Assuming a Ðrst-order phase transition from nuclear to quark matter in neutron stars and in supernova cores, we have studied the phase transition from two-Ñavor quark matter to strange matter. This transition has bearing on the cooling of neutron stars and may lead to observable signals in the form of a second neutrino burst. In the case of transition occurring in a supernova core, it has the e †ect of raising the core temperature and the energy of the shock wave and thus a †ecting the evolution of the core. In this study we have systematically taken into account the e †ect of strong interactions perturbatively to order and the e †ect of Ðnite temperature and strange quark mass.a c

We have studied the bulk viscosity of strange quark matter in the density dependent quark mass model (DDQM) and compared results with calculations done earlier in the MIT bag model where Ù, masses were neglected and first order interactions were taken into account. We find that at low temperatures and high relative perturbations, the bulk viscosity is higher by 2 to 3 orders of magnitude while at low perturbations the enhancement is by 1-2 order of magnitude as compared to earlier results. Also the damping time is 2-3 orders of magnitude lower implying that the star reaches stability much earlier than in MIT bag model calculations.

We report on the study of the mass-radius (M -R) relation and the radial oscillations of proto strange stars. For the quark matter we have employed the very recent modification, the temperature and density dependent quark mass model of the well known density dependent quark mass model. We find that the maximum mass the star can support increases significantly with the temperature of the star in this model which implies that transition to a black hole at the early stage of formation of the star is inhibited. As for the effect of the neutrinos, we find, contrary to expectation, that the values of mass, radius and oscillation frequencies are almost independent of the neutrino chemical potentials.

It has recently been pointed out that many models of dense nuclear matter allow the direct URCA process in highly dense systems like neutron stars. In view of this, we have calculated the energy loss in dense nuclear matter as a result of such processes at finite temperatures. For the description of nuclear matter we have used Walecka's mean-field theory. For the neutrino emissivity rates we have employed the Iwamoto model and evaluated the angular integrals involved exactly. We find that the emissivity rate is density dependent, being almost a constant at high densities, while at low densities it shows an exponential behaviour with temperature rather than the usual power law.

The eigenfrequencies of radial pulsations of quark stars are calculated in a general relativistic formalism given by Chandrasekhar in the density-dependent quark mass model in strong magnetic Ðelds. It is found that the squares of the frequencies are always decreasing functions of the central density of the strange star. The maximum mass, the radius, and gravitational redshift of the star are increasing functions of the magnetic Ðeld.

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Abstract.We rederive the bulk viscosity of strange quark matter from the dominant reaction u + s ~ d + u by taking the effect of temperature and quark-gluon coupling perturbatively to first order in the chemical composition of the quark matter. We also calculate the contribution from the [3-decay processes s(d)~u+e+~ and u + e --, s(d) + v and show that this contribution has different temperature dependence and can even be larger than the contribution from the former reaction at temperatures of the order of the electron Fermi energy. PACS" 97.60.Gb; 12.38.Mh; 97.60.Jd

The equation of state, the magnetic moment, as well as a Curie-type law for the susceptibility parameter of a system of particles of spin 1/2 in the presence of an external magnetic field are calculated assuming that these particles obey generalized statistics. Here one assumes that the occupation number of an energy state can take the values 0, 1, 2, ..., 1 and no more, where I is an integer greater than unity. This is a direct consequence of the generalized method of field quantization originally due to Green. Exact expressions for the thermodynamic and magnetic properties due to the generalized nature of the statistics are given, and the characteristic departures of the present results from those of the Fermi-Dirac case are pointed out. Our results are useful for investigating the thermodynamic state and spontaneous magnetization of the highly compressed state of matter, presumably of quarks, obtained in a star in gravitational collapse. The quarks are assumed to obey generalized F e m i statistics known in the literature as para-Fermi statistics.