arXiv subjects

Juan M. Z. Pretel

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

35 records · Page 2Linked to original sources

arXiv2024

Neutron stars in $f(R,L_m,T)$ gravity

This study explores the behavior of compact stars within the framework of $f(R,L_m,T)$ gravity, focusing on the functional form $f(R,L_m,T) = R + αTL_m$. The modified Tolman-Oppenheimer-Volkoff (TOV) equations are derived and numerically solved for several values of the free parameter $α$ by considering both quark and hadronic matter -- described by realistic equations of state (EoSs). Furthermore, the stellar structure equations are adapted for two different choices of the matter Lagrangian density (namely, $L_m= p$ and $L_m= -ρ$), laying the groundwork for our numerical analysis. As expected, we recover the traditional TOV equations in General Relativity (GR) when $α\rightarrow 0$. Remarkably, we found that the two choices for $L_m$ have appreciably different effects on the mass-radius diagrams. Results showcase the impact of $α$ on compact star properties, while final remarks summarize key findings and discuss implications, including compatibility with observational data from NGC 6397's neutron star. Overall, this research enhances comprehension of $f(R,L_m,T)$ gravity's effects on compact star internal structures, offering insights for future investigations.

gr-qc↗

arXiv2024

Normal oscillation modes and radial stability of neutron stars with a dark-energy core from the Chaplygin gas

As a potential candidate for the late-time accelerating expansion of the Universe, the Chaplygin gas and its generalized models have significant implications to modern cosmology. In this work we investigate the effects of dark energy on the internal structure of a neutron star composed of two phases, which leads us to wonder: Do stable neutron stars have a dark-energy core? To address this question, we focus on the radial stability of stellar configurations composed by a dark-energy core -- described by a Chaplygin-type equation of state (EoS) -- and an ordinary-matter external layer which is described by a polytropic EoS. We examine the impact of the rate of energy densities at the phase-splitting surface, defined as $α= ρ_{\rm dis}^-/ρ_{\rm dis}^+$, on the radius, total gravitational mass and oscillation spectrum. The resulting mass-radius diagrams are notably different from dark energy stars without a common-matter crust. Specifically, it is found that both the mass and the radius of the maximum-mass configuration decrease as $α$ becomes smaller. Furthermore, our theoretical predictions for mass-radius relations consistently describe the observational measurements of different massive millisecond pulsars as well as the central compact object within the supernova remnant HESS J1731-347. The analysis of the normal oscillation modes reveals that there are two regions of instability on the $M(ρ_c)$ curve when $α$ is small enough indicating that the usual stability criterion $dM/dρ_c>0$ still holds for rapid phase transitions. However, this is no longer true for the case of slow transitions.

astro-ph.HE↗

arXiv2023

Effects of anisotropic pressure on interacting quark star structure

Perturbative Quantum Chromodynamics (pQCD) corrections and color superconductivity predict that strongly interacting matter can reveal new physical phenomena under extreme conditions. Taking into account these interaction effects, we investigate the role of anisotropic pressure in quark stars composed of interacting quark matter. Adopting two physically well-motivated anisotropy profiles, we numerically solve the stellar structure equations in order to explore the consequences of anisotropic pressure on various macroscopic properties such as radius, gravitational mass, surface redshift, moment of inertia, tidal Love number and oscillation spectrum. Remarkably, for both anisotropy models, negative anisotropies increase the radial stability of interacting quark stars, while the opposite occurs for positive anisotropies. However, for the Bowers-Liang profile, the central density corresponding to the maximum-mass point does not coincide with the central density where the squared oscillation frequency vanishes, indicating that the existence of stable anisotropic interacting quark stars is possible beyond the maximum mass for negative anisotropies. Additionally, we compare our theoretical predictions with several observational mass-radius measurements and tidal deformability constraints, which suggest that both strong interaction effects and anisotropy effects play a crucial role in describing compact stars observed in the Universe.

gr-qc↗

arXiv2023

Compact stars in scalar-tensor theories with a single-well potential and the corresponding $f(R)$ theory

