Hybrid Stars Research Articles

Hybrid star model with Tolman–Buchdahl metric potentials in non-conservative theory of gravity

In this study, we propose a streamlined approach to analyze the characteristics of compact stars with a unique equation of state resulting from the hybridization of dark matter and baryonic matter within the framework of Rastall gravity. The impact of dark energy on the star’s interior is investigated by introducing a linear coupling parameter β (called the Rastall parameter). The interior space–time model is developed using metric potentials eν=A+Br2 and eλ(r)=a(1+br2)a−br2 for temporal and spatial coordinates, respectively. The parameter b is set to 0.005 km−2 for the specific stellar candidate PSRJ1614-2230. Physical features derived from the exact solutions of the Einstein field equations are analyzed by considering various choices of the Rastall parameter. The results indicate that the proposed model satisfies the necessary conditions for a stable and physically plausible stellar structure.

Hybrid star properties with the NJL and mean field approximation of QCD models: A Bayesian approach

Hybrid star properties with the NJL and mean field approximation of QCD models: A Bayesian approach

Role of local anisotropy in hybrid stars

Using the Bower–Liang model, we discuss how pressure anisotropies affect the microscopic and macroscopic properties of hybrid stars. We find that anisotropies affect the maximum mass, central density, and radius of the canonical stars. Anisotropies also affect the minimum neutron star mass that presents quarks in their core, as well as the total amount of quarks for the maximally massive stars. We also confront our results with standard constraints, such as the radius and the tidal parameter of the canonical star, as well as the mass and radius of the PSR J0740+6620 pulsar. We observe that moderate values for anisotropies could fulfill these constraints simultaneously. On the other hand, within more extreme degrees of anisotropies, more speculative constraints such as black widow pulsars PSR J0952-0607 and the mass-gap object in the GW190814 event can be explained as hybrid stars. We also investigate the role of anisotropies in the neutron stars’ moment of inertia.

Modeling and analyzing stability of hybrid stars within [formula omitted] gravity

This study uses the Krori–Barua type metric to represent hybrid stars within the f(Q) theory of gravity. We postulate that the hybrid star also contains strange quark matter in addition to regular baryonic matter. To investigate the physical viability of the hybrid star model, we present the graphical behavior of the energy density, radial pressure, and tangential pressure, equation of state parameters, anisotropy, and stability analysis, respectively, by choosing the compact star EXO 1785-248 with a mass of 1.3−0.2+0.2M⊙ and radius 8.849−0.4+0.4 km with five different values of the coupling constants as a=2, a=4, a=6, a=8, and a=10. The maximum allowed masses and corresponding radii have been calculated using the M−R curve for three different coupling parameter ’a’ values which match the observational data of three distinct compact stars, namely LMC X-4, SMC X-1, and 4U 1538-52. So, the f(Q) theory of gravity can provide results that are plausible for describing the macroscopical characteristics of hybrid star candidates.

Formation of a magnetized hybrid star with a purely toroidal field from phase-transition-induced collapse

ABSTRACT Strongly magnetized neutron stars are popular candidates for producing detectable electromagnetic and gravitational-wave signals. Gravitational collapses of neutron stars triggered by a phase transition from hadrons to deconfined quarks in the cores could also release a considerable amount of energy in the form of gravitational waves and neutrinos. Hence, the formation of a magnetized hybrid star from such a phase-transition-induced collapse is an interesting scenario for detecting all these signals. These detections may provide essential probes for the magnetic field and composition of such stars. Thus far, a dynamical study of the formation of a magnetized hybrid star from a phase-transition-induced collapse has yet to be realized. Here, we investigate the formation of a magnetized hybrid star with a purely toroidal field and its properties through dynamical simulations. We find that the maximum values of rest-mass density and magnetic field strength increase slightly and these two quantities are coupled in phase during the formation. We then demonstrate that all microscopic and macroscopic quantities of the resulting hybrid star vary drastically when the maximum magnetic field strength goes beyond a threshold of $\sim 5 \times 10^{17}$ G, but they are insensitive to the magnetic field below this threshold. Specifically, the magnetic deformation makes the rest-mass density drop significantly, suppressing the matter fraction in the mixed phase. These behaviours agree with those in the equilibrium models of previous studies. Therefore, this work provides a solid support for the magnetic effects on a hybrid star.

