Showing papers on "Quark star published in 2020"

Anisotropic strange star with Tolman–Kuchowicz metric under f ( R , T ) gravity

Abstract: In the current article, we study anisotropic spherically symmetric strange star under the background of f(R, T) gravity using the metric potentials of Tolman–Kuchowicz type (Tolman in Phys Rev 55:364, 1939; Kuchowicz in Acta Phys Pol 33:541, 1968) as $$\lambda (r)=\ln (1+ar^2+br^4)$$ and $$ u (r)=Br^2+2\ln C$$ which are free from singularity, satisfy stability criteria and also well-behaved. We calculate the value of constants a, b, B and C using matching conditions and the observed values of the masses and radii of known samples. To describe the strange quark matter (SQM) distribution, here we have used the phenomenological MIT bag model equation of state (EOS) where the density profile ($$\rho $$) is related to the radial pressure ($$p_r$$) as $$p_r(r)=\frac{1}{3}(\rho -4B_g)$$. Here quark pressure is responsible for generation of bag constant $$B_g$$. Motivation behind this study lies in finding out a non-singular physically acceptable solution having various properties of strange stars. The model shows consistency with various energy conditions, TOV equation, Herrera’s cracking condition and also with Harrison–Zel$$'$$dovich–Novikov’s static stability criteria. Numerical values of EOS parameter and the adiabatic index also enhance the acceptability of our model.

Hybrid and quark star matter based on a nonperturbative equation of state

Abstract: With the recent dawn of the multimessenger astronomy era a new window has opened to explore the constituents of matter and their interactions under extreme conditions. One of the pending challenges of modern physics is to probe the microscopic equation of state (EoS) of cold and dense matter via macroscopic neutron star observations such as their masses and radii. Still unanswered issues concern the detailed composition of matter in the core of neutron stars at high pressure and the possible presence of, e.g., hyperons or quarks. By means of a nonperturbative functional renormalization group approach the influence of quantum and density fluctuations on the quark matter EoS in $\ensuremath{\beta}$-equilibrium is investigated within two- and three-flavor quark-meson model truncations and compared to results obtained with common mean-field approximations where important fluctuations are usually ignored. We find that they strongly impact the quark matter EoS.

Minimally deformed anisotropic dark stars in the framework of gravitational decoupling

Abstract: In this work an analytic fluid sphere built on the well-known Tolman IV space–time is obtained. This toy model is sourced by an imperfect fluid distribution with a dark matter component. The anisotropic behavior is introduced into the system via gravitational decoupling by means of minimal geometric deformation. In this regard, the temporal component of the $$\theta $$-sector has been interpreted as the dark side of the matter distribution. To validate the feasibility of the salient model a detailed graphical analysis is performed, supported by real observational data corresponding to some strange star candidates. Besides, the impacts of minimal geometric deformation approach on the main macro physical observables i.e, the total mass M, compactness factor u and surface gravitational red-shift $$z_{s}$$ are discussed.

Compact Star Physics

Abstract: This self-contained introduction to compact star physics explains important concepts from areas such as general relativity, thermodynamics, statistical mechanics, and nuclear physics Containing many tested exercises, and written by an international expert in the research field, the book provides important insights on the basic concepts of compact stars, discusses white dwarfs, neutron stars, quark stars and exotic compact stars Included are sections on astrophysical observations of compact stars, and present and future terrestrial experiments related to compact stars physics, as the study of exotic nuclei and relativistic heavy-ion collisions Major developments in the field such as the discovery of massive neutron stars, and a discussion of the recent gravitational wave measurement of a neutron star merger are also presented This book is ideal for graduate students and researchers working on the physics of compact stars, general relativity and nuclear physics

Quark Stars in 4D Einstein-Gauss-Bonnet gravity with an Interacting Quark Equation of State.