The macroscopic properties of compact stars in modified gravity theories can be significantly different from the general relativistic (GR) predictions. Within the gravitational context of scalar-tensor theories, with a scalar field $ϕ$ and coupling function $Φ(ϕ)= \exp[2ϕ/\sqrt{3}]$, we investigate the hydrostatic equilibrium structure of neutron stars for the simple potential $V(ϕ)= ωϕ^2/2$ defined in the Einstein frame (EF). From the scalar field in the EF, we also interpret such theories as $f(R)$ gravity in the corresponding Jordan frame (JF). The mass-radius relations, proper mass, and binding energy are obtained for a polytropic equation of state (EoS) in the JF. Our results reveal that the maximum-mass values increase substantially as $ω$ gets smaller, while the radius and mass decrease in the low-central-density region as we move further away from the pure GR scenario. Furthermore, a cusp is formed when the binding energy is plotted as a function of the proper mass, which indicates the appearance of instability. Specifically, we find that the central-density value where the binding energy is a minimum corresponds precisely to $dM/dρ_c^J = 0$ on the $M(ρ_c^J)$-curve.

gr-qc↗

arXiv2023

Relativistic structure of charged quark stars in energy-momentum squared gravity

Within the context of energy-momentum squared gravity (EMSG), where non-linear matter contributions appear in the gravitational action, we derive the modified TOV equations describing the hydrostatic equilibrium of charged compact stars. We adopt two different choices for the matter Lagrangian density ($\mathcal{L}_m= p$ versus $\mathcal{L}_m= -ρ$) and investigate the impact of each one on stellar structure. Furthermore, considering a charge profile where the electric charge density $ρ_{\rm ch}$ is proportional to the standard energy density $ρ$, we solve numerically the stellar structure equations in order to obtain the mass-radius diagrams for the MIT bag model equation of state (EoS). For $\mathcal{L}_m= p$ and given a specific value of $β$ (including the uncharged case when $β= 0$), the maximum-mass values increase (decrease) substantially as the gravity model parameter $α$ becomes more negative (positive). However, for uncharged configurations and considering $\mathcal{L}_m= -ρ$, our numerical results reveal that when we increase $α$ (from a negative value) the maximum mass first increases and after reaching a maximum value it starts to decrease. Remarkably, this makes it a less trivial behavior than that caused by the first choice when we take into account the presence of electric charge ($β\neq 0$).

gr-qc↗

arXiv2023

Fluid pulsation modes and tidal deformability of anisotropic strange stars in light of the GW$170817$ event

The effects of the anisotropy on the fluid pulsation modes adopting the so-called Cowling approximation and tidal deformability of strange quark stars are investigated by using the numerical integration of the hydrostatic equilibrium, nonradial oscillations, and tidal deformability equations, being these equations modified from their standard form to include the anisotropic effects. The fluid matter inside the compact stars is described by the MIT bag model equation of state. For the anisotropy profile, we consider a local anisotropy that is both regular at the center and null at the star's surface. We find that the effect of the anisotropy is reflected in the fluid pulsation modes and tidal deformability. Finally, we analyze the correlation between the tidal deformability of the GW$170817$ event with the anisotropy.

astro-ph.HE↗

arXiv2023

Radial pulsations, moment of inertia and tidal deformability of dark energy stars

We construct dark energy stars with Chaplygin-type equation of state (EoS) in the presence of anisotropic pressure within the framework of Einstein gravity. From the classification established by Iyer et al. [Class. Quantum Grav. 2, 219 (1985)], we discuss the possible existence of isotropic dark energy stars as compact objects. However, there is the possibility of constructing ultra-compact stars for sufficiently large anisotropies. We investigate the stellar stability against radial oscillations, and we also determine the moment of inertia and tidal deformability of these stars. We find that the usual static criterion for radial stability $dM/dρ_c >0$ still holds for dark energy stars since the squared frequency of the fundamental pulsation mode vanishes at the critical central density corresponding to the maximum-mass configuration. The dependence of the tidal Love number on the anisotropy parameter $α$ is also examined. We show that the surface gravitational redshift, moment of inertia and dimensionless tidal deformability undergo significant changes due to anisotropic pressure, primarily in the high-mass region. Furthermore, in light of the detection of gravitational waves GW190814, we explore the possibility of describing the secondary component of such event as a stable dark energy star in the presence of anisotropy.

gr-qc↗

arXiv2023

Moment of inertia of slowly rotating anisotropic neutron stars in $f(R,T)$ gravity