Effect of dark matter interaction on hybrid star in the light of the recent astrophysical observations

We have explored the effect of dark matter interaction on hybrid star (HS) in the light of recent astrophysical observational constraints. The presence of dark matter is assumed to be there in both the hadron as well as the quark sector. The dark matter particle interacts with both hadron and quark matter through the exchange of a scalar as well as a vector meson. The equation of state (EOS) of the hadron part is computed using the NL3 version of the relativistic mean field(RMF) model, whereas the quark part is taken care of using the well-known MIT Bag model with the vector interaction. We investigate the effect of the dark matter density and the mass of the dark matter particle on various observables like mass, radius, tidal deformability of the dark matter admixed hybrid star(DMAHS).In this study, we have noted an intriguing aspect that is the speed of sound in the DMAHS is insensitive to both the mass as well as the density of dark matter. We also observe a striking similarity in the variation of transition mass and its corresponding radius, as well as the maximum mass of neutron stars, with dark matter density and mass. We employ observational constraints from neutron stars to narrow down the allowed range of the parameters of dark matter.

Love–C relations for elastic hybrid stars

Love–C relations for elastic hybrid stars

Recent progresses in strange quark stars

According to the hypothesis that strange quark matter may be the true ground state of matter at extremely high densities, strange quark stars should be stable and could exist in the Universe. It is possible that pulsars may actually be strange stars, but not neutron stars. Here we present a short review on recent progresses in the field of strange quark stars. First, three popular phenomenological models widely used to describe strange quark matter are introduced, with special attention being paid on the corresponding equation of state in each model. Combining the equation of state with the Tolman-Oppenheimer-Volkov equations, the inner structure and mass-radius relation can be obtained for the whole sequence of strange stars. Tidal deformability and oscillations (both radial and non-radial oscillations), which are sensitive to the composition and the equations of state, are then described. Hybrid stars as a special kind of quark stars are discussed. Several other interesting aspects of strange stars are also included. For example, strong gravitational wave emissions may be generated by strange stars through various mechanisms, which may help identify strange stars via observations. Especially, close-in strange quark planets with respect to their hosts may provide a unique test for the existence of strange quark objects. Fierce electromagnetic bursts could also be generated by strange stars. The energy may come from the phase transition of neutron stars to strange stars, or from the merger of binary strange stars. The collapse of the strange star crust can also release a huge amount of energy. It is shown that strange quark stars may be involved in short gamma-ray bursts and fast radio bursts.

Could a slow stable hybrid star explain the central compact object in HESS J1731-347?

Could a slow stable hybrid star explain the central compact object in HESS J1731-347?

Constraining the Abundance of Spinning Deformed Galactic Compact Objects with Continuous Gravitational Waves

Galactic spinning compact objects (COs) with nonzero ellipticity are expected to be sources of continuous gravitational waves (CGWs). Certain classes of hypothetical COs, such as neutron stars with quark cores (hybrid stars) and quark stars, are thought to be capable of sustaining large ellipticities from theoretical considerations. Such exotic COs (eCOs) with large ellipticities and spins should produce CGWs detectable by the current LIGO-Virgo-KAGRA GW detector network. Since no detections for CGWs from searches in LIGO-Virgo data have so far been reported, we place constraints on the abundance of highly elliptical, rapidly spinning eCOs in our Galaxy. We formulate a Bayesian framework to place upper limits on the number count N tot of highly deformed Galactic eCOs. We divide our constraints into two classes: an “agnostic” set of upper limits on N tot evaluated on a CGW frequency and ellipticity grid that depend only on the choice of spatial distribution of COs; and a model-dependent set that additionally assumes prior information on the distribution of frequencies. We find that COs with ellipticities ϵ ≳ 10−5 have abundance upper limits at 90% confidence, of Ntot90%≲100 , and those with ϵ ≳ 10−6 have Ntot90%≲104 . We additionally place upper limits on the ellipticity of Galactic COs informed by our choices of spatial distributions, given different abundances N tot.