Abstract: The detection of gravitational waves (GWs) from the binary neutron star (BNS) has opened a new window on the gravitational wave astronomy. With current sensitivities, detectable signals coming from compact objects like neutron stars turn out to be a crucial ingredient for probing their structure, composition, and evolution. Moreover, the astronomical observations on the pulsars and their mass-radius relations put important constraints on the dense matter equation of state (EoS). In this paper, we consider a homogeneous and unpaired charge-neutral $3$-flavor interacting quark matter with $\mathcal{O}(m_s^4)$ corrections that account for the moderately heavy strange quark instead of the naive MIT bag model. In this article, we perform a detailed analysis of strange quark star in the context of recently proposed $4D$ Einstein-Gauss-Bonnet (EGB) theory of gravity. However, this theory does not have standard four-dimensional field equations. Thus, we thoroughly show that the equivalence of the actions in the regularized $4D$ EGB theory and in the original one is satisfied for a spherically symmetric spacetime. We pay particular attention to the possible existence of massive neutron stars of mass compatible with $M \sim 2 M_{\odot}$. Our findings suggest that the fourth-order corrections parameter ($a_4$) of the QCD perturbation and coupling constant $\alpha$ of the GB term play an important role in the mass-radius relation as well as the stability of the quark star. Finally, we compare the results with the well-measured limits of the pulsars and their mass and radius extracted from the spectra of several X-ray compact sources.

Study on Anisotropic Strange Stars in f ( T , T ) Gravity

Abstract: In this work, we study the existence of strange stars in the background of f(T,T) gravity in the Einstein spacetime geometry, where T is the torsion tensor and T is the trace of the energy-momentum tensor. The equations of motion are derived for anisotropic pressure within the spherically symmetric strange star. We explore the physical features like energy conditions, mass-radius relations, modified Tolman–Oppenheimer–Volkoff (TOV) equations, principal of causality, adiabatic index, redshift and stability analysis of our model. These features are realistic and appealing to further investigation of properties of compact objects in f(T,T) gravity as well as their observational signatures.

Comment on “Tidal Love numbers of neutron and self-bound quark stars”

Abstract: We comment on the paper of S. Postnikov et al. [Phys. Rev. D 82, 024016 (2010)] and give a modified formula that needs to be taken into account when calculating the tidal Love number of neutron stars in case a first order phase-transition occurs at nonzero pressure. We show that the error made when using the original formula tends to zero as $p\ensuremath{\rightarrow}0$, and we estimate the maximum relative error to be $\ensuremath{\sim}5%$ if the density discontinuity is at larger densities.

Quark Stars in Massive Brans–Dicke Gravity with Tolman–Kuchowicz Spacetime

Abstract: In this paper, we construct anisotropic model representing salient features of strange stars in the framework of massive Brans–Dicke gravity. We formulate the field equations for Tolman–Kuchowicz ansatz by incorporating the MIT bag model. Junction conditions are applied on the boundary of the stellar model to evaluate the unknown constants in terms of mass and radius of the star. The radius of the strange star candidate PSR J1614-2230 is predicted by assuming maximum anisotropy at the surface of the star for different values of the coupling parameter, mass of the scalar field and bag constant. We examine various properties as well as the viability and stability of the anisotropic sphere. We conclude that the astrophysical model agrees with the essential criteria of a physically realistic model for higher values of the coupling parameter as well as mass of the scalar field.

Why can hadronic stars convert into strange quark stars with larger radii

Abstract: The total binding energy of compact stars is the sum of the gravitational binding energy $(BE{)}_{g}$ and the nuclear binding energy $(BE{)}_{n}$, the last being related to the microphysics of the interactions. While the first is positive (binding) both for hadronic stars and for strange quark stars, the second is large and negative for hadronic stars (antibinding) and either small and negative (antibinding) or positive (binding) for strange quark stars. A hadronic star can convert into a strange quark star with a larger radius because the consequent reduction of $(BE{)}_{g}$ is overcompensated by the large increase in $(BE{)}_{n}$. Thus, the total binding energy increases due to the conversion and the process is exothermic. Depending on the equations of state of hadronic matter and quark matter and on the baryonic mass of the star, the contrary is obviously also possible, namely, the conversion of hadronic stars into strange quark stars having smaller radii, a situation more often discussed in the literature. We provide a condition that is sufficient and in most of the phenomenologically relevant cases also necessary in order to form strange quark stars with larger radii while satisfying the exothermicity request. Finally, we compare the two schemes in which quark stars are produced (one having large quark stars and the other having small quark stars) among themselves and with the third-family scenario and we discuss how present and future data can discriminate among them.

