Maximum Central Density Research Articles

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.

Unified neutron star equations of state calibrated to nuclear properties

Recently, a dataset of several equations of state (EOSs) for purely nucleonic stellar matter based on a nonlinear relativistic mean-field model prescription and constrained to properties of nuclear matter, state-of-the-art chiral effective-field theory calculations for low-density neutron matter, and astrophysical data were proposed. In this work, 21 unified neutron star EOSs were chosen from that dataset in such a way that a large range of values of the slope of the symmetry energy at saturation is covered. Several quantities are calculated and discussed, such as the proton fraction and the direct Urca behavior, the density dependence of the speed of sound and the trace anomaly, the crust-core transition properties, the compatibility with astrophysical observations, and the neutron matter properties from chiral effective-field theory calculations and pQCD constraints. We construct unified EOSs where the outer crust is given by the BSk22 functional and the inner crust is calculated from a compressible liquid drop approximation. The core is purely nucleonic; made of protons, neutrons, electrons, and muons; under charge neutrality; and in beta -equilibrium conditions. The correlation of the slope of the symmetry energy at saturation with the crust-core transition density and proton fraction is analyzed, and equations that translate these relations are proposed. Moreover, the spectral representation for all the EOSs is given, which is a convenient representation to study quasi-periodic oscillations with realistic EOSs. We show that several of these EOSs have in the center of the most massive neutron star a speed of sound squared on the order of $ 0.5$. Most of the EOSs predict a maximum central density on the order of about six times the nuclear saturation density. Three of the EOSs satisfy all of the constraints imposed. The 21 unified EOSs are available in the zenodo platform.

Constraining the Equation of State of Hybrid Stars Using Recent Information from Multidisciplinary Physics

At the ultrahigh densities existing in the core of neutron stars (NSs), it is expected that a phase transition from baryonic to deconfined quark matter may occur. Such a phase transition would affect the underlying equation of state (EoS) as well as the observable astrophysical properties of NSs. Comparison of EoS model predictions with astronomical data from multimessenger signals then provides us an opportunity to probe the behavior of dense matter. In this work, we restrict the allowed parameter space of EoS models in NSs for both nucleonic (relativistic mean field model) and quark matter (MIT bag model) sectors by imposing state-of-the-art constraints from nuclear calculations, multimessenger astrophysical data, and perturbative quantum chromodynamics (pQCD). We systematically investigate the effect of each constraint on the parameter space of uncertainties using a cutoff filter scheme, as well as the correlations among the parameters and with NS astrophysical observables. Using the constraints, we obtain limits for maximum NS mass, maximum central density, as well as for NS radii and tidal deformability. Although pQCD constraints are only effective at very high densities, they significantly reduce the parameter space of the quark model. We also conclude that astrophysical data supports high values of the bag parameter B and disfavors the existence of a pure quark matter core in hybrid stars.

Gravitational Waves from Accretion-Induced Descalarization in Massive Scalar-Tensor Theory.

Many classes of extended scalar-tensor theories predict that dynamical instabilities can take place at high energies, leading to the formation of scalarized neutron stars. Depending on the theory parameters, stars in a scalarized state can form a solution-space branch that shares a lot of similarities with the so-called mass twins in general relativity appearing for equations of state containing first-order phase transitions. Members of this scalarized branch have a lower maximum mass and central energy density compared to Einstein ones. In such cases, a scalarized star could potentially overaccrete beyond the critical mass limit, thus triggering a gravitational phase transition where the star sheds its scalar hair and migrates over to its nonscalarized counterpart. Such an event resembles, but is distinct from, a nuclear or thermodynamic phase transition. We dynamically track a gravitational transition by first constructing hydrostatic, scalarized equilibria for realistic equations of state, and then allowing additional material to fall onto the stellar surface. The resulting bursts of monopolar radiation are dispersively stretched to form a quasicontinuous signal that persists for decades, carrying strains of order ≳10^{-22} (kpc/L)^{3/2} Hz^{-1/2} at frequencies of ≲300 Hz, detectable with the existing interferometer network out to distances of L≲10 kpc, and out to a few hundred kpc with the inclusion of the Einstein Telescope. Electromagnetic signatures of such events, involving gamma-ray and neutrino bursts, are also considered.