Hadron-quark phase transition in the neutron star with vector MIT bag model and Korea-IBS-Daegu-SKKU functional

Employing the Korea-IBS-Daegu-SKKU (KIDS) density functional for the hadron phase and the MIT bag model with vector (vBag) model for the quark phase, we obtain hadron-quark phase transition in neutron stars considering Maxwell construction. The structural properties of the resultant hybrid stars are computed for three different values of bag constant (B) in the range B1/4=145−160 MeV). We study the effects of symmetry energy (J) on the hybrid star properties with the different KIDS model and found that J has important influence not only on the transition properties like the transition mass, transition radius and jump in density due to phase transition, but also on the stability of the hybrid stars. The vector repulsion of the quark phase via the parameter GV has profound influence in obtaining reasonable hybrid star configurations, consistent with the recent astrophysical constraints on the structural properties of compact stars. Within the aforesaid range of B, the value of GV is constrained to be 0.3 ≲GV≲ 0.4 in order to obtain reasonable hybrid star configurations.

Physically viable solutions of anisotropic hybrid stars in f(T) gravity: an embedding approach

Physically viable solutions of anisotropic hybrid stars in f(T) gravity: an embedding approach

Analysis of MW-Level Offshore Wind Turbine Generators with Dual Star–Delta Fractional-Slot Windings

This paper investigates the use of fractional-slot concentrated windings (FSCWs) in large-scale (MW level) offshore wind generators. It focuses specifically on a power rating of 3 MW and uses an existing direct-drive synchronous PM machine (DD-SPM) with 480s/160p and dual three-phase integer-slot winding (ISW) as a baseline. A multiple of the common 12s/10p FSCW machine is used that matches the electrical frequency of the ISW machine, yielding a 192s/160p dual three-phase machine. The hybrid star–delta connection has grown increasingly popular owing to its unique harmonic cancellation properties, which can help reduce rotor and PM eddy current losses in FSCW machines. In this paper, two dual three-phase star–delta-wound machines are scaled to 3 MW and included in the investigation. Specifically, a 384s/160p dual three-phase and dual star–delta winding machine, which is a multiplication of the 24s/10p machine, and a 192s/176p dual three-phase and dual star–delta winding machine, which is a multiplication of the 24s/22p machine, are used. These machines are investigated using finite element analysis (FEA) and compared on the basis of their air-gap flux density harmonics, open-circuit electro-motive force (EMF), torque performance, and losses and power. It is found that the proposed 384s/160p dual star–delta winding machine has the best electromagnetic performance of all, with a stator power that is 1.2% greater than that of the baseline ISW machine. However, this machine has a coil pitch of 2 and so loses the manufacturing and fault-tolerant advantage of having concentrated windings. If concentrated windings are desired, then the proposed 192s/176p dual star–delta winding machine is the best choice, with the stator power only 2.6% less than that of the baseline ISW machine, but unfortunately still has significant rotor and PM eddy current losses.