Anisotropic strange stars through embedding technique in massive Brans–Dicke gravity

Abstract: This paper investigates the existence and properties of anisotropic strange quark stars in the context of massive Brans–Dicke theory. The field equations are constructed in Jordan frame by assuming a suitable potential function with MIT bag model. We employ the embedding class-one approach as well as junction conditions to determine the unknown metric functions. Radius of the strange star candidate, LMC X-4, is predicted through its observed mass for different values of the bag constant. We analyze the effects of coupling parameter as well as mass of scalar field on state determinants and execute multiple checks on the stability and viability of the spherical system. It is concluded that the resulting stellar structure is physically viable and stable as it satisfies the energy conditions as well as essential stability criteria.

Anisotropic strange star with Tolman-Kuchowicz metric under $f(R,T)$ gravity

Abstract: In the current article, we study anisotropic spherically symmetric strange star under the background of $f(R,T)$ gravity using the metric potentials of Tolman-Kuchowicz type~\cite{Tolman1939,Kuchowicz1968} as $\lambda(r)=\ln(1+ar^2+br^4)$ and $ u(r)=Br^2+2\ln C$ which are free from singularity, satisfy stability criteria and also well behaved. We calculate the value of constants $a$, $b$, $B$ and $C$ using matching conditions and the observed values of the masses and radii of known samples. To describe the strange quark matter (SQM) distribution, here we have used the phenomenological MIT bag model equation of state (EOS) where the density profile ($\rho$) is related to the radial pressure ($p_r$) as $p_r(r)=\frac{1}{3}(\rho-4B_g)$. Here quark pressure is responsible for generation of bag constant $B_g$. Motivation behind this study lies in finding out a non-singular physically acceptable solution having various properties of strange stars. The model shows consistency with various energy conditions, TOV equation, Herrera's cracking condition and also with Harrison-Zel$'$dovich-Novikov's static stability criteria. Numerical values of EOS parameter and the adiabatic index also enhance the acceptability of our model.

Color-flavor locked quark stars in energy-momentum squared gravity

Abstract: Several attempts have been made in the past decades to search for the true ground state of the dense matter at sufficiently large densities and low temperatures via compact astrophysical objects. Focusing on strange stars, we derive the hydrostatic equilibrium assuming a maximally symmetric phase of homogeneous superconducting quark matter called the \textit{color-flavor-locked} (CFL) phase in the background of energy-momentum squared gravity (EMSG). Theoretical and experimental investigations show that strange quark matter (SQM) in a CFL state can be the true ground state of hadronic matter at least for asymptotic densities, and even if the unequal quark masses. Motivated by these theoretical models, we explore the structure of stellar objects in recently proposed EMSG, which allows a correction term $T_{\mu u}T^{\mu u}$ in the action functional of the theory. Interestingly, EMSG may be effective to resolve the problems at high energy densities, e.g., relevant to the early universe and dense compact astrophysical objects without invoking some new forms of fluid stress, such as bulk viscosity or scalar fields. Finally, we solve the complicated field equations numerically to obtain the mass-radius relations for strange stars in CFL equation of state.

Strange Quark Stars in 4D Einstein-Gauss-Bonnet Gravity

Abstract: The existence of strange matter in compact stars may pose striking sequels of the various physical phenomena. As an alternative to neutron stars, a new class of compact stars called strange stars should exist if the strange matter hypothesis is true. In the present article, we investigate the possible construction of the strange stars in quark matter phases based on the MIT bag model. We consider scenarios in which strange stars have no crusts. Then we apply two types of equations of state to quantify the mass-radius diagram for static strange star models performing the numerical calculation to the modified Tolman-Oppenheimer-Volkoff (TOV) equations in the context of $4D$ Einstein-Gauss-Bonnet gravity. It is worth noting that the GB term gives rise to a non-trivial contribution to the gravitational dynamics in the limit $D \to 4$. However, the claim that the resulting theory is of pure graviton was cast in doubt by several grounds. Thus, we begin our discussion with showing the regularized $4D$ EGB theory has an equivalent action as the novel $4D$ EGB in a spherically symmetric spacetime. We also study the effects of coupling constant $\alpha$ on the physical properties of the constructed strange stars including the compactness and criterion of adiabatic stability. Finally, we compare our results to those obtained from the standard GR.

