New Insights into r-process Nucleosynthesis Highlight Heavy Element Formation
The formation of elements heavier than iron has long intrigued the physics community. While stars can fuse elements up to iron, the creation of heavier elements is hindered by Coulomb repulsion. However, in explosive environments, the necessary temperature and density for generating heavy elements may be achieved. The r-process, which occurs in such environments, is thought to be responsible for approximately half of the elements heavier than iron.
The 2017 detection of gravitational waves and their afterglow from the neutron star merger event GW081708 provided the first direct evidence of r-process nucleosynthesis. Yet, subsequent studies have questioned whether neutron star mergers are the sole sites of r-process, as the lanthanide abundance observed from these events is significantly less than what is found in metal-poor stars. This suggests that other sites, such as collapsars and magnetohydrodynamic supernovae, may also play crucial roles in the r-process.
Dr. Jin Shilun, from the Institute of Modern Physics (IMP) of the Chinese Academy of Sciences (CAS), along with a collaborator from Technion in Israel, has introduced the r-process characteristics within the novel CEJSNe scenario for the first time.
CEJSNe describes a scenario involving a neutron star remnant from a supernova and a red supergiant in the late stages of a massive binary system. As the red supergiant expands, it engulfs the neutron star, which spirals inward through the supergiant's envelope and into its core. Upon entering the core, the neutron star accretes mass at a high rate via an accretion disk, producing energetic, dense jets that create favorable conditions for r-process nucleosynthesis.
The researchers demonstrated that CEJSNe can generate the highest abundance of elements heavier than lanthanides among all known r-process scenarios. By comparing log(XLa) with log(Ir/Eu), a new metric showing the relative strength of lanthanides and third-peak elements, they discovered an anti-correlation between CEJSNe and other r-process models.
"This finding means that abundant lanthanide and heavier elements can't be generated in a single event, which would be a critical feature for further research on r-process. CEJSNe is also critical for explaining the characteristics of the r-enhanced metal-poor stars," said Dr. Jin.
This study not only advances the understanding of r-process nucleosynthesis but also sets the stage for future experimental investigations. With exotic isotopes soon to be available at the High Intensity Accelerator Facility (HIAF) in China, the key nuclear properties relevant to the r-process in CEJSNe are expected to be revealed.