Hybrid Stars Research Articles

Fate of twin stars on the unstable branch: Implications for the formation of twin stars

Hybrid hadron-quark equations of state that give rise a third family of stable compact stars have been shown to be compatible with the LIGO-Virgo event GW170817. Stable configurations in the third family are called hybrid hadron-quark stars. The equilibrium stable hybrid hadron-quark star branch is separated by the stable neutron star branch with a branch of unstable hybrid hadron-quark stars. The end-state of these unstable configurations has not been studied, yet, and it could have implications for the formation and existence of twin stars -- hybrid stars with the same mass as neutron stars but different radii. We modify existing hybrid hadron-quark equations of state with a first-order phase transition in order to guarantee a well-posed initial value problem of the equations of general relativistic hydrodynamics, and study the dynamics of non-rotating or rotating unstable twin stars via 3-dimensional simulations in full general relativity. We find that unstable twin stars naturally migrate toward the hadronic branch. Before settling into the hadronic regime, these stars undergo (quasi)radial oscillations on a dynamical timescale while the core bounces between the two phases. Our study suggests that it may be difficult to form stable twin stars if the phase transition is sustained over a large jump in energy density, and hence it may be more likely that astrophysical hybrid hadron-quark stars have masses above the twin star regime. We also study the minimum-mass instability for hybrid stars, and find that these configurations do not explode, unlike the minimum-mass instability for neutron stars. Additionally, our results suggest that oscillations between the two Quantum Chromodynamic phases could provide gravitational wave signals associated with such phase transitions in core-collapse supernovae and white dwarf-neutron star mergers.

Electrically charged supermassive twin stars

By assuming that ultra dense hybrid neutron stars are endowed with a distribution of electric charge, we study the corresponding twin star solutions and their properties resulting from a sharp first order transition from confined hadronic to a deconfined quark phase. Two distinct quark matter equations of state with increasing stiffness are considered and the values for the maximum gravitational masses of the hadronic and hybrid twin configurations are obtained for different values of the total electric charge. Interestingly, our calculations indicate that sharp transitions make charged twin hybrid stars more massive than their neutral counterparts, and that the {2},{hbox {M}_{odot }} constraint from PSR J0740+6620 is surpassed for standard values of electric charge and can be considered stable only satisfying partial M/partial epsilon _0 > 0. In particular, our charged stellar models reach masses even higher than the unknown compact object measured in the GW190814 event.

Horndeski fermion–boson stars

We establish the existence of static and spherically symmetric fermion–boson stars, in a low energy effective model of (beyond) Horndeski theories. These stars are in equilibrium, and are composed by a mixing of scalar and fermionic matters that only interact gravitationally one with each other. Properties such as mass, radius, and compactness are studied, highlighting the existence of two families of configurations defined by the parameter c 4. These families have distinctive properties, although in certain limits both are reduced to their counterparts in general relativity (GR). Finally, by assuming the same conditions used in GR, we find the maximum compactness of these hybrid stars and determine that it remains below the so-called Buchdahl’s limit.

Hybrid stars from a constrained equation of state

We determine, within a meta-model, the properties of the nuclear matter equation of state (EoS) that allow for a phase transition to deconfinement matter. It is shown that the properties that define the isoscalar channel are the ones that are affected, in particular, a phase transition implies much larger values of the skewness and kurtosis. The effect of multi-quark interaction channels in the description of the quark phase in hybrid stars is also studied. NS properties, such as the mass and radius of the quark core, show an interplay dependence between the 8-quark vector and the 4-quark isovector-vector interactions. We show that low mass NS, M ~ 1.4M⊙, may already contain a quark core, and satisfy all existing NS observational constraints. We discuss the strangeness content of the quark core and its influence on the speed of sound.

Gravitational decoupling of generalized Horndeski hybrid stars

Gravitational decoupled compact polytropic hybrid stars are here addressed in generalized Horndeski scalar-tensor gravity. Additional physical properties of hybrid stars are scrutinized and discussed in the gravitational decoupling setup. The asymptotic value of the mass function, the compactness, and the effective radius of gravitational decoupled hybrid stars are studied for both cases of a bosonic and a fermionic prevalent core. These quantities are presented and discussed as functions of Horndeski parameters, the decoupling parameter, the adiabatic index, and the polytropic constant. Important corrections to general relativity and generalized Horndeski scalar-tensor gravity, induced by the gravitational decoupling, comply with available observational data. Particular cases involving white dwarfs, boson stellar configurations, neutron stars, and Einstein–Klein–Gordon solutions, formulated in the gravitational decoupling context, are also scrutinized.

