Chiral Invariant Mass Constraints from HESS J1731–347 in an Extended Parity Doublet Model with Isovector Scalar Meson

The recent discovery of an extremely light and small central compact object (CCO) within the supernova remnant HESS J1731-347, with mass 0.77−0.17+0.20M⊙ and radius 10.4−0.78+0.86 km, is challenging our understanding of neutron stars. In this article, we identify it as an ultra-light neutron star (NS) and constrain the chiral invariant mass of nucleon m0 from the observational data of NS using an extended parity doublet model with the isovector scalar meson a0(980). We show that the 1σ data from the HESS J1731-347 impose a very narrow constraint on the allowed values of m0 and L0 in the crossover model: 830MeV≲m0≲900MeV for L0=40 MeV, and 850MeV≲m0≲890MeV for L0=45 MeV. We also study the higher-order asymmertic matter properties such as the symmetry incompressibility Ksym and the symmetry skewness Qsym in the presence of the a0 meson. We find that Ksym and Qsym are very sensitive to m0 in the presence of the a0 meson.