Determine whether nucleons in neutron star cores undergo hyperonization

Determine whether nucleons in the cores of neutron stars undergo hyperonization at supranuclear densities, and quantify the implications of hyperon formation for the neutron-star equation of state, particle composition distributions, thermal evolution, and observable signatures.

Background

The paper investigates how the presence of hyperons affects neutrino emissivity and luminosity in nucleonic direct Urca processes within neutron stars, using relativistic mean field theory with GM1 and NL3 parameter sets under SU(3) and SU(6) symmetries. A central motivation is the long-standing uncertainty about whether hyperons appear in neutron star cores, which would soften the equation of state and alter macroscopic properties.

The authors emphasize that, despite numerous pulsar observations and NICER mass–radius inferences, direct observational constraints on core composition are lacking. Establishing whether nucleons hyperonize is thus a key open question linked to dense matter physics and the cooling behavior of neutron stars.

References

However, in stark contrast to these observational breakthroughs, the core questions about the internal composition of neutron stars remain unresolved. Although observations clearly confirm the extreme compactness of neutron stars — with radii of $\sim$ 10 km and masses up to 2 $M_{\odot}$ — direct observational constraints on their core composition, such as whether nucleons undergo hyperonization, deconfinement phase transitions, or form meson condensates, are still lacking .

— A Study on the Triggering of Nucleonic Direct Urca Processes in Neutron Stars of Specific Masses and Their Hyperon Dependence  (2509.02988 - Xu et al., 3 Sep 2025) in Section 1 (Introduction), first paragraph