Narrow Results

A realistic EOS (equation of state) leads to strange stars (ReSS) which are compact in the mass–radius (M–R) plot, close to the Schwarzschild limiting line.¹ Many of the observed stars fit in with this kind of compactness, irrespective of whether they are X-ray pulsars, bursters or soft γ repeaters or even radio pulsars. We point out that a change in the radius of a star can be small or large, when its mass is increasing and this depends on the position of a particular star on the M–R curve. We carry out a stability analysis against radial oscillations and compare with the EOS of other SS models. We find that the ReSS is stable and an M–R region can be identified to that effect.

Skin vibration of ReSS and consequent resonance absorption can account for the absorption lines in the spectrum of X-ray emission from many compact stellar objects and in particular, the stars J1210-5226 and RXJ1856-3754. Observations of the X-ray spectrum of these stars is difficult to explain, if they are neutron stars.

We describe a problem associated with the role the ω meson at short distance in applying the Skyrme model to the dense matter: the ω coupling to the baryon density prevents the scale-anomaly dilaton field from developing a vanishing vacuum expectation value at the chiral restoration, which is at variance with standard scenario of QCD at the phase transition. We suggest a possible phenomenological solution to this puzzle and discuss its physical consequences.

The role of the Coulomb and isovector effects in dense stellar matter with neutrino trapping in the region of perturbative momentum q ≤ 0.6 fm^-1 has been investigated. The result might have an impact on the neutrino transport in stellar matter.

Effects of the ω meson self coupling (OMSC) on the thermal properties of asymmetric nuclear matter (ANM) are studied within the framework of relativistic mean field (RMF) model that includes contributions of all possible mixed interactions among meson fields involved up to quartic order. In particular, we study the mechanical and chemical instabilities (spinodal), as well as the liquid-gas phase transition (binodal) at finite temperature. It is found that the onset of spinodal instabilities and the binodal curve are only marginally affected by variation of the OMSC parameter, whereas the binodal curve shows a strong correlation to the symmetry energy. Comparison with other ERMF parameter sets is also performed.

Understanding the equation of state of dense nuclear matter is a fundamental challenge for nuclear physics. It is an especially timely and interesting challenge as we have reached a period where neutron stars, which contain the most dense nuclear matter in the Universe, are now being studied in completely new ways, from gravitational waves to satellite-based telescopes. We review the theoretical approaches to calculating this equation of state which involve a change in the structure of the baryons, along with their predictions for neutron star properties.

The HADES experiment, installed at GSI, Darmstadt, measures di-electron production in A + A, p/π + N and p/π + A collisions. Here, the π⁰ and η Dalitz decays have been reconstruced in the exclusive p + p reaction at 2.2 GeV to form a reference cocktail for long-lived di-electron sources. In the C+C reaction at 1 and 2 GeV/u, these long-lived sources have been subtracted from the measured inclusive e⁺e^- yield to exhibit the signal from the early phase of the collision. The results suggest that resonances play an important role in dense nuclear matter.

We study the interaction between two B = 1 states in the Chiral Dilaton Model where baryons are described as nontopological solitons arising from the interaction of chiral mesons and quarks. By using the hedgehog solution for B = 1 states we construct, via a product ansatz, three possible B = 2 configurations to analyse the role of the relative orientation of the hedgehog quills in the dynamics of the soliton–soliton interaction and investigate the behavior of these solutions in the range of long/intermediate distance. One of the solutions is quite binding due to the dynamics of the π and σ fields at intermediate distance and should be used for nuclear matter studies. Since the product ansatz break down as the two solitons get close, we explore the short range distance regime with a model that describes the interaction via a six-quark bag ansatz. We calculate the interaction energy as a function of the inter-soliton distance and show that for small separations the six quarks bag, assuming a hedgehog structure, provides a stable bound state that at large separations connects with a special configuration coming from the product ansatz.