What if the neutron star maximum mass is beyond ∼2.3 M⊙?

Abstract: By assuming the formation of a black hole soon after the merger event of GW170817, the maximum mass of non-rotating stable neutron star, M_TOV ≃ 2.3 M_⊙, is proposed by numerical relativity, but there is no solid evidence to rule out M_TOV > 2.3 M_⊙ from the point of both microphysical and astrophysical views. It is naturally expected that the equation of state (EOS) would become stiffer beyond a specific density to explain massive pulsars. We consider the possibility of EOSs with M_TOV > 2.3 M_⊙, investigating the stiffness and the transition density in a polytropic model, for two kinds of neutron stars (i.e. gravity-bound and strong-bound stars on surface). Only two parameters are input in both cases: (ρ_t, γ) for gravity-bound neutron stars, while (ρ_s, γ) for strong-bound strange stars, with ρ_t the transition density, ρ_s the surface density, and γ the polytropic exponent. In the matter of M_TOV > 2.3 M_⊙ for the maximum mass and 70 ≤ Λ_1.4 ≤ 580 for the tidal deformability, it is found that the smallest ρ_t and γ should be ∼0.50 ρ_0 and ∼2.65 for neutron stars, respectively, whereas for strange star, we have γ > 1.40 if ρ_s > 1.0 ρ_0 (ρ_0 is the nuclear saturation density). These parametric results could guide further research of the real EOS with any foundation of microphysics if a pulsar mass higher than 2.3 M_⊙ is measured in the future, especially for an essential comparison of allowed parameter space between gravity-bound and strong-bound compact stars.

Tidal deformability and gravitational-wave phase evolution of magnetized compact-star binaries

Abstract: The evolution of the gravitational-wave phase in the signal produced by inspiralling binaries of compact stars is modified by the nonzero deformability of the two stars. Hence, the measurement of these corrections has the potential of providing important information on the equation of state of nuclear matter. Extensive work has been carried out over the last decade to quantify these corrections, but it has so far been restricted to stars with zero intrinsic magnetic fields. While the corrections introduced by the magnetic tension and magnetic pressure are expected to be subdominant, it is nevertheless useful to determine the precise conditions under which these corrections become important. To address this question, we have carried out a second-order perturbative analysis of the tidal deformability of magnetized compact stars under a variety of magnetic-field strengths and equations of state describing either neutron stars or quark stars. Overall, we find that magnetically induced corrections to the tidal deformability will produce changes in the gravitational-wave phase evolution that are unlikely to be detected for a realistic magnetic field i.e., $B\ensuremath{\sim}{10}^{10}--{10}^{12}\text{ }\text{ }\mathrm{G}$. At the same time, if the magnetic field is unrealistically large, i.e., $B\ensuremath{\sim}{10}^{16}\text{ }\text{ }\mathrm{G}$, these corrections would produce a sizeable contribution to the phase evolution, especially for quark stars. In the latter case, and if the neglected higher-order terms will remain negligible also for very high magnetic fields, the induced phase differences would represent a unique tool to measure the properties of the magnetic fields, providing information that is otherwise hard to quantify.

From Neutron and Quark Stars to Black Holes

Abstract: New physics and models for the most compact astronomical objects - neutron / quark stars and black holes are proposed. Under the new supersymmetric mirror models, neutron stars at least heavy ones could be born from hot deconfined quark matter in the core with a mass limit less than $2.5 M_\odot$. Even heavier cores will inevitably collapse into black holes as quark matter with more deconfined quark flavors becomes ever softer during the staged restoration of flavor symmetry. With new understanding of gravity as mean field theories emergent from the underlying quantum theories for providing the smooth background spacetime geometry for quantum particles, the black hole interior can be described well as a perfect fluid of free massless Majorana fermions and gauge bosons under the new genuine 2-d model. In particular, the conformal invariance on a 2-d torus for the black hole gives rise to desired consistent results for the interior microphysics and structures including its temperature, density, and entropy. Conjectures for further studies of the black hole and the early universe are also discussed in the new framework.

