The role of isolated binaries in forming binary-neutron-star mergers

• Applicant: Dr. Xiaotian Xu
• Subject Area: Astrophysics and Astronomy
• Term: since 2025
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 568468452

Massive stars end their lives as neutron stars or black holes (compact objects). Two compact objects orbiting each other forms a binary compact object system, which produces gravitational-wave radiation, resulting in a merger. About one hundred merger events have been observed through gravitational-wave observations, which has fostered a wide interest in understanding the formation of binary compact objects. Several scenarios have been proposed, including the evolution of isolated massive binaries. However, many key factors regarding massive binary evolution are poorly understood, rendering the predictions of merging binary compact objects uncertain. To address these uncertainties, I will perform a detailed theoretical study on the formation of binary neutron stars with a focus on the Be/X-ray binary stage. My work will improve the modeling of accretion efficiency and mass transfer stability in current detailed models to address two key discrepancies in the literature. 1) Previous rapid population synthesis calculations, based on fitting formulas and tabulated data, have suggested an accretion efficiency of 50% for neutron star-forming binaries, while current detailed binary evolution models have near-zero accretion efficiencies due to the limitation of critical rotation. 2) Compared to current theoretical expectations, there is an apparent lack of long-period evolved massive binaries in observations. With new mechanisms implemented in the detailed binary evolution models, I will perform comprehensive population synthesis predictions covering all the evolutionary stages up to the Be/X-ray binary stage and make comparisons with observed populations to constrain model uncertainties, which will set a new benchmark in massive binary evolution.

• DFG Programme: WBP Position