Manju Prakash

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.

Non-equilibrium properties of hadronic mixtures

Abstract The equilibration of hot hadronic matter is studied in the framework of relativistic kinetic theory. Various non-equilibrium properties of a mixture comprised of pions, kaons and nucleons are calculated in the dilute limit for small deviations from local thermal equilibrium. Interactions between these constituents are specified through the empirical phase shifts. The properties calculated include the relaxation/collision times, momentum and energy persistence ratios in elastic collisions, and transport properties such as the viscosity, the thermal conductivity, and the diffusion and thermal diffusion coefficients. The Chapman-Enskog formalism is extended to extract relaxation times associated with shear and heat flows, and drag and diffusion flows in a mixture. The equilibrium number concentration of the constituents is chosen to mimic those expected in the mid-rapidity interval of CERN and RHIC experiments. In this case, kaons and nucleons are found to equilibrate significantly more slowly than pions. These results shed new light on the influence of collective flow effects on the transverse momentum distributions of kaons and nucleons versus those of pions in ultra-relativistic nuclear collisions.

RAPID COOLING AND THE STRUCTURE OF NEUTRON STARS

This report discusses the following topics on neutron stars: direct URCA neutrino emission; thermal evolution models; analytic model for diffusion through the crust; and core superfluidity. (LSP).

Rapid cooling of neutron stars by hyperons and Delta isobars

It is shown that direct Urca processes with hyperons and/or nucleon isobars can occur in dense matter as long as the concentration of Λ hyperons exceeds a critical value that is less than 3% and is typically about 0.1%. The neutrino luminosities from the hyperon Urca processes are about 5-100 times less than the typical luminosity from the nucleon direct Urca process, if the latter process is not forbidden, but they are larger than those expected from other sources. These new direct Urca processes provide avenues for rapid cooling of neutron stars which invoke neither exotic states nor the large proton fraction (of order 0.11-0.15) required for the nucleon direct Urca process

Jψ interactions with hot hadronic matter

Abstract We consider the possibility that the suppression of the J ψ signal observed in oxygen-uranium collisions at CERN is due to interactions with a hot hadron gas. Assuming a longitudinally expanding meson gas, we obtain an overall suppression compatible with the CERN data. This suppression, however, varies only slowly with the J ψ transverse momentum. We also discuss the possibility of J ψ regeneration from the more abundantly produced χ and ηc states, an effect that might become important at RHIC energies.

Quark-hadron phase transitions in Young and old neutron stars

Abstract The mixed phase of quarks and hadrons which might exist in the dense matter encountered in the varying conditions of temperature and trapped neutrino fraction in proto-neutron stars is studied. We show that hadronic equations of state that maximize the quark content of matter at a given density generally minimize the extent of the mixed phase region in a neutron star of a given mass, and that only in extreme cases could a pure quark star result. Neutrino trapping inhibits the appearance of a mixed phase which leads to possible proto-neutron star metastability. We also demonstrate that the temperature along adiabats in the quark-hadron mixed phase is much smaller than what is found for the kaon condensate-hadron mixed phase. This could lead to core temperatures which are significantly lower in stars containing quarks than in those not containing quarks.

Loop Corrections and Other Many Body Effects in Relativistic Field Theories

Abstract Incorporation of effective masses into negative energy states (nucleon loop corrections) gives rise to repulsive many-body forces, as has been known for some time. Rather than renormalizing away the three- and four-body terms, we introduce medium corrections into the effective σ-exchange, which roughly cancel the nucleon loop terms for densities ϱ ∼ ϱ nm , where ϱ nm is nuclear matter density. Going to higher densities, the repulsive contributions tend to saturate whereas the attractive ones keep on growing in magnitude. The latter is achieved through use of a density-dependent effective mass for the σ-particle, m σ = mσ ( ϱ ), such that m σ ( ϱ ) decreases with increasing density. Such a behavior is seen e.g. in the Nambu-Jona-Lasinio model. It is argued that a smooth transition to chiral restoration implies a similar behavior. The resulting nuclear equation of state is, because of the self-consistency in the problem, immensely insensitive to changes in the mass or coupling constant of the σ-particle.

Knock out for subthreshold pion production

Abstract The contribution of nucleon-nucleon-single collisions to subthreshold pion production in hadron-nucleus and nucleus-nucleus collisions, E lab MeV nucleon but decrease strongly with decreasing beam energy.

Parity doubling of the nucleon and first-order chiral transition in dense matter

Abstract A new kind of first-order phase transition at finite baryon density is shown to occur using an effective lagrangian which simulates the parity doubling of the nucleon found recently in lattice QCD. Within the mean field approximation, we obtain a critical density of about five times the saturation density of nuclear matter. The implications of this phase transition to neutron star structure are discussed.

Rotation of stars containing strange quark matter

Abstract We calculate the maximum keplerian frequencies of stars containing strange matter that are in uniform rotation. We consider both self-bound stars and stars containing quark matter cores but normal surfaces. For both cases studied, use of perturbative QCD results for the equation of state of quark matter implies a limit on the maximum keplerian frequency of ∼1×104s−1.