Supernuclear Densities Research Articles

Analytical derivation of expressions for the specific heat of superfluid Fermi liquids with anisotropic spin-triplet p-wave pairing at finite temperatures

Analytical expressions for the specific heat of superfluid Fermi liquids (SFLs) with anisotropic spin-triplet p-wave pairing of the 3He-A type have been obtained based on a generalised Fermi-liquid approach both at the low temperatures of 0 < T ≪Tc0 (n) and near the temperature of the phase transition [Tc0 (n)] from a normal to a superfluid phase in the absence of a magnetic field. Apart from liquid 3He-A, we also study dense superfluid neutron matter (SNM) with anisotropic spin-triplet p-wave pairing (similar to 3He-A) at subnuclear (n < n0, where n0 = 0.17 fm–3 is the nuclear density) and supernuclear densities (n > n0) in view of generalised Skyrme forces (with additional terms depending on the density n). At 0 < T ≪Tc0 (n), we have obtained an asymptotic expansion for the specific heat CSNM(SFL)(T,n), in which, apart from the main term ∼ T 3 (known for 3He-A in the limit of T → 0) there is an additional correction term (∼ T5), which may reach several percent of the contribution from the main term to the decomposition of the specific heat of SNM (or SFLs). An analytical formula has also been derived for calculating the specific heat CSNM(SFL)(T,n) at temperatures near Tc0(n). The expressions obtained for the CSNM(T,n) functions (valid for an arbitrary parametrisation of the effective Skyrme interaction in neutron matter) have been defined for SNM with the generalised BSk21 parametrisation of Skyrme forces. In addition, dependency graphs were plotted for the specific heat CBSk21(t, y) in the reduced temperature range of 0 < t ≡ T/Tc0(n) ≪ 1 and at t ≲ 1 for dense SNM (at 0.1 ≤ y ≡ n/n0 ≤ 1.7). These results may be of interest for neutron star physics in connection with neutron star cooling (in the presence of neutron superfluidity with anisotropic spin-triplet p-wave Cooper pairing in the outer part of dense liquid neutron star cores).

PSR J1311–3430: A HEAVYWEIGHT NEUTRON STAR WITH A FLYWEIGHT HELIUM COMPANION

We have obtained initial spectroscopic observations and additional photometry of the newly discovered Pb=94min gamma-ray black-widow pulsar PSR J1311-3430. The Keck spectra show a He-dominated, nearly H-free photosphere and a large radial-velocity amplitude of 609.5+/-7.5km/s. Simultaneous seven-color GROND photometry further probes the heating of this companion, and shows the presence of a flaring infrared excess. We have modeled the quiescent light curve, constraining the orbital inclination and masses. Simple heated light-curve fits give M_NS=2.7Msun, but show systematic light-curve differences. Adding extra components allows a larger mass range to be fit, but all viable solutions have M_NS>2.1Msun. If confirmed, such a large M_NS substantially constrains the equation of state of matter at supernuclear densities.

New eigen-mode of spin oscillations in the triplet superfluid condensate in neutron stars

The eigen-mode of spin oscillations with ω≃58/35Δ is predicted to exist besides already known spin waves with ω≃Δ/5 in the triplet superfluid neutron condensate in the inner core of neutron stars. The new mode is kinematically able to decay into neutrino pairs through neutral weak currents. The problem is considered in BCS approximation for the case of P23–F23 pairing with a projection of the total angular momentum mj=0 which is conventionally considered as preferable one at supernuclear densities.

Neutron stars and the dense matter equation of state

Neutron stars provide a unique laboratory with which to study cold, dense matter. The observational quantities of primary astrophysics interest are the maximum mass and the typical radius of a neutron star. These quantities are related to the relative stiffness of neutron-rich matter at supernuclear densities and the density dependence of the nuclear symmetry energy near the nuclear saturation density. The measurements of these nuclear properties via nuclear systematics and structure, heavy-ion collisions and parity-violating electron scattering from neutron-rich nuclei, are discussed. Several new observations, including mass measurements of binary pulsars and a confirmed distance determination for a nearby cooling neutron star, will be summarized. Additionally addressed will be observations of thermal emissions from cooling neutron stars in globular clusters and thermonuclear explosions from accreting stars. It will be demonstrated how this astrophysical data is shedding light on the pressure-density relation of extremely dense matter.

