Ignazio Bombaci

Composition and structure of protoneutron stars

Abstract We investigate the structure of neutron stars shortly after they are born, when the entropy per baryon is of order 1 or 2 and neutrinos are trapped on dynamical timescales. We find that the structure depends more sensitively on the composition of the star than on its entropy, and that the number of trapped neutrinos play an important role in determining the composition. Since the structure is chiefly determined by the pressure of the strongly interacting constituents and the nature of the strong interactions is poorly understood at high density, we consider several models of dense matter, including matter with strangeness-rich hyperons, a kaon condensate and quark matter. In all cases, the thermal effects for an entropy per baryon of order 2 or less are small when considering the maximum neutron star mass. Neutrino trapping, however, significantly changes the maximum mass due to the abundance of electrons. When matter is allowed to contain only nucleons and leptons, trapping decreases the maximum mass by an amount comparable to, but somewhat larger than, the increase due to finite entropy. When matter is allowed to contain strongly interacting negatively charged particles, in the form of strange baryons, a kaon condensate, or quarks, trapping instead results in an increase in the maximum mass, which adds to the effects of finite entropy. A net increase of order 0.2 M ⊙ occurs. The presence of negatively-charged particles has two major implications for the neutrino signature of gravitational collapse supernovae. First, the value of the maximum mass will decrease during the early evolution of a neutron star as it loses trapped neutrinos, so that if a black hole forms, it either does so immediately after the bounce (accretion being completed in a second or two) or it is delayed for a neutrino diffusion timescale of ~ 10 s . The latter case is most likely if the maximum mass of the hot star with trapped neutrinos is near 1.5 M ⊙ . In the absence of negatively-charged hadrons, black hole formation would be due to accretion and therefore is likely to occur only immediately after bounce. Second, the appearance of hadronic negative charges results in a general softening of the equation of state that may be observable in the neutrino luminosities and average energies. Further, these additional negative charges decrease the electron fraction and may be observed in the relative excess of electron neutrinos compared to other neutrinos.

Strange stars with realistic quark vector interaction and phenomenological density-dependent scalar potential

Abstract We derive an equation of state (EOS) for strange matter, starting from an interquark potential which (i) has asymptotic freedom built into it, (ii) shows confinement at zero density (ρB=0) and deconfinement at high ρB, and (iii) gives a stable configuration for chargeless, β-stable quark matter. This EOS is then used to calculate the structure of Strange Stars, and in particular their mass-radius relation. Our present results confirm and reinforce the recent claim 1 , 2 that the compact objects associated with the X-ray pulsar Her X-1, and with the X-ray burster 4U 1820-30 are strange stars.

Is SAX J1808.4-3658 a strange star?

The possibility of strange stars is one of the most important issues in the study of compact objects. Here we use the observations of the newly discovered millisecond x-ray pulsar SAX J1808.4-3658 to constrain the radius of the compact star. Comparing the mass-radius relation of SAX J1808.4-3658 with theoretical models for both neutron stars and strange stars, we argue that a strange star model could be more consistent with SAX J1808.4-3658, and suggest that it is a likely strange star candidate. Our results are useful in constraining microscopic chiral symmetry restoration parameters in the quantum chromodynamics (QCD) modeling of strange matter.

Conversion of neutron stars to strange stars as the central engine of gamma-ray bursts

We study the conversion of a neutron star to a strange star as a possible energy source for gamma-ray bursts. We use different recent models for the equation of state of neutron star matter and strange quark matter. We show that the total amount of energy liberated in the conversion is in the range of &parl0;1-4&parr0;x1053 ergs (1 order of magnitude larger than previous estimates) and is in agreement with the energy required to power gamma-ray burst sources at cosmological distances.