Special point "trains" in the M-R diagram of hybrid stars

We present a systematic investigation of the possible locations for the special point (SP), a unique feature of hybrid neutron stars in the massradius diagram. The study is performed within the two-phase approach where the high-density (quark matter) phase is described by the covariant nonlocal Nambu–Jona-Lasinio (nlNJL) model equation of state (EOS) which is shown to be equivalent to a constant-sound-speed (CSS) EOS. For the nuclear matter phase around saturation density different relativistic density functional EOSs are used: DD2p00, its excluded-volume modification DD2p40 and the hypernuclear EOS DD2Y-T. In the present contribution we apply the Maxwell construction scheme for the deconfinement transition and demonstrate that a simultaneous variation of the vector and diquark coupling constants results in the occurrence of SP "trains" which are invariant against changing the nuclear matter EOS. We propose that the SP train corresponding to a variation of the diquark coupling at constant vector coupling is special since it serves as a lower bound for the line of maximum masses and accessible radii of massive hybrid stars.

Core-collapse Supernova Simulations and the Formation of Neutron Stars, Hybrid Stars, and Black Holes

Abstract We investigate observable signatures of a first-order quantum chromodynamics (QCD) phase transition in the context of core-collapse supernovae. To this end, we conduct axially symmetric numerical relativity simulations with multi-energy neutrino transport, using a hadron–quark hybrid equation of state (EOS). We consider four nonrotating progenitor models, whose masses range from 9.6 to 70 M ⊙. We find that the two less-massive progenitor stars (9.6 and 11.2 M ⊙) show a successful explosion, which is driven by the neutrino heating. They do not undergo the QCD phase transition and leave behind a neutron star. As for the more massive progenitor stars (50 and 70 M ⊙), the proto-neutron star (PNS) core enters the phase transition region and experiences the second collapse. Because of a sudden stiffening of the EOS entering to the pure quark matter regime, a strong shock wave is formed and blows off the PNS envelope in the 50 M ⊙ model. Consequently the remnant becomes a quark core surrounded by hadronic matter, leading to the formation of the hybrid star. However, for the 70 M ⊙ model, the shock wave cannot overcome the continuous mass accretion and it readily becomes a black hole. We find that the neutrino and gravitational wave (GW) signals from supernova explosions driven by the hadron–quark phase transition are detectable for the present generation of neutrino and GW detectors. Furthermore, the analysis of the GW detector response reveals unique kHz signatures, which will allow us to distinguish this class of supernova explosions from failed and neutrino-driven explosions.

Hybrid stars with large strange quark cores

The possible existence of hybrid stars is studied using several multiquark interaction channels. The hadronic phase consists of an equation of state (EoS) with presently accepted nuclear matter properties and the quark model is constrained by the vacuum properties of several light mesons. The dependence of several NS properties on the different quark interactions is analyzed. We show that the present constraints from neutron star observations allow for the existence of hybrid stars with large strangeness content and large quark cores.

Locating the special point of hybrid neutron stars

The special point is a feature unique to models of hybrid neutron stars. It represents a location on their mass–radius sequences that is insensitive to the phase transition density. We consider hybrid neutron stars with a core of deconfined quark matter that obeys a constant–sound–speed (CSS) equation of state model and provide a fit formula for the coordinates of the special point as functions of the squared sound speed (cs2) and pressure scale (A) parameters. Using the special point mass as a proxy for the maximum mass of the hybrid stars we derive limits for the CSS model parameters based on the recent NICER constraint on mass and radius of pulsar PSR J0740+6620, 0.36 < Cs min2 < 0.43 and 80 < A[MeV/fm3] < 160. The upper limit for the maximum mass of hybrid stars depends on the upper limit for cs2 so that choosing cs,max2 = 0.6 results in Mmax < 2.7 M⊙, within the mass range of GW190814.