We discuss observable aspects of neutron stars and supernova that are influenced by the properties of matter at extreme density. In particular, we explore the possible role phase transitions to quark matter phases at supra nuclear density. The competition between the strange quark mass and the pairing energy in quark matter, and show that it leads to a rich phase structure at densities of relevance to neutron stars. The equation of state and transport properties of quark matter is shown to be strongly influenced by pairing correlations at the Fermi surface.

Recent observational data suggests a high compacticity (the quotient M/R) of some "neutron" stars. Motivated by these works we revisit models based on quark–diquark degrees of freedom and address the question of whether that matter is stable against diquark disassembling and hadronization within the different models. We find that equations of state modeled as effective λϕ⁴ theories do not generally produce stable self-bound matter and are not suitable for constructing very compact star models, that is the matter would decay into neutron matter. We also discuss some insights obtained by including hard sphere terms in the equation of state to model repulsive interactions. We finally compare the resulting equations of state with previous models and emphasize the role of the boundary conditions at the surface of compact self-bound stars, features of a possible normal crust of the latter and related topics.

We present the equation of state (EOS) of quark-diquark matter in the quark mass-density-dependent model. The region of the 2-D parameter space inside which this quark-diquark matter is stable against diquark disassembling and hadronization is determined. Motivated by observational data suggesting a high compactness of some neutron stars (NS) we present models based on the present EOS and compare the results with those obtained with previous works addressing a quark-diquark composition based on the MIT Bag model. We show that very compact self-bound stars (yet having ≥ 1M_⊙) are allowed by our EOS even if the diquark itself is unbound.

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.

We study the conversion of neutron matter into strange matter as a detonation wave. The detonation is assumed to originate from a central region in a spherically symmetric background of neutrons with a varying radial density distribution. We present self-similar solutions for the propagation of detonation in static and collapsing backgrounds of neutron matter. The solutions are obtained in the framework of general relativistic hydrodynamics, and are relevant for the possible transition of neutron into strange stars. Conditions for the formation of either bare or crusted strange stars are discussed.

The components of the diffusive thermal conductivity tensor of superfluid neutron stars are calculated by using anisotropic energy gap for ³p₂ pairing and approximation collision integrals at temperatures where . The contribution from neutron–neutron collisions is taken into account. Nonrelativistic effects for pairing will be studied. A comparison with the corresponding relativistic case is also made.

We present explicit examples to show that the "compatibility criterion" (recently obtained by us toward providing equilibrium configurations compatible with the structure of general relativity) which states that for a given value of σ[≡ (P₀/E₀) ≡ the ratio of central pressure to central energy-density], the compactness ratio u[≡ (M/R), where M is the total mass and R is the radius of the configuration] of any static configuration cannot exceed the compactness ratio, u_h, of the homogeneous density sphere (i.e., u ≤ u_h) is capable of providing a necessary and sufficient condition for any regular configuration to be compatible with the state of hydrostatic equilibrium. This conclusion is drawn on the basis of the finding that the M–R relation gives the necessary and sufficient condition for dynamical stability of equilibrium configurations only when the compatibility criterion for these configurations is appropriately satisfied. In this regard, we construct an appropriate sequence composed of core-envelope models on the basis of compatibility criterion such that each member of this sequence satisfies the extreme case of causality condition v = c = 1 at the center. The maximum stable value of u ≃ 0.3389 (which occurs for the model corresponding to the maximum value of mass in the mass–radius relation) and the corresponding central value of the local adiabatic index, (Γ₁)₀ ≃ 2.5911, of this model are found fully consistent with those of the corresponding absolute values, u_max ≤ 0.3406 and (Γ₁)₀ ≤ 2.5946, which impose strong constraints on these parameters of such models. In addition to this example, we also study dynamical stability of pure adiabatic polytropic configurations on the basis of variational method for the choice of the "trial function," ξ = re^ν/4, as well as the mass–central density relation, since the compatibility criterion is appropriately satisfied for these models. The results of this example provide additional proof in favor of the statement regarding compatibility criterion mentioned above. Together with other results, this study also confirms the previous claim that just the choice of the "trial function," ξ = re^ν/4, is capable of providing the necessary and sufficient condition for dynamical stability of a mass on the basis of variational method. Obviously, the upper bound on the compactness ratio of neutron stars, u ≅ 0.3389, which belongs to two-density model studied here, turns out to be much stronger than the corresponding "absolute" upper bound mentioned in the literature.

