Impact of hyperons on structural properties of neutron stars and hybrid stars within the regularized four-dimensional Einstein-Gauss-Bonnet gravity

TL;DR

This study assesses how hyperons and a hadron–quark phase transition influence neutron and hybrid star structure within regularized 4D Einstein-Gauss-Bonnet gravity. By combining DDME2 hadronic matter with hyperons and DDQM quark matter, the authors build hadronic and hybrid EoSs and solve the modified TOV equations across a range of Gauss-Bonnet couplings $\alpha$. They find that positive $\alpha$ stiffens the mass–radius relation, enabling $2\,M_{\odot}$ stars and NICER-compatible radii, while negative $\alpha$ often fails to reach the maximum mass, especially with phase transitions; the speed of sound remains positive and subluminal, and anisotropy can offset softening effects, yielding a degeneracy between $\alpha$ and the anisotropy parameter. The work demonstrates that astrophysical measurements can constrain the allowed range of $\alpha$ in this modified gravity framework and highlights anisotropy as a potential mechanism to reconcile soft EoSs with observations.

Abstract

We investigate the impact of hyperons and phase transition to quark matter on the structural properties of neutron stars within the regularized four-dimensional Einstein-Gauss-Bonnet gravity (4DEGB). We employ the density-dependent relativistic mean-field model (DDME2) for the hadronic phase and the density-dependent quark mass (DDQM) model for the quark phase to construct hadronic and hybrid equations-of-state (EoSs) that are consistent with the astrophysical constraints. The presence of hyperons softens the EoS and with a phase transition, the EoS further softens, and the speed of sound squared drops to around 0.2 for the maximum mass configuration, which lies in the pure quark phase. Adjusting the Gauss-Bonnet coupling constant, $α$, within its allowed range results in a decrease in the mass-radius relationship for negative $α$, and an increase for positive $α$. In addition, functions are fitted to the maximum mass and its associated radius as a function of the constant $α$ to observe its impact on these properties. We find that positive values of $α$ support massive stars consistent with the 2\,$M_{\odot}$ constraint and NICER measurements, while negative values, although compatible with low-mass radius observations, fail to reach the observed maximum mass, particularly for EoSs involving phase transitions. Therefore, astrophysical observations may be used to effectively constrain the allowed range of $α$.

Figures (7)

• Figure 1: Energy density and pressure variation for the given DD-ME2 parameter set without and with a phase transition to the quark matter at different quark model parameters ($C, D^{1/2}$). The solid (dashed) line represents the pure nucleonic matter (nucleonic with hyperons) without a phase transition, while as dash-dotted (dotted) line represents the EoS for the nucleonic matter (nucleonic with hyperons) with a phase transition.
• Figure 2: Speed of sound squared as a function of number density for the different hadronic composition EoS without (N, N + H) and with phase transition ( N (0.90, 125), N + H (0.65, 133)) to the quark matter. The dotted lines in the right plot correspond to the mixed-phase region where $c_s^{2}$ drops to zero. The green dashed line in both plots represents the conformal limit $c_s^{2}$ = 1/3.
• Figure 3: Left: Mass-Radius relation for the nucleonic matter (left) and nucleons with hyperons (right) at different values of $\alpha$. The various shaded areas are credibility regions from the mass and radius inferred from the analysis of GW190814, PSR J0952-0607, PSR J0740+6620, PSR J0030+0451, PSR J0437-4715, and HESS J1731-347 as discussed in the text.
• Figure 4: Same as Figure \ref{['figmr1']}, but with a phase transition to the quark matter at different quark model parameters ($C$, $D^{1/2}$).
• Figure 5: Variation of the maximum mass for different compositions of the EoS without and with phase transition at different values of the GB constant $\alpha$.