arXiv subjects

Juan M. Z. Pretel

Publications and source records attributed to Juan M. Z. Pretel.

At least 19 recordsLinked to original sources

arXiv2026

Structure and astrophysical properties of dark energy admixed neutron stars

The presence of a dark energy (DE) core, made of a Chaplygin dark fluid, is capable of influencing the diverse observable properties of neutron stars (NSs). However, there is also the possibility that NSs could contain a mixture of ordinary nuclear matter and DE. In this perspective, we propose an equation of state (EoS) that includes both normal matter and DE, where a free parameter $f$ quantifies the ratio of DE density with respect to the total energy density. As expected, in the limit $f \rightarrow 0$ we recover the already known results corresponding to pure nuclear matter. Remarkably, increasing this fraction $f$ leads to a softer EoS and thus strongly favors the fulfillment of the causality condition. As a consequence of our mass--radius results, the central compact object in HESS J1731-347 can be satisfactorily described as a DE admixed NS. For sufficiently high stellar masses, our calculations further indicate that both the gravitational redshift and the fundamental nonradial oscillation frequency increase significantly with an increasing DE fraction, whereas the opposite trend is observed for the total gravitational mass and tidal deformability. As is typical for compact stars composed purely of ordinary matter, we find that in our DE admixed stellar models the central density corresponding to the maximum-mass configuration coincides with the point at which the squared radial vibration frequency vanishes. Moreover, higher DE concentrations lead to enhanced stability of these configurations.

gr-qc↗

arXiv2026

Bayesian constraints on quadratic $f(Q)$ gravity from the ADM mass of white dwarfs

We calculate static equilibrium sequences for white dwarfs (WDs) in quadratic symmetric teleparallel gravity, f(Q)=Q+alpha Q^2, and perform a Bayesian comparison between the negative-coupling sector, general relativity (GR), and the positive-coupling sector using mass-radius measurements. Regular interior solutions are obtained by imposing the affine field equation and selecting the analytic branch C=r exp(B/2), and are matched to asymptotically flat exterior spacetimes to determine the ADM mass. The matter sector is described by a cold carbon-12 equation of state including relativistic electron degeneracy, nuclear rest energy, and Coulomb lattice corrections. Central density is marginalized at fixed alpha with a log-uniform prior. The sequences span 10^14 <= |alpha|/cm^2 <= 10^21. Positive couplings shift the compact branch toward larger radii and lower masses, whereas negative couplings produce high-mass extensions, including an admissible equilibrium configuration with M_ADM=14.34 solar masses. Using Sirius B, QS Vir, V471 Tau B, ZTF J1901+1458, and LHS 4033, we obtain conditional posterior medians alpha_-= -1.12x10^16 cm^2 and alpha_+=1.85x10^17 cm^2. With equal prior weights, the posterior probabilities for the negative, GR, and positive sectors are 0.228, 0.378, and 0.395, respectively, with ln(B_+/GR)=0.043. Current data therefore provide conditional constraints on the coupling but no direct evidence for modified gravity, emphasizing that the ADM mass prescription and admissible stellar domain must be consistently defined when using WD observations to constrain the theory.

gr-qc↗

arXiv2026

Nucleon spectra and wave functions from holographic models with dual Einstein-dilaton and Starobinsky-dilaton gravities

We study the nucleon spectra in two holographic set-ups: Einstein-dilaton and Starobinsky-dilaton gravity models. The Einstein-dilaton holographic model, also known as improved holographic QCD, have been proposed some time ago to describe confinement and glueball spectra. Recently, it has been applied to the case of mesons and nucleons. In this work, we reconsider the Einstein-dilaton holographic model to discuss the nucleon spectra introducing new parameters that allow us to improve the comparison with experimental data. Then, we extend this idea to the context of Starobinsky-dilaton gravity defining another improved holographic model. We use this new holographic model to reanalyse the nucleon spectra and also compare them with soft-wall model and experimental data.

hep-th↗

arXiv2026

Magnetic pressure anisotropy in ultra-dense white dwarfs: Landau quantization and neutronization effects

