Hadronic Equation of State of Low-Mass Neutron Stars from a Relativistic Mean-Field Model with Tensor Couplings

A recent report has identified a central compact object (CCO) within the supernova remnant HESS J1731-347, with a mass and radius of M = 0.77+0.20-0.17M circle dot and R = 10.4+0.86 this light compact star, a density-dependent relativistic mean-field (DDRMF) model, specifically the DDVT model, has been employed. The DDVT model incorporates tensor couplings of vector mesons, which can successfully describe the properties of finite nuclei, such as charge radius, binding energy, and spin-orbit splitting. The introduction of tensor coupling reduces the influence of scalar mesons and generates a softer equation of state (EOS) in the outer core of the neutron star. Moreover, it has been found that the crust segment plays a crucial role in reproducing the mass-radius relation of HESS J1731-347, indicating a preference for a soft crust EOS. By manipulating the coupling strength of the isovector meson in the DDVT parameter set, a reasonable hadronic EOS has been obtained, satisfying the constraints from the gravitational-wave signal GW170817, the simultaneous mass-radius measurements from the NICER collaboration, and the properties of finite nuclei. Notably, the mass-radius relations derived from this hadronic EOS also accurately describe the observables of HESS J1731-347. Therefore, based on our estimation, the CCO in HESS J1731-347 may represent the lightest known neutron star, within the estimations on the uncertainty from the carbon atmosphere model and the inhomogeneous temperatures on the stellar surface of the neutron star.