Relativistic neutron star merger simulations with non-zero temperature equations of state

An extended set of binary neutron star (NS) merger simulations is performed with an approximative treatment of general relativity to systematically investigate the influence of the nuclear equation of state (EoS), the NS masses, and the NS spin states prior to merging. The general relativistic hydrodynamics simulations are based on a conformally flat approximation to the Einstein equations and a Smoothed Particle Hydrodynamics code for the gas treatment. We employ the two non-zero temperature EoSs of Shen et al. (1998a, Nucl. Phys. A, 637, 435; 1998b, Prog. Theor. Phys., 100, 1013) and Lattimer & Swesty (1991, Nucl. Phys. A, 535, 331), which represent a “harder” and a “softer” behavior, respectively, with characteristic differences in the incompressibility at supernuclear densities and in the maximum mass of nonrotating, cold neutron stars. In addition, we use the cold EoS of Akmal et al. (1998, Phys. Rev. C, 58, 1804) with a simple ideal-gas-like extension according to Shibata & Taniguchi (2006, Phys. Rev. D, 73, 064027), in order to compare with their results, and an ideal-gas EoS with parameters fitted to the supernuclear part of the Shen-EoS. We estimate the mass sitting in a dilute “torus” around the future black hole (BH) by requiring the specific angular momentum of the torus matter to be larger than the angular momentum of the ISCO around a Kerr BH with the mass and spin parameter of the compact central remnant. The dynamics and outcome of the models is found to depend strongly on the EoS and on the binary parameters. Larger torus masses are found for asymmetric systems (up to ~ for a mass ratio of 0.55), for large initial NSs, and for a NS spin state which corresponds to a larger total angular momentum. We find that the postmerger remnant collapses either immediately or after a short time when employing the soft EoS of Lattimer& Swesty, whereas no sign of post-merging collapse is found within tens of dynamical timescales for all other EoSs used. The typical temperatures in the torus are found to be about MeV depending on the strength of the shear motion at the collision interface between the NSs and thus depending on the initial NS spins. About of NS matter become gravitationally unbound during or right after the merging process. This matter consists of a hot/high-entropy component from the collision interface and (only in case of asymmetric systems) of a cool/low-entropy component from the spiral arm tips.

BLACK HOLES AND WORMHOLES IN RS2 TYPE BRANE WORLDS

We review some recent results concerning the properties of static, spherically symmetric configurations in RS2 type brane worlds: 1) Solving the trace of the effective 4D gravity equations, we obtain general classes of black hole and wormhole metrics, including families which unify symmetric wormholes, extremal black holes and non-extremal non-singular black holes. 2) Assuming [Formula: see text] (where [Formula: see text] is a part of the 5D Weyl tensor contributing to the 4D gravity equations), we show that material scalar fields confined on the brane may violate the no-hair theorem, support wormholes or form particlelike objects, but only at super-nuclear matter densities in the strong field region. 3) Solutions to the bulk field equations are found in case [Formula: see text], among them solutions with a nonzero 4D cosmological constant, generalizing the so-called "black string".

Neutron star cooling

The impact of nuclear physics theories on cooling of isolated neutron stars is analyzed. Physical properties of neutron star matter important for cooling are reviewed such as composition, the equation of state, superfluidity of various baryon species, neutrino emission mechanisms. Theoretical results are compared with observations of thermal radiation from neutron stars. Current constraints on theoretical models of dense matter, derived from such a comparison, are formulated.

Neutron Star Cooling

▪ Abstract Observation of cooling neutron stars can potentially provide information about the states of matter at supernuclear densities. We review physical properties important for cooling such as neutrino emission processes and superfluidity in the stellar interior, surface envelopes of light elements owing to accretion of matter, and strong surface magnetic fields. The neutrino processes include the modified Urca process and the direct Urca process for nucleons and exotic states of matter, such as a pion condensate, kaon condensate, or quark matter. The dependence of theoretical cooling curves on physical input and observations of thermal radiation from isolated neutron stars are described. The comparison of observation and theory leads to a unified interpretation in terms of three characteristic types of neutron stars: high-mass stars, which cool primarily by some version of the direct Urca process; low-mass stars, which cool via slower processes; and medium-mass stars, which have an intermediate behavior. The related problem of thermal states of transiently accreting neutron stars with deep crustal burning of accreted matter is discussed in connection with observations of soft X-ray transients. Observations imply that some stars cool more rapidly than can be explained on the basis of nonsuperfluid neutron star models cooling via the modified Urca process, whereas other star cool less rapidly. We describe possible theoretical models that are consistent with observations.

