Scalarized Hot Neutron Stars Containing Hyperons and $Δ$-Resonances in Different Evolution Regimes

TL;DR

The paper investigates scalarization in scalar-tensor gravity for hot neutron stars containing hyperons and $Δ$-resonances, across evolution stages from neutrino-trapped to cold-catalyzed. Using a relativistic mean-field EoS with DDME2 couplings for N, NH, and NH$Δ$ matter, and the generalized TOV equations in the Einstein frame with $a(Φ)=\exp\left(\frac{1}{2}\beta(Φ-Φ_0)^2\right)$, it analyses how composition and temperature affect scalarization via the mass-radius relation, central scalar field, and scalar charge. The results show that hyperons and $Δ$-resonances soften the EoS and shift scalarization thresholds to higher densities, generally reducing the scalar charge, with the evolution stage significantly altering the extent of scalarization; lower $|\beta|$ strengthens scalarization and broadens its density range. These findings underscore the importance of finite-temperature effects and exotic degrees of freedom in testing scalar-tensor theories against NS observations such as NICER radius measurements and gravitational-wave tidal deformabilities, and motivate future work including rotation, magnetic fields, and possible quark phases. $\,$

Abstract

Scalar-tensor gravity models are among the prime candidates to explain cosmic acceleration, and compact stars provide unique laboratories for testing such theories. Predictions of scalar-tensor gravity in compact stars can be examined during the evolution of neutron stars. Spontaneous scalarization in relativistic stars is influenced by different properties of stellar matter in various evolution regimes. In the present study, we investigate the scalarization of neutron stars in different stages of the evolvement. For this aim, we apply the isentropic equations of state for the neutron star matter including nucleons, hyperons, and $Δ$- resonances in neutrino-trapped, neutrino diffusion, and neutrino-transparent stages as well as cold-catalyzed neutron star. Our equations of state are based on the relativistic model within the mean-field approximation. To emphasize the role of scalar-tensor theories in exploring the properties and structure of compact stars, we calculate the structure of neutron stars with hyperons and $Δ$-resonances in different snapshots of the neutron star evolution in the scalar-tensor gravity. Our calculations confirm that the neutron star scalarization is affected by the hyperons as well as the $Δ$-resonances. Moreover, the properties of scalarized neutron stars depend on the stage of the star evolution.

Figures (8)

• Figure 1: Neutron star matter equation of state in four stages of the star's evolution, Neutrino-Trapped Regime ($s_B = 1, Y_L = 0.4$), Neutrino Diffusion Regime ($s_B = 2, Y_L = 0.2$), Neutrino-Transparent Regime ($s_B = 2, Y_{\nu_e} = 0$), and Cold-Catalyzed Regime (T=0) in three cases with N, NH, and NH$\Delta$ matter.
• Figure 2: Comparison of neutron star matter equation of state containing N, NH, and NH$\Delta$ matter for different regimes of the star evolution.
• Figure 3: The temperature distributions in the stellar matter are shown as a function of the ratio of the baryon density ($n_B$) to the nuclear saturation density ($n_0$). The left panel shows the neutrino-trapped and neutrino diffusion stages and the right panel shows the neutrino-transparent stage.
• Figure 4: Neutron star mass versus the central density, $\tilde{\rho}_{c}$, considering different contents, i.e. N, NH, and NH$\Delta$ matter, in the scalar-tensor theory (STT) and general relativity (GR) with three values of the coupling constant $\beta$ in four stages of the star's evolution, Neutrino-Trapped Regime ($s_B = 1, Y_L = 0.4$), Neutrino Diffusion Regime ($s_B = 2, Y_L = 0.2$), Neutrino-Transparent Regime ($s_B = 2, Y_{\nu_e} = 0$), and Cold-Catalyzed Regime (T=0). We have considered the value $\rho_0=1.66\times10^{14}g/cm^3$ to present the dimensionless density.
• Figure 5: Same as Fig. \ref{['mro1']} but for the mass versus the star radius.