Within the framework of $f(R,T)$ theories of gravity, we investigate the hydrostatic equilibrium of anisotropic neutron stars with a physically relevant equation of state (EoS) for the radial pressure. In particular, we focus on the $f(R,T) = R+ 2βT$ model, where $β$ is a minimal coupling constant. In the slowly rotating approximation, we derive the modified TOV equations and the expression for the relativistic moment of inertia. The main properties of neutron stars, such as radius, mass and moment of inertia, are studied in detail. Our results revel that the main consequence of the $2βT$ term is a substantial increase in the surface radius for low enough central densities. Nevertheless, such a term slightly modifies the total gravitational mass and moment of inertia of the slowly rotating stars. Furthermore, the changes are noticeable when anisotropy is incorporated into the stellar fluid, and it is possible to obtain higher masses that are consistent with the current observational data.

gr-qc↗

arXiv2022

Charged quark stars in metric $f(R)$ gravity

We provide the modified TOV equations for the hydrostatic equilibrium of charged compact stars within the metric $f(R)$ gravitational background. We adopt the MIT bag model EoS for the dense matter and assume a charge distribution where the electric charge density $ρ_{\rm ch}$ is proportional to the standard energy density $ρ$. Using the Starobinsky model, we explore the role of the $αR^2$ term, where $α$ is a free constant and $R$ the Ricci scalar, on the global properties of charged stars such as radius, mass and total charge. We present the dependence of the structure of the star for several values of $α$ and for different values of the constant parameter $β\equiv ρ_{\rm ch}/ρ$. Remarkably, we find that the radius decreases with respect to its GR value for low central densities, while the opposite occurs in the high-central-density region. The mass measured at the surface always decreases and the maximum-total charge undergoes a substantial increase as the parameter $α$ increases. We also illustrate the variations of the asymptotic mass as a consequence of the electric charge and the extra quadratic term.

gr-qc↗

arXiv2022

Charged quark stars in $f(R,T)$ gravity

Recent advances in nuclear theory combined with new astrophysical observations have led to the need for specific theoretical models that actually apply to phenomena on dense-matter physics. At the same time, quantum chromodynamics (QCD) predicts the existence of non-nucleonic degrees of freedom at high densities in neutron-star matter, such as quark matter. Within a confining quark matter model, which consists of homogeneous, neutral 3-flavor interacting quark matter with $\mathcal{O}(m_s^4)$ corrections, we study the structure of compact stars made of a charged perfect fluid in the context of $f(R,T)$ gravity. The system of differential equations that describe the structure of charged compact stars have been derived and solved numerically for a gravity model with $f(R,T)= R+ 2βT$. For simplicity, we assume that the charge density is proportional to the energy density, namely, $ρ_{\rm ch} = αρ$. It is demonstrated that matter-geometry coupling constant $β$ and the charge parameter $α$ affect the total gravitational mass and the radius of the star.

gr-qc↗

arXiv2022

Anisotropic quark stars in $f(R)= R^{1+ε}$ gravity

Within the metric formalism of $f(R)$ theories of gravity, where $R$ is the Ricci scalar, we study the hydrostatic equilibrium structure of compact stars with the inclusion of anisotropic pressure. In particular, we focus on the $f(R)= R^{1+ε}$ model and we examine small deviations from General Relativity (GR) for $\vert ε\vert \ll 1$. A suitable definition of mass function is explicitly formulated from the field equations and the value of the Ricci scalar at the center of each star is chosen such that it satisfies the asymptotic flatness requirement. We find that both the mass and the radius of a compact star are larger with respect to the general relativistic counterpart. Furthermore, we remark that the substantial changes due to anisotropy occur mainly in the high-central-density region.

gr-qc↗

arXiv2022

$R^2$-gravity quark stars from perturbative QCD

We investigate the structure of quark stars in the framework of $f(R)= R+ αR^2$ gravity using an equation of state for cold quark matter obtained from perturbative QCD, parametrized only by the renormalization scale. We show that a considerably large range of the free parameter $α$, within and even beyond the constraints previously reported in the literature, yield non-negligible modifications in the mass and radius of stars with large central mass densities. Besides, their stability against baryon evaporation is analyzed through the behavior of the associated total binding energies for which we show that these energies are slightly affected by the modified gravity term in the regime of high proper (baryon) masses.