EQUATION OF STATE OF NEUTRON STAR MATTER

The physical properties of neutron star matter at supernuclear densities are highly uncertain and the associated equation of state is only very poorly known. This has its origin in numerous sources which are discussed in this paper.

Mapping deconfinement with a compact star phase diagram

We have found correlations between properties of the equation of state for stellar matter with a phase transition at supernuclear densities and two characteristic features of a phase diagram for rotating compact stars in the angular velocity — baryon number plane: 1) the critical dividing line between mono- and two-phase star configurations and 2) the maximum mass line. The second line corresponds to the minimum mass function for black hole candidates whereas the first one is observable by a population statistics, e.g. for Z-sources in low-mass X-ray binaries. The observation of a population gap in the mass distribution for the latter is suggested as an astrophysical verification of the existence of a first order phase transition in QCD at high densities such as the deconfinement.

Quark-Hadron Hybrid Stars as Remnants of Supernovae Explosions

It is expected that at explosion of Supernovae the mechanism of producing a shock wave is based on subatomic interactions, and the remnant of the supernovae explosion can be a hybrid quark-hadron star. Since the temperature in the centre of collapsing stars reaches of order T = 6 − 8 · 1010K and density electrons from the top of the Fermi sea can be captured and convert protons into neutrons via e− + p → n + ve. The capture of electrons results in a neutronization burst (V.S. Imshennik 1988). Core collapse of the progenitor star becomes essentially a free fall with a time scale . When the central density of the core reaches supernuclear densities the repulsive QCD forces becomes essential. This can bring about manifestations of quark-hadron phase transitions (A. Dar 1997). After the explosion, from the remaining matter is probably formed a hybrid star.

An effective equation of state for dense matter with strangeness

An effective equation of state which generalizes the Lattimer-Swesty equation for nuclear matter is presented for matter at supernuclear densities including strange baryons. It contains an adjustable baryon potential energy density, based on models of local potentials for the baryon-baryon interactions. The features of the equation rely on the properties of nuclei for the nucleon-nucleon interactions, and mainly on experimental data from hypernuclei for the hyperon-nucleon and hyperon-hyperon interactions. The equation is used to calculate equilibrium compositions and thermodynamic properties of high density matter with strangeness in two astrophysical contexts: neutron star matter (transparent to neutrinos) and proto-neutron star matter (opaque to neutrinos). The effective equation of state reproduces typical properties of high density matter found in theoretical microscopic models. Of these, the main result is that hyperons appear in both types of matter at about twice the nuclear saturation density, and that their appearance significantly softens the equation of state. The range of maximal masses of neutron stars found in a comprehensive parameter survey is 1.4--1.7 M ⊙. Another typical result is that the maximal mass of a proto-neutron star with strange baryons is higher than that of an evolved neutron star (opposite to the case of nuclear matter), setting the stage for a “delayed collapse” scenario.

Thermal evolution of compact stars

A collection of modern, field-theoretical equations of state is applied to the investigation of cooling properties of compact stars. These comprise neutron stars as well as hypothetical strange-matter stars, made up of absolutely stable 3-flavor strange-quark matter. Various uncertainties in the behavior of matter at supernuclear densities, e.g., hyperonic degrees of freedom, behavior of coupling strengths in matter, pion and meson condensation, superfluidity, transition to quark matter, absolute stability of strange-quark matter, and last but not least the many-body technique itself are tested against the body of observed cooling data.

Gravitational radiation from type-II supernovae: The effect of the high-density equation of state.

Odd-parity gravitational perturbations of detailed spherically symmetric type-II supernova models, which include realistic equations of state and neutrino transport, are studied in the linearized Einstein theory. To investigate the parameters affecting gravitational-wave production during core collapse, we have performed simulations in which we vary the equation of state at supernuclear densities, the free proton mass fraction, and the precollapse iron core. These variations span much of the range of conditions currently thought to occur during stellar collapse and lead to the production of core-bounce shock waves that vary from the very strong to those that stall almost immediately. We find little difference in the gravitational radiation output among any of these models, though models that use stiffer equations of state or higher-mass iron cores give somewhat more radiation output.