Strong magnetic fields modify the thermodynamics of white dwarf (WD) matter by quantizing the transverse motion of degenerate electrons into Landau levels and splitting the pressure into components parallel and perpendicular to the magnetic field. We investigate how this microscopic pressure anisotropy affects ultra-dense WD sequences when Coulomb lattice contributions, electron capture thresholds, neutron drip limits, and nuclear composition are treated consistently in the equation of state (EoS). The stellar equilibrium structure is obtained by solving an anisotropic extension of the Tolman--Oppenheimer--Volkoff (TOV) equations, where $P_r = P_\parallel$ and $P_t = P_\perp$ denote the radial and tangential pressures, respectively. We compare the post-electron-capture branches with their corresponding pre-capture sequences for pure $^{12}\rm{C}$, $^{16}\rm{O}$, $^{20}\rm{Ne}$, and $^{24}\rm{Mg}$ compositions, as well as for fixed 50/50 C/O, Ne/O, and Mg/Ne mixtures. The EoS predicts $P_\perp < P_\parallel$ in density ranges where only a few Landau levels are occupied. Consequently, pressure anisotropy shifts the stellar sequences toward smaller radii, with only minor changes in the maximum masses. We also find that post-capture configurations cannot be interpreted from the mass--radius plane alone, since the corresponding $M(ρ_c)$ curves reveal unstable portions after the maximum mass is reached. Our results show that strong magnetic fields significantly reduce the radii of low-mass WDs, yielding better agreement with the observational data available in the Montreal White Dwarf Database and suggesting that future high-precision observations of massive magnetic WDs could help assess the role of magnetic pressure anisotropy in determining their masses and radii.

astro-ph.SR↗

arXiv2026

Examining the influence of anisotropy on the fundamental mode of nonradial oscillation in neutron stars on a complete general relativistic scheme

The anisotropic influence on the $f$-mode frequency of oscillations and dimensionless tidal deformability of neutron stars is analyzed by employing the nonradial oscillation equations for the complete general relativity frame and tidal deformability equations, which are derived and modified from their standard form to introduce the anisotropic factor. The fluid inside the compact star obeys an equation of state constructed by matching microscopic nuclear and perturbative QCD calculations through a piecewise polytropic interpolating scheme. For the anisotropic profile, we use a local anisotropy which is regular along the whole star and vanishes both at the center and on the star's surface. We show that the $f$-mode oscillation frequency and dimensionless tidal deformability are noticeably affected by anisotropy. Finally, we investigate the correlation between the dimensionless tidal deformability inferred from the GW$170817$ event and the anisotropy parameter.

gr-qc↗

arXiv2026

Vector mesons from a holographic QCD model in $f(R)$-dilaton gravity

In this paper we investigate the spectrum and decay constants of vector mesons based on $f(R)$-dilaton gravity. We focus particularly on the well-known Starobinsky model, given by the function $f(R)= R+ αR^2$ in the metric formalism, and we examine the deviations from the pure Einstein-dilaton case. We thus provide the field equations for Starobinsky-dilaton gravity in $\rm AdS_{5}$ spacetime and calculate the spectral decomposition of the vector mesons by means of a Sturm-Liouville problem. Remarkably, for the rho mesons $m_{ρ^2}$ and $m_{ρ^3}$, our results are in good agreement with the experimental data when $α= 10^{-8}$. Our work also generalizes previous studies and recovers standard results when $α= 0$.

hep-th↗

arXiv2026

Dark energy stars from the modified Chaplygin gas: $C-I-Λ-E_g-f$ universal relations

Dark energy stars (DESs), described by the modified Chaplygin gas (MCG), can be dynamically stable and fall within different observational measurements. In this work, we employ diverse macroscopic properties, such as compactness $C$, moment of inertia $I$, tidal deformability $Λ$, gravitational binding energy $E_g$ and $f$-mode nonradial pulsation frequency, to explore whether they are correlated by universal relations (URs). Remarkably, our stellar configurations always obey the causality condition and are compatible with several observational mass-radius constraints. Via the $C-I-\text{Love}-f$ URs, our results reveal that we cannot distinguish quark stars (QSs) from DESs in the sense that DESs satisfy several URs very similar to those of QSs. However, when we involve $E_g$, DESs and QSs can be strongly distinguished through the $I-E_g^{-2}$, $Λ-E_g^{-5}$ and $f-E_g^{-2}$ URs. We also make use of these findings and the tidal deformability constraint from the GW170817 event to forecast the canonical properties of a $1.4\, M_\odot$ compact star. Furthermore, we present a set of fine empirical correlations involving the tidal deformability, obtained from an extensive scan of the parameter space of our DE stellar models.