An isotropic analytical model for charged stars

In this investigation report, we present a perfect charged fluid solution for a static and spherically symmetric spacetime; for its construction, we suppose a metric potential, [Formula: see text], and a specific form of the electric field’s intensity, [Formula: see text], in such a manner that the resulting stellar model is physically acceptable and stable. The model presented depends on two parameters [Formula: see text] related to the compactness and the magnitude of the electric field and these same parameters will generate different possibilities for the behavior of the speed of sound. For the particular case in which [Formula: see text], we obtain once more a chargeless model constructed previously, the compactness for the charged model case is greater than in the chargeless case. As an effect of the charge, the model admits two regions for the parameter [Formula: see text], in one of these the speed of sound is a monotonic decreasing function and in the other it is a monotonic increasing function. By means of a numerical analysis, it is shown that the orders of magnitude associated to the pressure and density are characteristic of the compact stars. In particular for [Formula: see text], the range of [Formula: see text], which implies that the radius of an object with mass [Formula: see text] is found between 6554.620 m and 7672.702 m with a maximum central density of [Formula: see text].

A uniparametric perfect fluid solution to represent compact stars

In the description of neutron stars, it is very important to consider gravitational theories as general relativity, due to the determining influence on the behavior of the different types of stars, since some objects show densities even bigger than nuclear density. This paper starts with Einstein’s equations for a perfect fluid and then we present a uniparametric stellar model which allows to describe compact objects like neutron stars with compactness ratio [Formula: see text]. The pressure and density are monotone decreasing regular functions, the speed of sound satisfies the causality condition, while the value for its adiabatic index [Formula: see text] guarantees the stability. In addition, the graph of [Formula: see text] versus [Formula: see text] shows a quasi-linear relationship for the equation of state [Formula: see text], which is similar to the so-called MIT Bag equation when we have the interaction between quarks. In our case it is due to the interaction of the different components found inside the star, such as electrons and neutrons. As an application of the model, we describe the star PSR J1614-2230 with a observed mass of [Formula: see text] and a radius [Formula: see text], the model shows that the maximum central density occurs for a maximal compactness value [Formula: see text].

A simple geometry to model fluid spheres in general relativity

A simple geometry which is static and spherically symmetrical is proposed, useful to model a fluid sphere in general relativity. This allows us to describe the interior of stellar objects both neutral and charged. It is shown that in both cases the solution is stable, which means that the behavior of the density and radial pressure is monotonic decreasing functions. Both models depend on two parameters, in the neutral case, these are the compactness rate $$u=GM/c^2R$$ and w related to the monotonically increasing or decreasing behavior of the speed of sound, and in the charged case, they are the compactness rate and the rate between the charge Q and the radius R, $$q=Q/R$$ . A comparison is done between the case of a charged perfect fluid and the case of an anisotropic fluid; from there we can show that, as a result of the presence of the charge and its repulsion effect that it generates at a local and universal level, the density for the anisotropic case is greater than the density in the charged case. For both models, we obtain physical values of objects with a compactness rate $$u=0.35578$$ for the particular cases of masses and radios for $$(M_\odot ,R=4150.1\,\mathrm{m})$$ , $$(1.5M_\odot ,R=6225\,\mathrm{m})$$ , $$(2M_\odot ,R=8300 m)$$ . In the neutral case, the greater central density $$\rho _c=5.0840\times 10^{19} \,\mathrm{kg/m}^3$$ matches the object of $$1.5M_\odot $$ and for the charged case, the maximum central density is of $$\rho _c=8.8439\times 10^{18} \,\mathrm{kg/m}^3$$ , associated with the star with $$1M_\odot $$ . Additionally, by using the proposed model, we obtain the values of density and pressure for the star PSR J1614–223, the maximum value of the central density $$\rho _c=1.0573\times 10^{18} \,\mathrm{kg/m}^3$$ .

Quintessences compact star with Durgapal potential

A compact star model formed by quintessence and ordinary matter is presented, both sources have anisotropic pressures and are described by linear state equations, also the state equation of the tangential pressure for the ordinary matter incorporates the effect of the quintessence. It is shown that depending on the compactness of the star [Formula: see text] the constant of proportionality [Formula: see text] between the density of the ordinary matter and the radial pressure, [Formula: see text], has an interval of values which is consistent with the possibility that the matter is formed by a mixture of particles like quarks, neutrons and electrons and not only by one type of them. The geometry is described by the Durgapal metric for [Formula: see text] and each one of the pressures and densities is positive, finite and monotonic decreasing, as well as satisfying the condition of causality and of stability [Formula: see text], which makes our model physically acceptable. The maximum compactness that we have is [Formula: see text], so we can apply our solution considering the observational data of mass and radii [Formula: see text], [Formula: see text] km which generate a compactness [Formula: see text] associated to the star PSR J0348[Formula: see text]+[Formula: see text]0432. In this case, the interval of [Formula: see text] and its maximum central density [Formula: see text] and in the surface [Formula: see text] of the star are [Formula: see text] and [Formula: see text], respectively, meanwhile the central density of the quintessence [Formula: see text].