G modes of neutron stars with hadron-to-quark crossover transitions

We perform the first study of the principal core $g$-mode oscillation in hybrid stars containing quark matter, utilizing a crossover model for the hadron-to-quark transition inspired by lattice QCD. The ensuing results are compared with our recent findings of $g$-mode frequencies in hybrid stars with a first-order phase transition using Gibbs constructions. We find that models using Gibbs construction yield $g$-mode amplitudes and the associated gravitational energy radiated that dominate over those of the chosen crossover model owing to the distinct behaviors of the equilibrium and adiabatic sound speeds in the various models. Based on our results, we conclude that were $g$-modes to be detected in upgraded LIGO and Virgo detectors it would indicate a first-order phase transition akin to a Gibbs construction.

Relativistic hybrid stars in light of the NICER PSR J0740+6620 radius measurement

We explore the implications of the recent radius determination of PSR J0740+6620 by the NICER experiment combined with the neutron skin measurement by the PREX-II experiment and the associated inference of the slope of symmetry energy, for the structure of hybrid stars with a strong first-order phase transition from nucleonic to quark matter. We combine a covariant density-functional nucleonic equation of state (EOS) with a constant-speed-of-sound EOS for quark matter. We show that the radius and tidal deformability ranges obtained from GW170817 can be reconciled with the implication of the PREX-II experiment if there is a phase transition to quark matter in the low-mass compact star. In the high-mass segment, the EoS needs to be stiff to comply with the large-radius inference for PSR J0740+6620 and J0030+0451 with masses $M\simeq 2M_{\odot}$ and $M\simeq 1.4M_{\odot}$. We show that twin stars are not excluded, but the mass and radius ranges (with $M \geq M_\odot$) are restricted to narrow domains $\Delta M_{\rm twin} \lesssim 0.05 M_\odot$ and $\Delta R_{\rm twin} \sim 1.0$~km. We also show that the existence of twin configurations is compatible with the light companion in the GW190814 event being a hybrid star in the case of values of the sound-speed square $s=0.6$ and $s=1/3$.

Phase Conversions in Neutron Stars: Implications for Stellar Stability and Gravitational Wave Astrophysics

We review the properties of hybrid stars with a quark matter core and a hadronic mantle, focusing on the role of key micro-physical properties such as the quark/hadron surface and curvature tensions and the conversion speed at the interface between both phases. We summarize the results of works that have determined the surface and curvature tensions from microscopic calculations. If these quantities are large enough, mixed phases are energetically suppressed and the quark core would be separated from the hadronic mantle by a sharp interface. If the conversion speed at the interface is slow, a new class of dynamically stable hybrid objects is possible. Densities tens of times larger than the nuclear saturation density can be attained at the center of these objects. We discuss possible formation mechanisms for the new class of hybrid stars and smoking guns for their observational identification.

Bayesian analysis of multimessenger M-R data with interpolated hybrid EoS

We introduce a family of equations of state (EoS) for hybrid neutron star (NS) matter that is obtained by a two-zone parabolic interpolation between a soft hadronic EoS at low densities and a set of stiff quark matter EoS at high densities within a finite region of chemical potentials mu _H< mu < mu _Q. Fixing the hadronic EoS as the APR one and choosing the color-superconducting, nonlocal NJL model with two free parameters for the quark phase, we perform Bayesian analyses with this two-parameter family of hybrid EoS. Using three different sets of observational constraints that include the mass of PSR J0740+6620, the tidal deformability for GW170817, and the mass-radius relation for PSR J0030+0451 from NICER as obligatory (set 1), while set 2 uses the possible upper limit on the maximum mass from GW170817 as an additional constraint and set 3 instead of the possibility that the lighter object in the asymmetric binary merger GW190814 is a neutron star. We confirm that in any case, the quark matter phase has to be color superconducting with the dimensionless diquark coupling approximately fulfilling the Fierz relation eta _D=0.75 and the most probable solutions exhibiting a proportionality between eta _D and eta _V, the coupling of the repulsive vector interaction that is required for a sufficiently large maximum mass. We used the Bayesian analysis to investigate with the method of fictitious measurements the consequences of anticipating different radii for the massive 2~M_odot PSR J0740+6220 for the most likely equation of state. With the actual outcome of the NICER radius measurement on PSR J0740+6220 we could conclude that for the most likely hybrid star EoS would not support a maximum mass as large as 2.5~M_odot so that the event GW190814 was a binary black hole merger.