GW190814: Circumstantial Evidence for Up-Down Quark Star

Abstract: Within a confining quark matter model which considers phenomenologically the quark confinement and asymptotic freedom as well as the chiral symmetry restoration and quark deconfinement at high baryon density, we find that if the up-down quark matter ($ud$QM) is more stable than nuclear matter and strange quark matter (SQM), the maximum mass of static quark stars with $ud$QM is $2.77M_{\odot}$ under agreement with both the constraints on star tidal deformability from gravitational wave signal GW170817 and the mass-radius of PSR J0030+045 measured simultaneously by NICER. In contrast, the conventional strange quark star with SQM has a maximum static mass of only $2.05M_{\odot}$ and its radius significantly deviates from NICER's constraint. Our results thus provide circumstantial evidence suggesting the recently reported GW190814's secondary component with a mass of $2.59^{+0.08}_{-0.09}M_\odot$ could be an up-down quark star.

Searching for Strange Quark Matter Objects Among White Dwarfs

Abstract: It has long been argued that the ground state of matter may be strange quark matter (SQM), not hadronic matter. A whole sequence of SQM objects, ranging from strange quark stars and strange quark dwarfs to strange quark planets, can stably exist according to this SQM hypothesis. A strange dwarf has a mass similar to that of a normal white dwarf but could harbor an extremely dense SQM core (with a density as large as $\sim \rm 4\times10^{14}\,g\,cm^{-3} $) at the center so that its radius can be correspondingly smaller. In this study, we try to search for strange dwarfs among the observed "white dwarfs" by considering their difference in the mass-radius relation. Eight strange dwarf candidates are identified in this way, whose masses are in the range of $\sim 0.02 M_{\odot}$ -- $0.12 M_{\odot}$, with the radii narrowly distributed in $\sim$ 9,000 km -- 15,000 km. The eight objects are SDSS J165143.45+364647.6, LSPM J0815+1633, LP 240-30, BD+20 5125B, LP 462-12, WD J1257+5428, 2MASS J13453297+4200437, and SDSS J085557.46+053524.5. Comparing with white dwarfs of similar mass, these candidates are obviously smaller in radius. Further observations with large radio/IR/optical telescopes on these interesting candidates are solicited.

Charged anisotropic compact objects obeying Karmarkar condition

Abstract: This research develops a well-established analytical solution of the Einstein-Maxwell field equations. We analyze the behavior of a spherically symmetric and static interior driven by a charged anisotropic matter distribution. The class I methodology is used to close the system of equations and a suitable relation between the anisotropy factor and the electric field is imposed. The inner geometry of this toy model is described using an ansatz for the radial metric potential corresponding to the well-known isotropic Buchdahl space-time. The main properties are explored in order to determine if the obtained model is appropriate to represent a real compact body such as neutron or quark star. {We have fixed the mass and radii using the data of the compact objects} SMC X-1 and LMC X-4. It was found that the electric field and electric charge have magnitudes of the order of $\sim 10^{21}\ [V/cm]$ and $\sim 10^{20}\ [C]$, respectively. The magnitude of the electric field and electric charge depends on the dimensionless parameter $\chi$. To observe these effects on the total mass, mass-radius ratio and surface gravitational red-shift, we computed numerical data for different values of $\chi$.

Anisotropic strange star inspired by Finsler geometry

Abstract: In this paper, we report on a study of the anisotropic strange stars under Finsler geometry. Keeping in mind that Finsler spacetime is not merely a generalization of Riemannian geometry rather the ...