A possible representation for the neutron star PSR J0437-4715

A construction and analysis of an exact solution to Einstein's field equations with the source of matter being an anisotropic fluid that allows to model compact stars with a rate of compactness $ u\le 0.2061897680$ of which the orders of magnitude of pressure and density match those of neutron stars. The model is physically acceptable due to its regular geometry, the density and radial pressure are monotonically decreasing as function of the radial distance, and the speeds of the radial and tangential sounds do not break the condition of causality and as such according to the concept of “cracking” the solution is potentially stable. In a particular manner for the observational data of the star PSR J0437-4715 with a mass of $ 1.44 M_\odot$ and radius R = 13.1 km a description of its internal behaviour and range of values for the hydrostatic variables is presented. Their compactness is u = 0.162299, while their maximum central density $ \rho_c=5.949601 \times 10^{17}$ kg/m3 and the maximum density on the surface $ \rho_b=1.958845\times 10^{17}$ kg/m3, which is consistent with the expected values for the neutron stars.

Effect of electric charge on conformal compact stars

We investigate the effect of electric charge in anisotropic compact stars with conformal symmetry. We assume that the pressure and the density of the matter inside the stellar structure are large with strong gravitational fields. The strong electric field produces significant effects on the phenomenology of the stellar objects, in order of 10^{20}~text {V m}^{-1} in MKSA units. The conformal symmetry condition produces an integral relationship between the metric functions. We use this condition to find a new anisotropic solution to the Einstein–Maxwell field equations. This solution is relevant in modelling a relativistic compact star. Radii and masses are consistent with stellar objects such PSR J1614-2230, Vela X1, PSR J1903+327 and Cen X-3. The mass-radius ratio and the surface red shift are in agreement with realistic constraints. Also our model displays constraint on the maximum stellar mass, central density and radius for the upper bound redshift requirements.

Compact stars

An analysis and construction is presented for a stellar model characterized by two parameters (w, n) associated with the compactness ratio and anisotropy, respectively. The reliability range for the parameter w [Formula: see text] 1.97981225149 corresponds with a compactness ratio u [Formula: see text] 0.2644959374, the density and pressures are positive, regular and monotonic decrescent functions, the radial and tangential speed of sound are lower than the light speed, moreover, than the plausible stability. The behavior of the speeds of sound are determinate for the anisotropy parameter n, admitting a subinterval where the speeds are monotonic crescent functions and other where we have monotonic decrescent functions for the same speeds, both cases describing a compact object that is also potentially stable. In the bigger value for the observational mass M = 2.05 M[Formula: see text] and radii R = 12.957 Km for the star PSR J0348+0432, the model indicates that the maximum central density [Formula: see text] = 1.283820319 × 10[Formula: see text] Kg/m3 corresponds to the maximum value of the anisotropy parameter and the radial and tangential speed of the sound are monotonic decrescent functions.

Relativistic stars in degenerate higher-order scalar-tensor theories after GW170817

We study relativistic stars in degenerate higher-order scalar-tensor theories that evade the constraint on the speed of gravitational waves imposed by GW170817. It is shown that the exterior metric is given by the usual Schwarzschild solution if the lower order Horndeski terms are ignored in the Lagrangian and a shift symmetry is assumed. However, this class of theories exhibits partial breaking of Vainshtein screening in the stellar interior and thus modifies the structure of a star. Employing a simple concrete model, we show that for high-density stars the mass-radius relation is altered significantly even if the parameters are chosen so that only a tiny correction is expected in the Newtonian regime. We also find that, depending on the parameters, there is a maximum central density above which solutions cease to exist.

Neutron-star properties from the postmerger gravitational wave signal of binary neutron stars

We discuss the equation-of-state dependence of the gravitational-wave signal of neutron-star mergers. For a given binary mass the dominant oscillation frequency of the postmerger remnant scales tightly with neutron star radii of a given mass. This relation means that a measurement of the dominant GW frequency, when combined with information from the inspiral phase, implies a simultaneous mass and radius measurement. Moreover, we derive a procedure by which the detection of two merger events with somewhat different binary masses can be employed for an estimate of the maximum mass of nonrotating neutron stars, the radius of the maximum-mass configuration and the maximum central density of neutron stars. Compared to our method recently described in [7] we here employ a slightly modified procedure.