gr-qc↗

arXiv2022

Electrically Charged Quark Stars in $4D$ Einstein-Gauss-Bonnet Gravity

In this work we study the properties of compact spheres made of a charged perfect fluid with a MIT bag model EoS for quark matter. Considering static spherically symmetric spacetime we derive the hydrostatic equilibrium equations in the recently formulated four dimensional Einstein-Gauss-Bonnet ($4D$ EGB) gravity theory. In this setting, the modified TOV equations are solved numerically with the aim to investigate the impact of electric charge on the stellar structure. A nice feature of $4D$ EGB theory is that the Gauss-Bonnet term has a non-vanishing contribution to the gravitational dynamics in $4D$ spacetime. We therefore analyse the effects of Gauss-Bonnet coupling constant $α$ and the charge fraction $β$ on the mass-radius ($M-R$) diagram and also the mass-central density $(M-ρ_c)$ relation of quark stars. Finally, we conclude that depending on the choice of coupling constant one could have larger mass and radius compared with GR and can also be relevant for more massive compact objects due to the effect of the repulsive Coulomb force.

gr-qc↗

arXiv2021

Neutron stars in $f(R,T)$ gravity with conserved energy-momentum tensor: Hydrostatic equilibrium and asteroseismology

We investigate the equilibrium and radial stability of spherically symmetric relativistic stars, considering a polytropic equation of state (EoS), within the framework of $f(R,T)$ gravity with a conservative energy-momentum tensor. Both modified stellar structure equations and Chandrasekhar's pulsation equations are derived for the $f(R,T)= R+ h(T)$ gravity model, where the function $h(T)$ assumes a specific form in order to safeguard the conservation equation for the energy-momentum tensor. The neutron star properties, such as radius, mass, binding energy and oscillation spectrum are studied in detail. Our results show that a cusp -- which signals the appearance of instability -- is formed when the binding energy is plotted as a function of the compact star proper mass. We find that the squared frequency of the fundamental vibration mode passes through zero at the central-density value corresponding to such a cusp where the binding energy is a minimum.

gr-qc↗

arXiv2021

Radial oscillations and stability of compact stars in $f(R, T) = R+ 2βT$ gravity

We examine the static structure configurations and radial stability of compact stars within the context of $f(R, T)$ gravity, with $R$ and $T$ standing for the Ricci scalar and trace of the energy-momentum tensor, respectively. Considering the $f(R, T)=R+2βT$ functional form, with $β$ being a constant, we derive the corresponding hydrostatic equilibrium equation and the modified Chandrasekhar's pulsation equation. The mass-radius relations and radial mode frequencies are obtained for some realistic equations of state. Our results show that the traditional stellar stability criteria, namely, the necessary condition $dM/dρ_c >0$ and sufficient condition $ω^2 >0$, still hold in this theory of gravity.

gr-qc↗

arXiv2020

Strongest constraint in $f(R) = R+ αR^2$ gravity: stellar stability

In the metric approach of $f(R)$ theories of gravity, the fourth-order field equations are often recast as effective Einstein equations in the presence of standard matter and a curvature fluid (which gathers all the extra terms), always in the Jordan frame. In this picture, we investigate the strong gravity regime of the $f(R) = R+ αR^2$ model. In particular, we focus on the stability of a compact star composed by a mixture of ordinary matter -- described by a polytropic equation of state -- and an effective curvature fluid in an otherwise standard Einstein gravity, so that we are able to apply the usual equations that govern the radial adiabatic oscillations of relativistic stars. Our new restriction on the free parameter is $α\lesssim 2.4 \times 10^8\ \text{cm}^2$ in order to guarantee stellar stability, about $100$ times more restrictive than previous results (based on mass-radius relations alone) in the literature.

gr-qc↗

arXiv2020

Equilibrium, radial stability and non-adiabatic gravitational collapse of anisotropic neutron stars

In this work we construct families of anisotropic neutron stars for an equation of state compatible with the constraints of the gravitational-wave event GW170817 and for four anisotropy ansatze. Such stars are subjected to a radial perturbation in order to study their stability against radial oscillations and we develop a dynamical model to describe the non-adiabatic gravitational collapse of the unstable anisotropic configurations whose ultimate fate is the formation of a black hole. We find that the standard criterion for radial stability $dM/dρ_c >0$ is not always compatible with the calculation of the oscillation frequencies for some anisotropy ansatze, and each anisotropy parameter is constrained taking into account the recent restriction of maximum mass of neutron stars. We further generalize the TOV equations within a non-adiabatic context and we investigate the dynamical behaviour of the equation of state, heat flux, anisotropy factor and mass function as an unstable anisotropic star collapses. After obtaining the evolution equations we recover, as a static limit, the background equations.

gr-qc↗