Hyperon effects in covariant density functional theory and recent astrophysical observations

Motivated by recent observational data, the equations of state with the inclusion of strangeness-bearing $\Lambda$ hyperons and the corresponding properties of neutron stars are studied based on the covariant density functional (CDF) theory. To this end, we specifically employ the density-dependent relativistic Hartree-Fock (DDRHF) theory and the relativistic mean field (RMF) theory. The inclusion of $\Lambda$ hyperons in neutron stars shows substantial effects in softening the equation of state. Because of the extra suppression effect originating from the Fock channel, large reductions on both the star mass and radius are predicted by the DDRHF calculations. It is also found that the mass-radius relations of neutron stars with $\Lambda$ hyperons determined by DDRHF with the PKA1 parameter set are in fairly good agreement with the observational data, where a relatively small neutron-star radius is required. Therefore, it is expected that the exotic degrees of freedom such as the strangeness-bearing structure may appear and play significant roles inside the neutron stars, which is supported further by the systematical investigations on the consistency between the maximum neutron-star mass and $\Lambda$-coupling strength.

Figure 1

(a) The pressure as a function of the baryonic density ${\rho }_b$ (fm${}^{-3}$) with different CDF effective interactions for the stellar matter containing nucleons, $\Lambda$ hyperons, electrons, and muons [red (gray) curves], compared to the one without $\Lambda$ hyperons (black curves). (b) The corresponding contribution from the Hartree and Fock channels with PKA1 compared with TW99. See the text for details.

Figure 2

(a) The symmetry energy $E_{\mathrm{sym}}$ (MeV) as a function of the baryonic density ${\rho }_b$ (fm${}^{-3}$) with different CDF effective interactions for the stellar matter containing nucleons, $\Lambda$ hyperons, electrons, and muons [red (gray) curves], as compared to the one without $\Lambda$ hyperons (black curves). (b) The corresponding contribution from Hartree and Fock channels with PKA1 compared with TW99. See the text for details.

Figure 3

(a) The mass-radius relations for the neutron stars with $\Lambda$ hyperons (solid lines) and without $\Lambda$ hyperons (dashed lines). The symbols denote the neutron stars with maximum masses. The region excluded by causality is shaded. For comparison the observational constraints from three neutron stars in the binaries 4U 160-8248 (light gray/white gray), EXO 1745-248 (dark gray/gray), and 4U 1820-30 (black/black gray) in Ref. [12] are shown as the 1$\sigma$ and 2$\sigma$ confidence contours, and the later analyses in Ref. [13] are denoted by the shaded areas with two different models of photospheric radius. (b) The mass and radius for the neutron stars with maximum masses determined with different $\Lambda$-hyperon coupling strengths (solid symbols), compared to the ones without $\Lambda$ hyperons (open symbols). See the text for details.