gr-qc↗

arXiv2026

White dwarf structure in $f(R,T,L_m)$ gravity: beyond the Chandrasekhar mass limit

In this work, we investigate the relativistic structure of white dwarfs (WDs) within the framework of modified gravity theory $f(R, T, L_m) = R + αT L_m$, which introduces a non-minimal coupling between matter and curvature. Using a realistic equation of state (EoS) that includes contributions from a relativistic degenerate electron gas and ionic lattice effects, we solve the modified Tolman-Oppenheimer-Volkoff (TOV) equations for two standard choices of the matter Lagrangian density: $L_m = p$ and $L_m = -ρ$. We show that the extra $αTL_m$ term significantly alters the mass-radius relation of WDs, especially at high central densities $( ρ_c \gtrsim 10^8 - 10^9\,\rm g/cm^3)$, allowing for stable super-Chandrasekhar configurations. In particular, depending on the sign and magnitude of the parameter $α$, the maximum mass can increase or decrease, and in some regimes, the usual critical point indicating the transition from stability to instability disappears. Our findings suggest that $f(R,T,L_m)$ gravity provides a viable framework to explain the existence of massive WDs beyond the classical Chandrasekhar limit. Using Bayesian inference with WD observational data, we further constrain the coupling parameter $α$ for the two choices of the Lagrangian density $L_m$.

gr-qc↗

arXiv2026

Self-bound hybrid stars with strong phase transitions can relieve major compact star observation tensions

Some recent pulsar observations cannot naturally fit into the conventional picture of neutron stars: the compact objects associated with HESS J1731-347 and XTE J1814-338 have too small radii in the low-mass regime, while the secondary component of GW190814 is too massive for neutron stars to be compatible with constraints from the GW170817 event. In this study, we demonstrate that all these anomalous observations and tensions, together with other conventional ones such as recent NICER observations of PSR J0740+6620, J0030+0451, and PSR J0437-4715, can be naturally explained simultaneously by a new general type of self-bound hybrid stars with large density discontinuities, and thus are radially stable in either the slow or rapid phase transition context. As a proof of concept, we use hybrid quark stars, inverted hybrid stars, and hybrid strangeon stars as benchmark examples to explicitly demonstrate the advantage and feasibility of self-bound hybrid stars with strong phase transitions in relieving all tensions related to compact stars' masses, radii, and tidal deformabilities.

astro-ph.HE↗

arXiv2025

Dark Matter in White Dwarfs: Implications for Their Structure

White Dwarfs (WDs), the final evolutionary stage of most stars, are frequently modeled considering only a dense plasma matter. However, their potential interaction with dark matter (DM), especially in galactic halos where DM is expected to be prevalent, may lead to significant consequences. This work proposes a novel EoS (EoS) that consistently incorporates both hot dense plasma and cold dark matter (CDM) contributions in hot WDs. The hot dense plasma EoS is extended to include thermal and radiative contributions. At the same time, the CDM component is modeled as a linear fluid, with the coupling constant $α$ determined self-consistently within the star. A smooth phase transition between hot dense plasma and CDM regimes is introduced via a hyperbolic mixing function that depends on local energy density and stellar temperature. Our results show that the inclusion of CDM leads to an increase in the WD radius by approximately $12\%$ and a total mass enhancement of $0.7\%$, compared to standard hot WD models without lattice effects. These results highlight the importance of considering CDM in stellar modeling and suggest that WDs may serve as indirect probes for the astrophysical properties of dark matter.