Influence of strong magnetic field on the structure properties of strange quark stars

Abstract: We investigate the thermodynamic properties of strange quark matter under the strong magnetic field in the framework of the MIT bag model in two cases of bag constants. We consider two cases of the magnetic field, the uniform magnetic field and the density-dependent magnetic field to calculate the equation of state of strange quark matter. For the case of density-dependent magnetic field, we use a Gaussian equation with two free parameters $$\beta $$ and $$\theta $$ and use two different sets of the parameters for the magnetic field changes (a slow and a fast decrease of the magnetic field from the center to the surface). Our results show that the energy conditions based on the limitation of the energy-momentum tensor, are satisfied in the corresponding conditions. We also show that the equation of state of strange quark matter becomes stiffer by increasing the magnetic field. In this paper, we also calculate the structure parameters of a pure strange quark star using the equation of state. We investigate the compactification factor (2M/R) and the surface redshift of star in different conditions. The results show that the strange quark star is denser than the neutron star and it is more compact in the presence of the stronger magnetic field. As another result, the compactification factor increases when we use a slow increase of the magnetic field from the surface to the center. Eventually, we compare our results with the observational results for some strange star candidates, and we find that the structure of the strange star candidates is comparable to that of the star in our model.

Quark stars with isotropic matter in Hořava gravity and Einstein–æther theory

Abstract: We study non-rotating and isotropic strange quark stars in Lorentz-violating theories of gravity, and in particular in Hořava gravity and Einstein-aether theory. For quark matter we adopt both linear and non-linear equations of state, corresponding to the MIT bag model and color flavor locked state, respectively. The new structure equations describing hydrostatic equilibrium generalize the usual Tolman–Oppenheimer–Volkoff (TOV) equations of Einstein’s general relativity. A dimensionless parameter $$ u $$ measures the deviation from the standard TOV equations, which are recovered in the limit $$ u \rightarrow 0$$. We compute the mass, the radius as well as the compactness of the stars, and we show graphically the impact of the parameter $$ u $$ on the mass-to-radius profiles for different equations of state describing quark matter. The energy conditions and stability criteria are also considered, and they are all found to be fulfilled.

New class of relativistic anisotropic strange star in Vaidya-Tikekar model

Abstract: A new class of compact cold star with strange matter is obtained in a spheroidal space-time with anisotropic pressure described by Vaidya-Tikekar metric. Considering strange matter equation of state namely $p=\frac{1}{3}(\rho -4B)$ , where $B$ is Bag constant in MIT Bag model, we determine the Mass-Radius relationship for Strange Star in four and higher dimensions using the value of surface density $\rho _{s}=4B$ with the allowed value of $B$ ( $145~\text{MeV} < B^{\frac{1}{4}} <164.4~\text{MeV}$ or equivalently $57.55~\text{MeV}/\text{fm}^{3} < B < 95.11~\text{MeV}/\text{fm}^{3}$ ) with respect to neutron for stable strange matter with zero pressure condition. We found that for a constant $B$ , a limiting value of the radius ( $b$ ) of the star for which geometrical parameter $R$ is real within the allowed range of $B$ as mentioned above. We further note that in four dimension the compactness (ratio of mass to radius) of the star corresponding to the maximum radius is greater than 0.33 in isotropic case and increases when anisotropy increases. In case of five dimensions, we note that same type of nature with different values of compactness is observed. Causality condition holds good through out the star in this model up to a certain values of radius $b$ for which $R$ is real. The information about Mass-to-Radius ratio leads to the determination of total mass, radius and other physical parameters of the stellar configuration.

A Brief Review of Chiral Chemical Potential and Its Physical Effects

Abstract: Nontrivial topological gluon configuration is one of the remarkable features of the Quantum Chromodynamics (QCD). Due to chiral anomaly, the chiral imbalance between right- and left-hand quarks can be induced by the transition of the nontrivial gluon configurations between different vacuums. In this review, we will introduce the origin of the chiral chemical potential and its physical effects. These include: (1) the chiral imbalance in the presence of strong magnetic and related physical phenomena; (2) the influence of chiral chemical potential on the QCD phase structure; and (3) the effects of chiral chemical potential on quark stars. Moreover, we propose for the first time that quark stars are likely to be a natural laboratory for testing the destruction of strong interaction CP.