Matter at super nuclear densities including pion condensates

The authors investigate the nature of cold matter, both neutron matter and beta-stable nucleon matter, as a function of density in a model which allows for the appearance of a pion condensate viewed as a spontaneous breaking of chiral symmetry. They also consider the effect of allowing for the creation of the Delta (1236) resonances. The possible existence of an isomeric state of nucleon matter is found at 2-3 times nuclear matter density and the implications for neutron-star structure and heavy-ion collisions are discussed.

Very massive neutron stars in Ni's theory of gravity

It is shown that in Ni's theory of gravity, which is identical to general relativity in the post-Newtonian limit, neutron stars of arbitrarily large mass are possible. This result is independent, within reasonable bounds, of the equation of state of matter at supernuclear densities.

Quark bags or interacting neutrons - A 'neutron' star test

view Abstract Citations (25) References (6) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Quark bags or interacting neutrons: a "neutron" star test. Brecher, K. Abstract The sensitivity of the ('neutron' star mass, gravitational redshift) relation to the composition of the underlying matter is shown. Detection of X-ray or gamma-ray lines from binary X-ray sources offers the best hope of determining whether matter at supernuclear densities behaves as neutrons, quarks, or some other state of matter. Publication: The Astrophysical Journal Pub Date: July 1977 DOI: 10.1086/182467 Bibcode: 1977ApJ...215L..17B Keywords: Neutron Stars; Red Shift; Stellar Mass; Stellar Structure; X Ray Stars; Binary Stars; Equations Of State; Gravitational Effects; Line Spectra; Quarks; Stellar Gravitation; X Ray Spectra; Astrophysics full text sources ADS |

Phase transition from baryon to quark matter

THE equation of state of matter compressed to supernuclear densities is of considerable interest in astrophysics. For example, pulsars are stars compressed to densities greater than the density of atomic nuclei and are thought to be composed primarily of neutrons, based on detailed calculations of the equation of state of baryonic matter assuming that interaction potentials for the baryons (nucleons and hyperons) are derived from low energy nuclear physics1. There is, however, considerable evidence from high energy physics that baryons have structure, and it is currently believed that this structure is due to the fact that all hadrons consist of quarks bound by vector gluons2. The fact that free, isolated quarks have not been observed has led to the speculation that quarks may be permanently bound inside hadrons3. Since the quarks inside the nucleon behave as free particles2, the forces confining the quarks apparently become strong only when the distance between quarks exceeds the radius of a nucleon, ∼ 10−13 cm. When the density of matter is increased beyond the point that the mean distance between quarks in different baryons is ≪ 10−13 cm the description of matter as interacting baryons becomes invalid, and instead, a description of matter composed of quarks becomes relevant. At sufficiently high densities matter will behave like a relativistic gas of free quarks4 with P ≃ ρ/3, where P is the pressure and ρ is the energy density of matter. The density at which the baryon to quark transition occurs is of crucial importance to the structure of pulsars. General relativity implies that for a given equation of state there is a maximum energy density for stable stars1. It is therefore of considerable interest to know whether the energy density of baryon matter at the baryon–quark phase transition is less than this maximum value. Indeed, it has recently been suggested5 that the existence of quark matter inside pulsars would allow larger masses for pulsars than was previously thought possible. We will show, however, that there are reasons to doubt this.

A Simple Equation of State of Matter at Super-Nuclear Densities

The qualitative features of the equation of state of matter at supernuclear densities are deduced through a careful examination of the nature of particle interactions at short distances and by the introduction of an ‘effective mass spectrum.’ it is found that the equation of state begins to take on a particularly simple form (the ‘asymptotic form’) at a relatively low matter density of 1017 g cm−3. A brief review of various approaches to calculate the equation of state for a neutron star is also given.

Equation of State of Matter at Supernuclear Densities

The qualitative features of the equation of state of matter at supernuclear densities are deduced through a careful examination of the nature of particle interactions at short distances and by the introduction of an 'effective baryon mass spectrum.' It is found that the equation of state begins to take on a particularly simple form (the 'asymptotic form') at a relatively low matter density of 100,000 teragrams per cu cm.