arXiv subjects

Madappa Prakash

Publications and source records attributed to Madappa Prakash.

66 records · Page 4Linked to original sources

arXiv2000

Nuclear Matter and its Role in Supernovae, Neutron Stars and Compact Object Binary Mergers

The equation of state (EOS) of dense matter plays an important role in the supernova phenomenon, the structure of neutron stars, and in the mergers of compact objects (neutron stars and black holes). During the collapse phase of a supernova, the EOS at subnuclear densities controls the collapse rate, the amount of deleptonization and thus the size of the collapsing core and the bounce density. Properties of nuclear matter that are especially crucial are the symmetry energy and the nuclear specific heat. The nuclear incompressibility, and the supernuclear EOS, play supporting roles. In a similar way, although the maximum masses of neutron stars are entirely dependent upon the supernuclear EOS, other important structural aspects are more sensitive to the equation of state at nuclear densities. The radii, moments of inertia, and the relative binding energies of neutron stars are, in particular, sensitive to the behavior of the nuclear symmetry energy. The dependence of the radius of a neutron star on its mass is shown to critically influence the outcome of the compact merger of two neutron stars or a neutron star with a small mass black hole. This latter topic is especially relevant to this volume, since it stems from research prompted by the tutoring of David Schramm a quarter century ago.

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arXiv1999

First Order Phase Transitions in Neutron star Matter: Droplets and Coherent Neutrino Scattering

A first order phase transition at high baryon density implies that a mixed phase can occupy a significant region of the interior of a neutron star. In this article we investigate the effect of a droplet phase on neutrino transport inside the core. Two specific scenarios of the phase transition are examined, one having a kaon condensate and the other having quark matter in the high density phase. The coherent scattering of neutrinos off the droplets greatly increases the neutrino opacity of the mixed phase. We comment on how the existence of such a phase will affect a supernova neutrino signal.

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arXiv1999

Baryonic contributions to e+e- yields in a hydrodynamic model of Pb+Au collisions at the SPS

We analyze e+e- yields from matter containing baryons in addition to mesons using a hydrodynamic approach to describe Pb+Au collisions at 158 A GeV/c. We use two distinctly different e+e- production rates to provide contrast. Although the presence of baryons leads to significant enhancement of e+e- emission relative to that from mesons-only matter, the calculated results fall below the data in the range 400 < M/MeV < 600. The calculated results are, however, only 1.3-1.5 standard deviations below the data, which may not be statistically significant.

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arXiv1999

e+e- yields in Pb+Au collisions at 158 AGeV/c: Assessment of baryonic contributions

Using a hydrodynamic approach to describe Pb+Au collisions at 158 AGeV/c, we analyze e+e- yields from matter containing baryons in addition to mesons. We employ e+e- production rates from two independent calculations, which differ both in their input physics and in their absolute magnitudes, especially in the mass range where significant enhancements over expected backgrounds exist in the CERES data. Although the presence of baryons leads to significant enhancement of e+e- emission relative to that from mesons-only matter, the calculated results fall below the data in the range 400 < M_ee/MeV < 600, by a factor of 2-3. Since the calculated e+e- spectra are relatively insensitive to the equation of state for initial conditions that fit the observed hadronic spectra, either in-medium modifications of the e+e- sources more significant than so far considered, or the presence of hitherto unidentified additional sources of e+e- is indicated.

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arXiv1998

Effects of Strong and Electromagnetic Correlations on Neutrino Interactions in Dense Matter

An extensive study of the effects of correlations on both charged and neutral current weak interaction rates in dense matter is performed. Both strong and electromagnetic correlations are considered.The propagation of particle-hole interactions in the medium plays an important role in determining the neutrino mean free paths. The effects due to Pauli-Blocking and density, spin, and isospin correlations in the medium significantly reduce the neutrino cross sections. Due to the lack of experimental information at high density, these correlations are necessarily model dependent. For example, spin correlations in nonrelativistic models are found to lead to larger suppressions of neutrino cross sections compared to those of relativistic models. This is due to the tendency of the nonrelativistic models to develop spin instabilities. Notwithstanding the above caveats, and the differences between nonrelativistic and relativistic approaches such as the spin- and isospin-dependent interactions and the nucleon effective masses, suppressions of order 2--3, relative to the case in which correlations are ignored, are obtained. Neutrino interactions in dense matter are especially important for supernova and early neutron star evolution calculations. The effects of correlations for protoneutron star evolution are calculated. Large effects on the internal thermodynamic properties of protoneutron stars, such as the temperature, are found. These translate into significant early enhancements in the emitted neutrino energies and fluxes, especially after a few seconds. At late times, beyond about 10 seconds, the emitted neutrino fluxes decrease more rapidly compared to simulations without the effects of correlations, due to the more rapid onset of neutrino transparency in the protoneutron star

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arXiv1997

Neutrino Interactions in Hot and Dense Matter

We study the charged and neutral current weak interaction rates relevant for the determination of neutrino opacities in dense matter found in supernovae and neutron stars. We establish an efficient formalism for calculating differential cross sections and mean free paths for interacting, asymmetric nuclear matter at arbitrary degeneracy. The formalism is valid for both charged and neutral current reactions. Strong interaction corrections are incorporated through the in-medium single particle energies at the relevant density and temperature. The effects of strong interactions on the weak interaction rates are investigated using both potential and effective field-theoretical models of matter. We investigate the relative importance of charged and neutral currents for different astrophysical situations, and also examine the influence of strangeness-bearing hyperons. Our findings show that the mean free paths are significantly altered by the effects of strong interactions and the multi-component nature of dense matter. The opacities are then discussed in the context of the evolution of the core of a protoneutron star.