Edge-to-center density ratios in low-temperature plasmas

The ion flux leaving a plasma at a boundary can be given by: Γi = hLn0uB, where n0 is the maximum central plasma density, uB is the Bohm velocity, and hL is the sheath edge-to-center plasma density ratio. Such hL factors have become synonymous with global modeling of plasma discharges, where they play a vital role in the prediction of plasma losses to bounding surfaces. By performing one-dimensional (1D) particle-in-cell simulations of inductively and capacitively coupled plasmas (ICPs and CCPs) over a wide pressure range, we explicitly test the validity of standard heuristic formulae commonly used to estimate hL. The ICP simulation results are found to be in very good agreement, while a large discrepancy is present for the CCP results at high pressures. The onset of this discrepancy is found to be correlated with the bulk-to-sheath edge ionization transition that occurs in CCPs at high pressures. Consequently, global models will strongly underestimate plasma losses in this regime.

Revealing the high-density equation of state through binary neutron star mergers

We present a novel method for revealing the equation of state of high-density neutron star matter through gravitational waves emitted during the postmerger phase of a binary neutron star system. The method relies on a small number of detections of the peak frequency in the postmerger phase for binaries of different (relatively low) masses, in the most likely range of expected detections. From such observations, one can construct the derivative of the peak frequency versus the binary mass, in this mass range. Through a detailed study of binary neutron star mergers for a large sample of equations of state, we show that one can extrapolate the above information to the highest possible mass (the threshold mass for black hole formation in a binary neutron star merger). In turn, this allows for an empirical determination of the maximum mass of cold, nonrotating neutron stars to within 0.1 M_sun, while the corresponding radius is determined to within a few percent. Combining this with the determination of the radius of cold, nonrotating neutron stars of 1.6 M_sun (to within a few percent, as was demonstrated in Bauswein et al., PRD, 86, 063001, 2012), allows for a clear distinction of a particular candidate equation of state among a large set of other candidates. Our method is particularly appealing because it reveals simultaneously the moderate and very high-density parts of the equation of state, enabling the distinction of mass-radius relations even if they are similar at typical neutron star masses. Furthermore, our method also allows to deduce the maximum central energy density and maximum central rest-mass density of cold, nonrotating neutron stars with an accuracy of a few per cent.

Nuclear dynamics at the geometry of vanishing flow in heavy-ion collisions

We study the time evolution and mass dependence of various quantities (such as average and maximum central density, collision dynamics, participant spectator matter, and average and maximum temperature) at the geometry of vanishing flow (GVF) throughout the mass range between 80 and 262 units. We find that the reaction time at 100 MeV/nucleon of the geometry of vanishing flow is smaller for lighter nuclei compared to heavier ones. All the quantities can be parameterized by a power law dependence. The maximal values of corresponding quantities are also shifted accordingly.

Bose-Einstein condensate general relativistic stars

We analyze the possibility that due to their superfluid properties some compact astrophysical objects may contain a significant part of their matter in the form of a Bose-Einstein condensate. To study the condensate we use the Gross-Pitaevskii equation, with arbitrary non-linearity. By introducing the Madelung representation of the wave function, we formulate the dynamics of the system in terms of the continuity equation and of the hydrodynamic Euler equations. The non-relativistic and Newtonian Bose-Einstein gravitational condensate can be described as a gas, whose density and pressure are related by a barotropic equation of state. In the case of a condensate with quartic non-linearity, the equation of state is polytropic with index one. In the framework of the Thomas-Fermi approximation the structure of the Newtonian gravitational condensate is described by the Lane-Emden equation, which can be exactly solved. The case of the rotating condensate is also discussed. General relativistic configurations with quartic non-linearity are studied numerically with both non-relativistic and relativistic equations of state, and the maximum mass of the stable configuration is determined. Condensates with particle masses of the order of two neutron masses (Cooper pair) and scattering length of the order of 10-20 fm have maximum masses of the order of 2 M_sun, maximum central density of the order of 0.1-0.3 10^16 g/cm^3 and minimum radii in the range of 10-20 km. In this way we obtain a large class of stable astrophysical objects, whose basic astrophysical parameters (mass and radius) sensitively depend on the mass of the condensed particle, and on the scattering length. We also propose that the recently observed neutron stars with masses in the range of 2-2.4 M_sun are Bose-Einstein Condensate stars.

