Systematic study of scalar, vector, and mixed density dependencies in relativistic mean-field descriptions of hyperonic matter in neutron stars

TL;DR

This paper addresses how the neutron star equation of state is affected by hyperons when described within density-dependent relativistic mean-field theory. The authors systematically explore scalar, vector, and mixed density dependencies in meson–baryon couplings and alternative rho-meson couplings, comparing with the standard DD2 EOS. They find that most new parameterizations yield stiffer EOSs than DD2, increasing radii and tidal deformabilities, while the inclusion of Lambda hyperons softens but preserves maximum NS masses above ~2 solar masses and remains compatible with NICER constraints. The work underscores the critical role of density-dependence choices in constraining dense matter with multi-messenger observations and points toward Bayesian analyses to further refine the parameter space.

Abstract

We investigate the equation of state (EOS) of hyperonic neutron star (NS) matter within a density-dependent relativistic mean-field (DDRMF) framework. The effects of scalar, vector, and mixed density dependencies in meson-baryon couplings are systematically examined along with alternative forms of the $ρ$-meson coupling. Several meson-nucleon parameter sets are explored here for the first time for neutron stars and compared with the standard DD2 EOS. Most new parameterizations produce stiffer EOSs, leading to neutron stars with larger radii and higher tidal deformabilities. However, the inclusion of $Λ$ hyperons softens these EOSs, and the resulting maximum masses still satisfy the two solar mass limits and agree with NICER measurements. These results highlight the importance of exploring alternative density dependencies in constraining dense matter through multi-messenger observations.

Figures (5)

• Figure 1: Equation of State (EOS) for various models. Left panel VZR, SZR & MZR sets. Right panel SZE, MZE, MPE and DD2 sets. Solid lines and dashed lines represent nucleonic (N) and hyperonic (N$\Lambda$) models, respectively. The colour scheme is indicated in the figure. Inset highlights the EOS behaviour at lower densities, where SZR (DD2-N) is softer than MZR-N$\Lambda$ (MZE-N$\Lambda$) EOS.
• Figure 2: Particle number fractions as a function of baryon number density for various RMF models. Dashed curve shows the $\Lambda$ hyperon fraction and dashed dot curve shows proton fraction in hyperonic matter (N$\Lambda$), solid curves represent the proton fraction in purely nucleonic matter (N). The left panel displays results for the VZR, SZR, and MZR models, while the right panel shows the SZE, MZE, and MPE models with black lines representing the results for DD2 model.
• Figure 3: Speed of sound $c_s^2$ as a function of normalized baryon density $n/n_{\rm sat}$. The left panel shows the results for the VZR, SZR, and MZR models, while the right panel presents the SZE, MZE, MPE, and DD2 models. Dashed lines correspond to nucleonic--hyperonic (N$\Lambda$) matter, while solid lines correspond to nucleonic (N) matter.
• Figure 4: Mass–Radius profiles for various RMF models with observational constraints from NICER. The shaded regions denote the 95% CI: J0030+0451 (blue), J0437–4715 (green), J0614–3329 (peach), and J0740+6620 (grey). The colour scheme for the EOS model curves is the same as in the previous figures.
• Figure 5: Dimensionless tidal deformability as a function of total gravitational mass for N (solid lines) and N$\Lambda$ (dashed lines). The canonical tidal deformability from GW170817 is shown by vertical blue line.