S. Singh

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.

An atomic beam fluorescence locked magneto-optical trap for krypton atoms

We report here an atomic beam loaded magneto-optical trap (MOT) for metastable krypton atoms in which the fluorescence signal from the atomic beam is used to lock the cooling laser frequency. The fluorescence signal is generated by exciting the metastable krypton atomic beam using a probe laser beam (i.e. part of the cooling laser beam) intersecting the atomic beam at an angle. A spectral shift in the fluorescence signal can be achieved by varying the angle between the probe laser beam and the atomic beam to obtain the desired frequency detuning to lock the cooling laser frequency. This has been used to optimize the number of cold atoms in the MOT. The dependence of the peak height and slope of the atomic beam fluorescence (ABF) locking signal on the RF power in the discharge tube and pressure in the observation chamber of the setup has been studied to correlate its effect on the number of atoms in the MOT.

A computational model for transient temperature rise in a dye laser gain medium pumped by a copper vapor laser

Spectrally stable dye lasers play an important role in techniques based on high resolution spectroscopy and atomic spectroscopy. The spectral purity of a dye laser is affected when the pump power to it is increased beyond the threshold. When the pump power is increased beyond the threshold, two mode oscillations occur which decrease the spectral purity of the dye laser. The effect of higher pump pulse energies on transient thermal effects has been studied using a computational fluid dynamics (CFD) model and the disturbances to the laser cavity have been studied using commercially available ray tracing software. The change in the cavity length was determined from the CFD model for several dye concentrations and pump powers. The results of the CFD model have been verified by published results and experimental results from our system. Our study shows that in the longitudinally pumped single mode laser change in the cavity length is a more dominant disturbance than thermal blooming. Our model is useful for the design of the dye cell.

Velocity selective bipolarization spectroscopy for laser cooling of metastable krypton atoms

We report a velocity selective bipolarization spectroscopy (VS-BPS) technique to generate a background-free, dispersion-like reference signal which is tunable over a wide range of frequency. In this technique, a pair of linearly polarized weak probe beams passing through a gas cell of metastable krypton (Kr*) atoms overlaps with a pair of counterpropagating circularly polarized strong pump beams derived from an independently tunable control laser. The polarization spectroscopy signals from the two probe beams, after subtraction, result in the VS-BPS signal. The spectral shifting in the VS-BPS signal can be achieved by tuning the frequency of the control laser. The dependence of the amplitude and slope of the VS-BPS signal on the RF power used for excitation of Kr atoms in the gas cell and on the power of pump beams has been studied. The frequency stability of a diode laser locked with VS-BPS signal has been found to be better than the frequency stability of the laser locked with a saturated absorption spectroscopy signal. The VS-BPS signal is finally used for stabilization and tuning of the cooling laser frequency for a magneto-optical trap for Kr* atoms.

Laser frequency stabilization and large detuning by Doppler-free dichroic lock technique: Application to atom cooling

We present results of a study of frequency stabilization of a diode laser (λ = 780 nm) using the Doppler-free dichroic lock (DFDL) technique and its use for laser cooling of atoms. Quantitative measurements of frequency stability were performed and the Allan variance was found to be 6.9 × 1011 for an averaging time of 10 s. The frequency-stabilized diode laser was used to obtain the trapping beams for a magneto-optic trap (MOT) for Rb atoms. Using the DFDL technique, the laser frequency could be locked over a wide range and this enabled measurement of detuning dependence of the number and temperature of cold atoms using a relatively simple experimental set-up.

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.