TIC 308396022: δ Scuti–γ Doradus hybrid with large-amplitude radial fundamental mode and regular g-mode period spacing

We analyse the pulsating behaviour of TIC 308396022, based on observations by the TESS mission. The star is a high-amplitude δ Sct star that shows a very rich amplitude spectrum using the 3-yr light curve. Among these frequencies, the strongest peak of f1 = 13.20362567(12) d−1 is identified as the radial fundamental mode, and we also find the first and second overtones (f2 and f3). In the low-frequency range (&lt; 2.5 d−1), 22 peaks are identified as gravity modes, which show a regular period spacing of about 2460 s and have an angular degree of l = 1. The period spacing pattern does not show a significant downward trend, suggesting the star is rotating slowly. We note that this is a δ Sct–γ Dor hybrid star containing a high-amplitude radial fundamental mode and a regular g-mode period spacing pattern. With the O − C analysis, we find the star shows a significant time delay, implying that the star has a companion and it is likely to be a white dwarf. The history of possible mass transfer provides a great opportunity for testing the current theories of binary evolution, mass transfer, and pulsation.

Hybrid Stars with Color Superconducting Cores in an Extended FCM Model

We investigate the influence of repulsive vector interactions and color superconductivity on the structure of neutron stars using an extended version of the field correlator method (FCM) for the description of quark matter. The hybrid equation of state is constructed using the Maxwell description, which assumes a sharp hadron-quark phase transition. The equation of state of hadronic matter is computed for a density-dependent relativistic lagrangian treated in the mean-field approximation, with parameters given by the SW4L nuclear model. This model described the interactions among baryons in terms of σ, ω, ρ, σ*, and ϕ mesons. Quark matter is assumed to be in either the CFL or the 2SC+s color superconducting phase. The possibility of sequential (hadron-quark, quark-quark) transitions in ultra-dense matter is investigated. Observed data related to massive pulsars, gravitational-wave events, and NICER are used to constrain the parameters of the extended FCM model. The successful equations of state are used to explore the mass-radius relationship, radii, and tidal deformabilities of hybrid stars. A special focus lies on investigating consequences that slow or fast conversions of quark-hadron matter have on the stability and the mass-radius relationship of hybrid stars. We find that if slow conversion should occur, a new branch of stable massive stars would exist whose members have radii that are up to 1.5 km smaller than those of conventional neutron stars of the same mass. Such objects could be possible candidates for the stellar high-mass object of the GW190425 binary system.

Constraints on the phase transition and nuclear symmetry parameters from PSR J0740+6620 and multimessenger data of other neutron stars

Recently, the radius of neutron star (NS) PSR J0740+6620 was measured by Neutron Star Interior Composition Explorer (NICER) and an updated measurement of neutron skin thickness of ${}^{208}$Pb ($R_{\rm skin}^{208}$) was reported by the PREX-II experiment. These new measurements can help us better understand the unknown equation of state (EOS) of dense matter. In this work, we adopt a hybrid parameterization method, which incorporates the nuclear empirical parameterization and some widely used phenomenological parameterizations, to analyze the results of nuclear experiments and astrophysical observations. With the joint Bayesian analysis of GW170817, PSR J0030+0451, and PSR J0740+6620, the parameters that characterize the ultradense matter EOS are constrained. We find that the slope parameter $L$ is approximately constrained to $70_{-18}^{+21}$ MeV, which predicts $R_{\rm skin}^{208}=0.204^{+0.030}_{-0.026}\,{\rm fm}$ by using the universal relation between $R_{\rm skin}^{208}$ and $L$. The bulk properties of canonical $1.4\,M_\odot$ NS (e.g., $R_{1.4}$ and $\Lambda_{1.4}$) as well as the pressure ($P_{2\rho_{\rm sat}}$) at two times the nuclear saturation density are well constrained by the data; i.e., $R_{1.4}$, $\Lambda_{1.4}$, and $P_{2\rho_{\rm sat}}$ are approximately constrained to $12.3\pm0.7$ km, $330_{-100}^{+140}$, and $4.1_{-1.2}^{+1.5}\times10^{34}\,{\rm dyn\,cm^{-2}}$, respectively. Besides, we find that the Bayes evidences of the hybrid star and normal NS assumptions are comparable, which indicates that current observation data are compatible with quarkyonic matter existing in the core of massive star. Finally, in the case of normal NS assumption, we obtain a constraint for the maximum mass of nonrotating NS $M_{\rm TOV}=2.30^{+0.30}_{-0.18}$ $M_\odot$. All of the uncertainties reported above are for 68.3% credible levels.

