J. D. Anand

Burning of Two-Flavor Quark Matter into Strange Matter in Neutron Stars and in Supernova Cores

Assuming a first-order phase transition from nuclear to quark matter in neutron stars and in supernova cores, we have studied the phase transition from two-flavor 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 effect of raising the core temperature and the energy of the shock wave and thus affecting the evolution of the core. In this study we have systematically taken into account the effect of strong interactions perturbatively to order ?c and the effect of finite temperature and strange quark mass.

Radial Oscillations of Quark Stars in Strong Magnetic Fields

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 fields. 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 field.

On the cooling of neutron stars

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 Waleckas 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.

On quark matter in a strong magnetic field

The effect of strong magnetic field on the bulk properties of quark matter is reinvestigated takingu, d ands-quarks as well as electrons in the presence of magnetic field. Here the bag pressure is chosen such that in the absence of magnetic field and at zero temperature the binding energy of theuds-system is <930 MeV while that ofud-system is greater than 940 MeV. It is observed that the equation of state changes significantly in a strong magnetic field. At finite temperature the electron chemical potential varies between 6 and 50 MeV. Thus the expansion of thermodynamical quantities in powers ofT/(Μi2-Mv(i)2)1/2 is valid only up to few MeV. For high temperatures ∼40 MeV the exact integral expressions are to be taken.

EFFECT OF MAGNETIC FIELD ON THE PHASE TRANSITION FROM NUCLEAR MATTER TO QUARK MATTER DURING PROTO-NEUTRON STAR EVOLUTION

We have studied phase transition from hadron matter to quark matter in the presence of high magnetic fields incorporating the trapped electron neutrinos at finite temperatures. We have used the density dependent quark mass (DDQM) model for the quark phase while the hadron phase is treated in the frame-work of relativistic mean field theory. It is seen that the energy density in the hadron phase at phase transition decreases with both magnetic field and temperature.

Bulk viscosity of neutron stars

Viscosity of neutron stars has been a continuing area of research for many years now. Recently interest in this field has revived because of the possibility of URCA processes in neutron stars. In this paper we report calculation of the bulk viscosity of neutron stars from these processes. For this purpose we have used theβ-decay rates which were calculated without making the usual approximations of neglecting the neutrino momentum and using the nuclear mean field theory for the description of interacting nuclear matter. Also we have not restricted our calculation to the linear regime which corresponds to the assumption that fluctuations in the chemical potential away fromβ-equilibrium remain small: Δμ/kT ≪ 1. We find that for large amplitude fluctuations, where the linear approximation is not valid, bulk viscosity increases by many orders of magnitude. Also, as against strange matter stars, where the viscosity first increases with increasing temperature and then starts decreasing beyond 0.1 MeV, we find that the viscosity increases uniformly with temperature at least up to 2MeV. We discuss the implications of these results for the stability of neutron stars.

STUDY OF PROTO STRANGE STARS IN TEMPERATURE AND DENSITY DEPENDENT QUARK MASS MODEL

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.

Quasi-radial oscillations of rotating strange stars in strong magnetic fields

In this paper we study quasi-radial oscillations of slowly rotating strange stars in strong magnetic fields in the density-dependent quark mass (DDQM) model. We see that the difference in frequency of rotating and non-rotating stars is more for higher magnetic fields. The change is small for low-mass stars but it increases with the mass of the star. This change of frequency is significant for maximum mass whereas it is marginal for a 1.4M⊙ star.

Neutrino emissivity of quark matter at finite temperatures

We evaluate the emissivity rates for d-decay and s-decay by exactly solving the angular integrals involved and without assuming the degeneracy of electrons. We have also studied the effects of QCD coupling constant as well as the s-quark mass on the emissivity rates. We find that these parameters are important in determining the threshold and extinction densities for d- and s-decays.

Radial oscillations of neutron stars in strong magnetic fields

The eigen frequencies of radial pulsations of neutron stars are calculated in a strong magnetic field. At low densities we use the magnetic BPS equation of state (EOS) similar to that obtained by Lai and Shapiro while at high densities the EOS obtained from the relativistic nuclear mean field theory is taken and extended to include strong magnetic field. It is found that magnetized neutron stars support higher maximum mass whereas the effect of magnetic field on radial stability for observed neutron star masses is minimal.