In General Relativity, all forms of energy contribute to gravity and not only just ordinary matter as in Newtonian Physics. This fact can be seen in the modified hydrostatic equilibrium equation for relativistic stars pervaded by magnetic (B) fields. It has an additional term coupled to the matter part as well as an anisotropic term which is purely of magnetic origin. That additional term coming from the pressure changed by the radial component of the diagonal electromagnetic field tensor weakens the gravitational force when B is strong enough and can even produce an unexpected change in the attractive nature of the force by reversing its sign. In an extreme case, this new general relativistic (GR) effect can even trigger an instability in the star as a consequence of the sudden reversal of the hydrostatic pressure gradient. We suggest here that this GR effect may be the possible central engine driving the transient giant outbursts observed in Soft Gamma-ray Repeaters (SGRs). In small regions of the neutron star (NS), strong magnetic condensation can take place. Beyond a critical limit, these highly magnetized bubbles may explode releasing the trapped energy as a burst of γ-rays of ~10^36–40 erg.

We study the quark deconfinement phase transition in cold (T = 0) and hot β-stable hadronic matter. Assuming a first-order phase transition, we calculate and compare the nucleation rate and the nucleation time due to thermal and quantum nucleation mechanisms. We show that above a threshold value of the central pressure a pure hadronic star (HS) is metastable to the conversion to a quark star (QS) (i.e. hybrid star or strange star). We introduce the concept of critical mass M_cr for cold HSs and proto-hadronic stars, and the concept of limiting conversion temperature for proto-hadronic stars. We show that proto-hadronic stars with a mass M < M_cr could survive the early stages of their evolution without decaying to QSs. Finally, we discuss the possible evolutionary paths of proto-hadronic stars.

The state of cold quark matter challenges both astrophysicists and particle physicists, and even many-body physicists. It is conventionally suggested that BCS-like color superconductivity occurs in cold quark matter; however, other scenarios with a ground state rather than of Fermi gas could still be possible. It is addressed that quarks are dressed and clustered in cold quark matter at realistic baryon densities of compact stars, since a weakly coupling treatment of the interaction between constituent quarks would not be reliable. Cold quark matter is conjectured to be in a solid state if thermal kinematic energy is much lower than the interaction energy of quark clusters, and such a state could be relevant to different manifestations of pulsar-like compact stars.

We present a simplified description of the gravitational collapse of a uniformly rotating neutron stellar core, which can represent the final stage of the protoneutron star formed after the supernova type II explosion. The system is treated as a compressible homogeneous spheroid and the collapse time-evolution is extracted from an effective Lagrangian description of the system geometric parameters. A sudden mass quadripole change of the system at the final stage of the collapse is shown during the core bounce.

The necessary and sufficient condition for dynamical stability is worked out for the sequences of relativistic star models which correspond to the well-defined and causal values of adiabatic sound speed, , at the center. On the basis of the conditions obtained in this study, we show that the mass–radius (M-R) relation corresponding to the MIT bag models of strange quark matter (SQM) and the models obtained by Dey et al. [Phys. Lett. B438 (1998) 123] does not provide the necessary and sufficient condition for dynamical stability for the equilibrium configurations. These findings will remain unaltered and can be extended to any other sequence of pure SQM. This study explicitly shows that though SQM may exist in the state of zero pressure and temperature, the models of pure strange quark "stars" cannot exist in the state of hydrostatic equilibrium. This study can affect the results which are claiming that various objects, like RX J1856.5-3754, SAX J1808.4-3658, 4U 1728-34 and PSR 0943+10, represent strange stars.