Cold quark matter with heavy quarks and the stability of charm stars

Abstract: We study the effects of heavy quarks on the equation of state for cold and dense quark matter obtained from perturbative QCD, yielding observables parametrized only by the renormalization scale. We investigate the thermodynamics of charm quark matter under the constraints of $\ensuremath{\beta}$ equilibrium and electric charge neutrality in a region of densities where perturbative QCD is, in principle, much more reliable. We also analyze the stability of charm stars, which might be realized as a new branch of ultradense hybrid compact stars, and find that such quark stars are unstable under radial oscillations.

Quantum nucleation of up-down quark matter and astrophysical implications

Abstract: Quark matter with only $u$ and $d$ quarks ($ud\mathrm{QM}$) might be the ground state of baryonic matter at large baryon number $Ag{A}_{\mathrm{min}}$. With ${A}_{\mathrm{min}}\ensuremath{\gtrsim}300$, this has no direct conflict with the stability of ordinary nuclei. An intriguing test of this scenario is to look for quantum nucleation of $ud\mathrm{QM}$ inside neutron stars due to their large baryon densities. In this paper, we study the transition rate of cold neutron stars to $ud$ quark stars ($ud\mathrm{QSs}$) and the astrophysical implications, considering the relevant theoretical uncertainties and observational constraints. It turns out that a large portion of parameter space predicts an instantaneous transition, and so the observed neutron stars are mostly $ud\mathrm{QSs}$. We find this possibility still viable under the recent gravitational wave and pulsar observations, although there are debates on its compatibility with some observations that involve some complex structures of quark matter. The tension could be partially relieved in the two-families scenario, where the high-mass stars ($M\ensuremath{\gtrsim}2\text{ }\text{ }{M}_{\ensuremath{\bigodot}}$) are all $ud\mathrm{QSs}$ and the low-mass ones ($M\ensuremath{\sim}1.4\text{ }\text{ }{M}_{\ensuremath{\bigodot}}$) are mostly hadronic stars. In this case, the slow transition of the low-mass hadronic stars points to a very specific class of hadronic models with moderately stiff EOSs, and $ud\mathrm{QM}$ properties are also strongly constrained.

Locating ergostar models in parameter space

Abstract: Recently, we have shown that dynamically stable ergostar solutions (equilibrium neutron stars that contain an ergoregion) with a compressible and causal equation of state exist [A. Tsokaros, M. Ruiz, L. Sun, S. L. Shapiro, and K. Uryū, Phys. Rev. Lett. 123, 231103 (2019)]. These stars are hypermassive, differentially rotating, and highly compact. In this work, we make a systematic study of equilibrium models in order to locate the position of ergostars in parameter space. We adopt four equations of state that differ in the matching density of a maximally stiff core. By constructing a large number of models both with uniform and differential rotation of different degrees, we identify the parameters for which ergostars appear. We find that the most favorable conditions for the appearance of dynamically stable ergostars are a significant finite density close to the surface of the star (i.e., similar to self-bound quark stars) and a small degree of differential rotation.

On quintessence star model and strange star

Abstract: The astronomical observations on the accelerated expansion of the universe generate the possibility that the internal matter of the stars is not only formed by ordinary matter but also by matter with negative pressure. We discuss the existence of stars formed by the coexistence of two types of fluids, one associated to quintessence dark matter described by the radial and tangential pressures $$(P_{rq},P_{tq})$$ and the density $$\rho _{q}$$ characterized by a parameter $$-1

Structure of Quark Star: A Comparative Analysis of Bayesian Inference and Neural Network Based Modeling

Abstract: In this work, we compare two powerful parameter estimation methods namely Bayesian inference and Neural Network based learning to study the quark matter equation of state with constant speed of sound parametrization and the structure of the quark stars within the two-family scenario. We use the mass and radius estimations from several X-ray sources and also the mass and tidal deformability measurements from gravitational wave events to constrain the parameters of our model. The results found from the two methods are consistent. The predicted speed of sound is compatible with the conformal limit.