Overview of results from the Large Helical Device

The physical understanding of net-current-free helical plasmas has progressed in the Large Helical Device (LHD) since the last Fusion Energy Conference in Geneva, 2008. The experimental results from LHD have promoted detailed physical documentation of features specific to net-current-free 3D helical plasmas as well as complementary to the tokamak approach. The primary heating source is neutral beam injection (NBI) with a heating power of 23 MW, and electron cyclotron heating with 3.7 MW plays an important role in local heating and power modulation in transport studies. The maximum central density has reached 1.2 × 1021 m−3 due to the formation of an internal diffusion barrier (IDB) at a magnetic field of 2.5 T. The IDB is maintained for 3 s by refuelling with repetitive pellet injection. In a different operational regime with moderate density less than 2 × 1019 m−3, a plasma with a central ion temperature reaching 5.6 keV exhibits the formation of an internal transport barrier (ITB). The ion thermal diffusivity decreases to the level predicted by neoclassical transport. In addition to the rotation driven by the momentum input due to tangential NBI, the existence of intrinsic torque to drive toroidal rotation is identified in the plasma with an ITB. This ITB is accompanied by an impurity hole which generates an impurity-free core. The impurity hole is due to a large outward convection of impurities in spite of the negative radial electric field. The magnitude of the impurity hole is enhanced in the magnetic configuration with a large helical ripple and for heavier atoms. Another mechanism for suppressing impurity contamination is identified at the plasma edge with a stochastic magnetic field. A helical system shares common physics issues with tokamaks such as 3D equilibria, transport in a stochastic magnetic field, plasma response to a resonant magnetic perturbation, divertor physics and the role of radial electric field and meso-scale structure.

Stability of the fragments and thermalization at the peak centre-of-mass energy

We simulated the central reactions of nearly symmetric and asymmetric systems, for energies at which maximum production of intermediate mass fragments (IMFs) occurred ( $E_{\rm c.m.}^{\rm peak}$ ). This study was carried out using hard EOS along with Cugnon cross-section employing MSTB method for clusterization. We studied the various properties of fragments. The stability of fragments was checked through persistence coefficient and gain term. The information about the thermalization and stopping in heavy-ion collisions was obtained via relative momentum, anisotropy ratio and rapidity distribution. We found that for a complete stopping of incoming nuclei very heavy systems are required. The mass dependence of various quantities (such as average and maximum central density, collision dynamics as well as the time zone for hot and dense nuclear matter) was also presented. In all cases (i.e., average and maximum central density, collision dynamics as well as the time zone for hot and dense nuclear matter) a power-law dependence was obtained.

Dynamics and depletion in thermally supercritical starless cores

In previous studies, we identified two classes of starless cores, thermally subcritical and supercritical, distinguished by different dynamical behaviour and internal structure. Here, we study the evolution of the dynamically unstable, thermally supercritical cores by means of a numerical hydrodynamic simulation that includes radiative equilibrium and simple molecular chemistry. From an initial state as an unstable Bonnor–Ebert (BE) sphere, a contracting core evolves towards the configuration of a singular isothermal sphere by inside–out collapse. We follow the gas temperature and abundance of CO during the contraction. The temperature is predominantly determined by radiative equilibrium, but in the rapidly contracting centre of the core compressive heating raises the gas temperature by a few degrees over its value in static equilibrium. The time-scale for the equilibration of CO depends on the gas density and is everywhere shorter than the dynamical time-scale. The result is that the dynamics do not much affect the abundance of CO which is always close to that of a static sphere of the same density profile, and CO cannot be used as a chemical clock in starless cores. We use our non-local thermodynamic equilibrium (non-LTE) radiative transfer code mollie to predict observable CO and N2H+ line spectra, including the non-LTE hyperfine ratios of N2H+, during the contraction. These are compared against observations of the starless core L1544. The comparison indicates that the dust in L1544 has an opacity consistent with ice-covered rather than bare grains, the cosmic ray ionization rate is about 1 × 10−17 s−1 and the density structure of L1544 is approximately that of a BE sphere with a maximum central density of 2 × 107 cm−3, equivalent to an average density of 3 × 106 cm−3 within a radius of 500 au. The observed CO linewidths and intensities are reproduced if the CO desorption rate is about 30 times higher than the rate expected from cosmic ray strikes alone, indicating that other desorption processes are also active.