Asymmetric nuclear matter and neutron star properties in relativistic ab initio theory in the full Dirac space

The long-standing controversy about the isospin dependence of the effective Dirac mass in ab initio calculations of asymmetric nuclear matter is clarified by solving the relativistic Brueckner-Hartree-Fock equations in the full Dirac space. The symmetry energy and its slope parameter at the saturation density are $E_{\text{sym}}\left({\rho }_0\right)=33.1$ MeV and $L=65.2$ MeV, in agreement with empirical and experimental values. Further applications predict the neutron star radius $R_{1.4M_{\odot }}\approx 12$ km and the maximum mass of a neutron star $M_{\text{max}}\le 2.4M_{\odot }$.

Figure 1

Effective Dirac mass for the neutron (solid lines) and the proton (dashed lines) as functions of the asymmetry parameter $\alpha$ at $\rho =0.16{\text{fm}}^{-3}$ calculated by the RBHF theory in the full Dirac space (left), in comparison with the results obtained by RBHF calculations with PESs only using the projection method [31] (middle) and the momentum-independence approximation [30] (right). The Bonn-A potential [49] is used.

Figure 2

The symmetry energy $E_{\text{sym}}$ as a function of the density $\rho$ calculated by the RBHF theory in the full Dirac space (red solid line), in comparison with the results obtained by the RBHF calculation with the projection method (olive dashed line) and the momentum-independence approximation (Mom.-ind. app., gray band). The constraints from the HIC [51], the IAS [52], and the ASY-EOS experiments [53] are depicted with blue, yellow, and gray shadows, respectively.

Figure 3

The mass-radius relations of the neutron star obtained from the EOS by RBHF theory in the full Dirac space with the potentials Bonn A, B, and C. Astrophysical constraints from massive neutron star observations taken from Refs. [61, 62, 63, 64, 65] are shown as horizontal shaded areas. The dark and light blue regions indicate the 68% and 95% confidence intervals constrained by the NICER analysis of PSR $\mathrm{J0030}+0451$ [57]. The recently inferred mass of the secondary compact object of GW190814 [66] is also shown. The gray region is excluded by causality that $R>2.9GM/c^2$. See the text for details.