Quark Deconfinement and Implications for the Radius and the Limiting Mass of Compact Stars

We study the consequences of the hadron-quark deconfinement phase transition in stellar compact objects when finite-size effects between the deconfined quark phase and the hadronic phase are taken into account. We show that above a threshold value of the central pressure (gravitational mass) a neutron star is metastable to the decay (conversion) to a hybrid neutron star or to a strange star. The mean lifetime of the metastable configuration dramatically depends on the value of the stellar central pressure. We explore the consequences of the metastability of massive neutron stars and of the existence of stable compact quark stars (hybrid neutron stars or strange stars) on the concept of the limiting mass of compact stars. We discuss the implications of our scenario for the interpretation of the stellar mass and radius extracted from the spectra of several X-ray compact sources. Finally, we show that our scenario implies, as a natural consequence, a two-step process that is able to explain the inferred delayed connection between supernova explosions and gamma-ray bursts (GRBs), giving also the correct energy to power GRBs.

On the Nature of the compact star in 4u 1728-34

The discovery of kilohertz quasi-periodic oscillations (kHz QPOs) in low-mass X-ray binaries (LMXBs) with the Rossi X-Ray Timing Explorer has stimulated extensive studies of these sources. Recently, Osherovich & Titarchuk suggested a new model for kHz QPOs and the related correlations between kHz QPOs and low-frequency features in LMXBs. Here we use their results to study the mass-radius relation for the atoll source 4U 1728-34. We find that, if this model is correct, 4U 1728-34 is possibly a strange star rather than a neutron star.

Estimation of the effect of hyperonic three-body forces on the maximum mass of neutron stars

A model based on a microscopic Brueckner-Hartree-Fock approach of hyperonic matter supplemented with additional simple phenomenological density-dependent contact terms is employed to estimate the effect of hyperonic three-body forces on the maximum mass of neutron stars. Our results show that although hyperonic three-body forces can reconcile the maximum mass of hyperonic stars with the current limit of 1.4–1.5M⊙, they are unable to provide the repulsion needed to make the maximum mass compatible with the observation of massive neutron stars, such as the recent measurements of the unusually high masses of the millisecond pulsars PSR J1614-2230 (1.97±0.04M⊙) and PSR J1903+0327 (1.667±0.021M⊙).

Equation of state and magnetic susceptibility of spin polarized isospin asymmetric nuclear matter

Properties of spin-polarized isospin asymmetric nuclear matter are studied within the framework of the Brueckner-Hartree-Fock formalism. The single-particle potentials of neutrons and protons with spin up and down are determined for several values of the neutron and proton spin polarizations and the asymmetry parameter. It is found that the single-particle potentials exhibit an almost linear and symmetric variation as a function of these parameters. An analytic parametrization of the total energy per particle as a function of the asymmetry and spin polarizations is constructed. This parametrization is employed to compute the magnetic susceptibility of nuclear matter for several values of the asymmetry from neutron to symmetric matter. The results show no indication of a ferromagnetic transition at any density for any asymmetry of nuclear matter.

Temperature and asymmetry dependence of nuclear incompressibility and supernova explosions

Abstract The properties of hot asymmetric nuclear matter are investigated in the framework of an extended Brueckner-Bethe-Goldstone theory, using the Paris nucleon-nucleon interaction. In particular, the asymmetry and temperature dependence of the incompressibility of nuclear matter are explored in connection with the models of type-II supernova explosions. The present calculations predict a strong decrease of the nuclear incompressibility coming from both the temperature and the asymmetry dependence. The nuclear equation of state (EOS) is calculated along an isentrope, and compared with the phenomenological nuclear EOS which has been commonly used in some hydrodynamical calculations of type-II supernova explosions.

Temperature profiles of accretion discs around rapidly rotating strange stars in general relativity: A comparison with neutron stars

We compute the temperature profiles of accretion discs around rapidly rotating strange stars, using constant gravitational mass equilibrium sequences of these objects, considering the full effect of general relativity. Beyond a certain critical value of stellar angular momentum (J), we observe the radius (