astro-ph.SR↗

arXiv2025

White dwarfs in regularized 4D Einstein-Gauss-Bonnet gravity

White dwarfs (WDs), as the remnants of low to intermediate-mass stars, provide a unique opportunity to explore the interplay between quantum mechanical degeneracy pressure and gravitational forces under extreme conditions. In this study, we examine the structure and macroscopic properties of WDs within the framework of 4D Einstein-Gauss-Bonnet (4DEGB) gravity, a modified theory that incorporates higher-order curvature corrections through the Gauss-Bonnet coupling constant $α$. Using the modified Tolman-Oppenheimer-Volkoff (TOV) equations tailored for 4DEGB gravity, we analyze the hydrostatic equilibrium of WDs modeled with a realistic equation of state (EoS). Our findings reveal that the inclusion of the Gauss-Bonnet (GB) term significantly influences the mass-radius ($M-R$) relation, allowing for deviations from the Chandrasekhar mass limit. In particular, we observe that such stars become more compact and slightly smaller with the increase of the parameter $α$. For WDs with $\vertα\vert \leq 500\, \rm km^2$, the impact of 4DEGB gravity appears to be negligible. However, a larger range for $α$ allows for appreciable changes in the $M-R$ diagram, mainly in the high-central-density region. Furthermore, we explore the role of anisotropic pressures, quantified by the parameter $β$, on such systems and demonstrate their impact on stability and compactness. For sufficiently large values of $\vertβ\vert$ keeping negative $β$ with a large and positive $α$, there exists a second stable branch according to the classical stability criterion $dM/dρ_c >0$. These results suggest that anisotropic WDs in 4DEGB gravity exhibit unique characteristics that distinguish them from their general relativistic counterparts, offering a novel testing ground for modified gravity theories in astrophysical settings.

gr-qc↗

arXiv2024

Neutron stars with a dark-energy core from the Chaplygin gas

We analyze the effect of a Chaplygin dark fluid (CDF) core on neutron stars (NSs). To address this study, we focus on the relativistic structure of stellar configurations composed by a dark-energy core, described by a Chaplygin-like equation of state (EoS), and an ordinary-matter crust which is described by a polytropic EoS. We examine the impact of the rate of energy densities at the discontinuous surface, defined as $α= ρ_{\rm dis}^-/ρ_{\rm dis}^+$, on the radius, total gravitational mass, oscillation spectrum and tidal deformability. Furthermore, we compare our theoretical predictions with several observational mass-radius measurements and tidal deformability constraints. These comparisons together with the radial stability analysis show that the existence of NSs with a dark-energy core is possible.

gr-qc↗

arXiv2024

Gravitational wave seismology of charged strange stars in the Cowling approximation: the fluid pulsation modes

In this work we study, within the framework of Cowling approximation, the effect of the electric charge on the gravitational wave frequency of fluid oscillation modes of strange quark stars. For this purpose, the dense matter of the stellar fluid is described by the MIT bag model equation of state (EoS), while for the electric charge profile, we consider that the electric charge density is proportional to the energy density. We find that the gravitational wave frequencies change with the increment of electric charge; these effects are more noticeable at higher total mass values. We obtain that the $f$-mode is very sensitive to the change in the electric charge of the star. Furthermore, in the case of the $p_1$ mode, the effect of the electric charge is not very significant. Our results reveal that the study of the fundamental pulsation mode of an electrically charged compact star is very important in distinguishing whether compact stars could contain electric charge. We also employ another electric charge distribution profile that follows a power law, and it is found that the $f$-mode change is more noticeable than the $p_1$-mode when the electric charge is incremented.

astro-ph.SR↗

arXiv2024

Dark energy effects on realistic neutron stars

By considering realistic equations of state (EoSs) to describe the ordinary matter of the stellar crust, in this study, we explore the effect of a dark energy core, made of Chaplygin Dark Fluid (CDF), on neutron stars (NSs). To accomplish this purpose, we solve the stellar structure equations and investigate the impact of the CDF parameters on the several macroscopic properties of NSs such as mass-radius ($M-R$) relation, and tidal deformabilities of a single star and of a binary system, the latter being of great importance when analyzing gravitational-wave signals coming from the merger of such compact objects. We also present an analysis of the radial oscillation modes for the rapid phase transition, with the aim of distinguishing regions consisting of dynamically stable stars from those of unstable ones. Specifically, our outcomes reveal that an increase in the energy density jump (controlled by a parameter $α$) leads to an increase in the radial stability of the NS with a CDF core. Furthermore, our theoretical results are consistent with the observational $M-R$ measurements of millisecond pulsars from NICER data and tidal deformability constraints from the GW170817 event.