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arXiv1996

Neutrino Scattering in a Newly Born Neutron Star

We calculate neutrino cross sections from neutral current reactions in the dense matter encountered in the evolution of a newly born neutron star. Effects of composition and of strong interactions in the deleptonization and cooling phases of the evolution are studied. The influence of the possible presence of strangeness-rich hyperons on the neutrino scattering cross sections is explored. Due to the large vector couplings of the Sigma-minus and Cascade-minus, |C_V|~2, these particles, if present in protoneutron star matter, give significant contributions to neutrino scattering. In the deleptonization phase, the presence of strangeness leads to large neutrino energies, which results in large enhancements in the cross sections compared to those in matter with nucleons only. In the cooling phase, in which matter is nearly neutrino free, the response of the Sigma-minus hyperons to thermal neutrinos is the most significant. Neutrinos couple relatively weakly to the Lambda hyperons and, hence, their contributions are significant only at high density.

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arXiv1996

Hydrodynamical Description of 200 A GeV/c S+Au Collisions: Hadron and Electromagnetic Spectra

We study relativistic S+Au collisions at 200 A GeV/c using a hydrodynamical approach. We test various equations of state (EOSs), which are used to describe the strongly interacting matter at densities attainable in the CERN-SPS heavy ion experiments. For each EOS, suitable initial conditions can be determined to reproduce the experimental hadron spectra; this emphasizes the ambiguity between the initial conditions and the EOS in such an approach. Simultaneously, we calculate the resulting thermal photon and dielectron spectra, and compare with experiments. If one allows the excitation of resonance states with increasing temperature, the electro-magnetic signals from scenarios with and without phase transition are very similar and are not resolvable within the current experimental resolution. With regard to the CERES dilepton data, none of the EOSs considered, in conjunction with the standard leading order dilepton rates, succeed in reproducing the observed excess of dileptons below the rho peak. Our work, however, suggests that an improved measurement of the photon and dilepton spectra has the potential to strongly constrain the EOS.

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arXiv1996

Composition and Structure of Protoneutron Stars

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. 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 of order $0.2M_\odot$, which adds to the effects of finite entropy. The presence of negatively-charged particles has two major implications. 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 or it is delayed for a neutrino diffusion timescale of $\sim 10$ s. The latter case is most likely if the maximum mass of the hot star with trapped neutrinos is near $1.5M_\odot$. 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.

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arXiv1995

Neutrino Scattering in Strangeness-Rich Stellar Matter

We calculate neutrino cross sections from neutral current reactions in dense matter containing hyperons. We show that Sigma- hyperons give significant contributions. To lowest order, the contributions from the neutral Lambda and Sigma0, which have zero hyper-charge, vanish. However, their presence furnishes baryon number which decreases the relative concentrations of nucleons. This leads to significant reductions in the cross sections. Due to the uncertainty in strong interactions at high density, the neutrino opacity may vary by a factor of about 2 depending on the behavior of the effective masses.

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arXiv1995

Neutrinos from Protoneutron Stars: A Probe of Hot and Dense matter

Neutrino processes in dense matter play a key role in the dynamics, deleptonization and early cooling of hot protoneutron stars formed in the gravitational collapse of massive stars. Here we calculate neutrino mean free paths from neutrino-hyperon interactions in dense matter containing hyperons. Significant contributions to the neutrino opacity arise from scattering and absorption reactions involving the sigma minus particle. The estimates given here emphasise the need for (a) opacities which incoparate many-body effects in a multi-component mixture, and (b) new calculations of thermal and leptonic evolution of protoneutron stars with neutrino transport and equations of state with strangeness-rich matter.

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arXiv1995

Kaon Condensation in Neutron Star Matter with Hyperons

Based on the Kaplan-Nelson Lagrangian, we investigate kaon condensation in dense neutron star matter allowing for the explicit presence of hyperons. Using various models we find that the condensate threshold is sensitive to the behavior of the scalar density; the more rapidly it increases with baryon density, the lower is the threshold for condensation. The presence of hyperons, particularly the $Σ^-$, shifts the threshold for $K^-$ condensation to a higher density. In the mean field approach, with hyperons, the condensate amplitude grows sufficiently rapidly that the nucleon effective mass vanishes at a finite density and a satisfactory treatment of the thermodynamics cannot be achieved. Thus, calculations of kaon-baryon interactions beyond the mean field level appear to be necessary.

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