Hybrid Star Models in the Light of New Multimessenger Data

Recent astrophysical mass inferences of compact stars HESS J1731-347 and PSR J0952-0607, with extremely small and large masses respectively, as well as the measurement of the neutron skin of Ca in the CREX experiment challenge and constrain the models of dense matter. We examine the concept of hybrid stars—objects containing quark cores surrounded by nucleonic envelopes—as models that account for these new data along with other inferences. We employ a family of 81 nucleonic equations of state (EOSs) with variable skewness and slope of symmetry energy at saturation density and a constant speed-of-sound EOS for quark matter. For each nucleonic EOS, a family of hybrid EOSs is generated by varying the transition density, the energy jump, and the speed of sound. These models are tested against the data from GW170817 and J1731-347, which favor low-density soft EOS and J0592-0607 and J0740+6620, which require high-density stiff EOS. The addition of J0592-0607's mass measurement to the constraints has no significant impact on the parameter space of the admissible EOS, but allows us to explore the potential effect of pulsars more massive than J0740+6620, if such exists. We then examine the occurrence of twin configurations and quantify the ranges of masses and radii that they can possess. It is shown that including J1731-347 data favors EOSs that predict low-mass twins with M ≲ 1.3 M ⊙ that can be realized if the deconfinement transition density is low. If combined with large speed of sound in quark matter such models allow for maximum masses of hybrid stars in 2.0–2.6 M ⊙.

Radial oscillations of hybrid stars and neutron stars including delta baryons: the effect of a slow quark phase transition

We study radial oscillations of hybrid neutron stars composed of hadronic external layers followed by a quark matter core. We employ a density-dependent relativistic mean-field model including hyperons and Δ baryons to describe hadronic matter, and a density-dependent quark model for quark matter. We obtain the ten lowest eigenfrequencies and the corresponding oscillation functions of N, N+Δ, N+H, and N+H+Δ equations-of-state with a phase transition to the quark matter at 1.4 and 1.8 M ⊙, focusing on the effects of a slow phase transition at the hadron-quark interface. We observe that the maximum mass is reached before the fundamental mode's frequency vanishes for slow phase transitions, suggesting that some stellar configurations with higher central densities than the maximum mass remain stable even when they undergo small radial perturbations. Future gravitational wave detectors and multi-messenger astronomy, complemented by robust microscopic models enabling exploration of various neutron star compositions, including hyperon content, are anticipated to impose precise limitations on the equation of state of baryonic matter under high-density conditions.

Correlation between the symmetry energy slope and the deconfinement phase transition

We study how the nuclear symmetry energy slope (L) can affect the hadron-quark phase transition and neutron star properties. We show that the main physical quantities as the critical chemical potential and pressure are strongly influenced by the symmetry energy slope. In extreme cases, the total amount of deconfined quarks can reach up to 99% of the hybrid star mass. Published by the American Physical Society 2024

Radial and nonradial oscillations of inverted hybrid stars

Radial and nonradial oscillations of inverted hybrid stars

Hybrid stars in light of the HESS J1731-347 remnant and the PREX-II experiment

Hybrid stars in light of the HESS J1731-347 remnant and the PREX-II experiment

Effects of a phase transition in hybrid stars from quark-meson coupling hadronic matter to deconfined quark matter

Effects of a phase transition in hybrid stars from quark-meson coupling hadronic matter to deconfined quark matter

Intrapreneurs, high performers, or hybrid stars? How individual entrepreneurial orientation affects employee performance

Recently, scholars have extended the concept of entrepreneurial orientation to the individual level. Yet, how individual entrepreneurial orientation (IEO) contributes to employee performance is not well understood. Building on role theory, we develop a novel typology of employee performance that distinguishes between employee behavior aimed at achieving in-role performance and employee engagement in intrapreneurial activities. To test how IEO affects the different performance prototypes of this typology, we collected survey and archival data on IEO, entrepreneurial self-efficacy (ESE), employee in-role performance, and engagement in intrapreneurship in a Dutch subsidiary of an international consultancy firm. Our results show that IEO is positively associated with in-role performance and intrapreneurship. ESE is also positively associated with in-role performance and intrapreneurship, albeit indirectly via IEO. These results mark an initial step in unraveling the impact of IEO on employee and firm performance, as well as why different outcomes occur as a result of IEO.