gr-qc↗

arXiv2024

Universal relations for anisotropic interacting quark stars

Interacting quark stars, which are entirely composed of interacting quark matter including perturbative QCD corrections and color superconductivity, can meet constraints from various pulsar observations. In realistic scenarios, pressure anisotropies are expected in the star's interior. Recently, the stellar structural properties of anisotropic interacting quark stars have been investigated. In this study, we further explore the universal relations (URs) related to the moment of inertia $I$, tidal deformability $Λ$, compactness $C$, and the $f$-mode nonradial pulsation frequency for such stars. Our results reveal that these approximate URs generally hold, being insensitive to both the EOS variations as well as to the presence of anisotropy. In contrast to previous studies on anisotropic neutron stars, we find that more positive anisotropy tends to enhance the $I-Λ$ and $I-C$ URs, but weakens the $C-Λ$ UR. For all the URs involving $f$-mode frequency, we find that they are enhanced by the inclusion of anisotropy (whether positive or negative). Utilizing these URs and the tidal deformability constraint from the GW170817 event, we put limits on the structural properties of isotropic and anisotropic quark stars, such as the moment of inertia $I_{1.4}$, the canonical radius $R_{1.4}$ and the canonical $f$-mode frequency $f_{f,1.4}$, all of which are very different compared to those of neutron stars.

nucl-th↗

arXiv2024

Five-dimensional compact stars in Einstein-Gauss-Bonnet gravity

Within the framework of Einstein-Gauss-Bonnet theory in five-dimensional spacetime ($5D$ EGB), we derive the hydrostatic equilibrium equations and solve them numerically to obtain neutron stars for both isotropic and anisotropic distribution of matter. The mass-radius relations are obtained for SLy equation of state, which describes both the solid crust and the liquid core of neutron stars, and for a wide range of the Gauss-Bonnet coupling parameter $α$. More specifically, we find that the contribution of the Gauss-Bonnet term leads to substantial deviations from Einstein gravity. We also discuss that after a certain value of $α$, the theory admits higher maximum masses compared with general relativity, however, the causality condition is violated in the high-mass region. Finally, our results are compared with the recent observations data on mass-radius diagram.

gr-qc↗

arXiv2024

Moment of inertia and compactness of quark stars within the context of $f(R,T,L_m)$ gravity

Within the metric formalism of $f(R,T,L_m)$ gravity theories, we investigate the hydrostatic equilibrium structure of compact stars taking into account both isotropic and anisotropic pressure. For this purpose, we focus on the $f(R,T,L_m)= R+ αTL_m$ model, where $α$ is a free parameter. We derive the modified TOV equations and the relativistic moment of inertia in the slowly rotating approximation. Using an equation of state (EoS) for color-superconducting quark stars, we examine the effects of the $αTL_m$ term on the different macroscopic properties of these stars. Our results reveal that the decrease of the parameter $α$ leads to a noticeable increase in the maximum-mass values. For negative $α$ with sufficiently small $\vertα\vert$, we obtain a qualitative behavior similar to the general relativistic (GR) context, namely, it is possible to obtain a critical stellar configuration such that the mass reaches its maximum. However, for sufficiently large values of $\vertα\vert$ keeping negative $α$, the critical point cannot be found on the mass-radius diagram. We also find that the inclusion of anisotropic pressure can provide masses and radii quite consistent with the current observational measurements, which opens an outstanding window onto the physics of anisotropic quark stars. By comparing the $I-C$ relations of isotropic quark stars in modified gravity, we show that such a correlation remains almost unchanged as the parameter $α$ varies from GR counterpart. On the other hand, given a fixed $α$, the $I-C$ relation is insensitive to variations of the anisotropy parameter $β$ to $\mathcal{O}(4\%)$.

gr-qc↗

arXiv2024

Compact stars in Rastall gravity: hydrostatic equilibrium and radial pulsations

Within the context of Rastall gravity, we investigate the hydrostatic equilibrium and dynamical stability against radial pulsations of compact stars, where a free parameter $β$ measures the deviations from General Relativity (GR). We derive both the modified Tolman-Oppenheimer-Volkoff (TOV) equations and the Sturm-Liouville differential equation governing the adiabatic radial oscillations. Such equations are solved numerically in order to obtain the compact-star properties for two realistic equations of state (EoSs). For hadronic matter, the fundamental mode frequency $ω_0$ becomes unstable almost at the critical central energy density corresponding to the maximum gravitational mass. However, for quark matter, where larger values of $\vertβ\vert$ are required to observe appreciable changes in the mass-radius diagram, there exist stable stars after the maximum-mass configuration for negative values of $β$. Using an independent analysis, our results reveal that the emergence of a cusp can be used as a criterion to indicate the onset of instability when the binding energy is plotted as a function of the proper mass. Specifically, we find that the central-density value where the binding energy is a minimum corresponds precisely to $ω_0^2 =0$.

gr-qc↗