Properties of hybrid stars with a density-dependent bag model

The phenomena of the deconfinement of hadronic matter into quark matter at high density, relevant to hybrid star (HS) cores, is studied in the present work. The effective chiral model describes the pure hadronic phase while for the quark phase the MIT bag model is chosen with density-dependent bag pressure. Phase transition is achieved using Maxwell construction. The effect of variation of the asymptotic value of the bag pressure (B as) is analyzed w.r.t. the mass and radius of the HSs. The presence of hyperons in the hadronic sector also has a significant effect on the choice of the value of B as. Both the hadronic composition and the choice of B as significantly affect the stability of the star. The gross structural properties of the resultant HS are calculated in a static condition and compared with the various constraints on them from different observational and empirical perspectives. The static properties, like the maximum gravitational mass of the HS obtained with B as = 80 MeV fm−3, is consistent with the limits imposed from the observational analysis of PSR J0348+0432 and PSR J0740+6620. The estimates of R 1.4 and R 1.6 of HSs are found to be within the range prescribed from GW170817 analysis. Also the M–R solutions of the HSs are in excellent agreement with the recently obtained NICER data for PSR J0030+0451. The results of maximum surface redshift, obtained with a hybrid equation of state, satisfy the constraints from 1E 1207.4-5209 and RX J0720.4-3125. The work is also extended to obtain the tidal deformation properties of the HSs. The obtained value of Λ1.4 is consistent with the bound obtained from GW170817 data analysis.

Impact of the PSR J0740+6620 radius constraint on the properties of high-density matter

X-ray pulse profile modeling of PSR J0740+6620, the most massive known pulsar, with data from the NICER and XMM-Newton observatories recently led to a measurement of its radius. We investigate this measurement's implications for the neutron star equation of state (EoS), employing a nonparametric EoS model based on Gaussian processes and combining information from other x-ray, radio and gravitational-wave observations of neutron stars. Our analysis mildly disfavors EoSs that support a disconnected hybrid star branch in the mass-radius relation, a proxy for strong phase transitions, with a Bayes factor of $6.9$. For such EoSs, the transition mass from the hadronic to the hybrid branch is constrained to lie outside ($1,2$) $M_{\odot}$. We also find that the conformal sound-speed bound is violated inside neutron star cores, which implies that the core matter is strongly interacting. The squared sound speed reaches a maximum of $0.75^{+0.25}_{-0.24}\, c^2$ at $3.60^{+2.25}_{-1.89}$ times nuclear saturation density at 90% credibility. Since all but the gravitational-wave observations prefer a relatively stiff EoS, PSR J0740+6620's central density is only $3.57^{+1.3}_{-1.3}$ times nuclear saturation, limiting the density range probed by observations of cold, nonrotating neutron stars in $\beta$-equilibrium.

Effects of strong magnetic fields on the hadron-quark deconfinement transition

The aim of the present work is to investigate the effects of strong magnetic fields on the hadron-quark phase transition point at zero temperature. To describe the hadronic phase, a relativistic mean field (RMF) model is used and to describe the quark phase a density dependent quark mass model (DDQM) is employed. As compared with the results obtained with non-magnetised matter, we observe a shift of the transition point towards higher pressures and, generally also towards higher chemical potentials. An investigation of the phase transitions that could sustain hybrid stars is also performed.

Hadron–quark phase transition in the context of GW190814

The properties of neutron stars are calculated for hadronic matter within the density-dependent relativistic mean-field model (DD-RMF). The phase transition to quark matter is studied and the hybrid star matter properties are systematically calculated using the vector-enhanced bag model. The maximum mass of a neutron star with DD-LZ1 and DD-RMF parameter sets is found to be around 2.55M ⊙ for the pure hadronic phase and around 2M ⊙ for the hadron–quark mixed phase using both Gibbs and Maxwell construction (MC). The variation in tidal deformability for the hybrid equation of state (EoS) at 1.4M ⊙ depends upon the construction of the phase transition. While it remains unchanged with MC, but decreases with the increasing neutron star mass for Gibbs construction. The pure hadron matter EoS satisfies the maximum mass constraint from recently observed GW190814 data, implying a stiff neutron star EoS. The phase transition to quark matter satisfies the maximum mass and radius constraints from the GW170817 event. Therefore, we cannot exclude the possibility of the secondary object in GW190814 as a neutron star with a phase transition to quark matter that satisfies the 2M ⊙ maximum mass limit.