The first detection of a neutron star (NS) merger (Abbot et al. 2017) and the simultaneous observation of r-elements (Tanvir et al. 2017) confirmed the understanding that the main scenario for the r-process *
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development is more likely associated with the ejecta that form during a NS merger at the end of the evolution of a close binary and not with supernova explosions (Hudepohl et al. 2010). The details of the NS merger scenario have long been known (Freiburghaus et al. 1999) and the conditions for the synthesis of heavy elements in the r-process have been determined on their basis. The NSs forming a close binary approach each other due to the loss of angular momentum by the binary through the radiation of gravitational waves. At the final stage
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