N. Riazi

Thermodynamics of rotating solutions in (n+1)-dimensional Einstein-Maxwell-dilaton gravity

We construct a class of charged, rotating solutions of (n+1)-dimensional Einstein-Maxwell-dilaton gravity with cylindrical or toroidal horizons in the presence of Liouville-type potentials and investigate their properties. These solutions are neither asymptotically flat nor (anti)-de Sitter. We find that these solutions can represent black brane, with inner and outer event horizons, an extreme black brane or a naked singularity provided the parameters of the solutions are chosen suitably. We also compute temperature, entropy, charge, electric potential, mass and angular momentum of the black brane solutions, and find that these quantities satisfy the first law of thermodynamics. We find a Smarr-type formula and perform a stability analysis by computing the heat capacity in the canonical ensemble. We find that the system is thermally stable when the coupling constant between the dilaton and matter field {alpha}{ 1 the system has an unstable phase. This shows that the dilaton field makes the solution unstable, while it is stable even in Lovelock gravity.

Magnetic branes in (n+1)-dimensional Einstein-Maxwell-dilaton gravity

We construct two new classes of spacetimes generated by spinning and traveling magnetic sources in (n+1)-dimensional Einstein-Maxwell-dilaton gravity with Liouville-type potential. These solutions are neither asymptotically flat nor (A)dS. The first class of solutions which yields a (n+1)-dimensional spacetime with a longitudinal magnetic field and k rotation parameters have no curvature singularity and no horizons, but have a conic geometry. We show that when one or more of the rotation parameters are nonzero, the spinning branes have a net electric charge that is proportional to the magnitude of the rotation parameters. The second class of solutions yields a static spacetime with an angular magnetic field and has no curvature singularity, no horizons, and no conical singularity. Although one may add linear momentum to the second class of solutions by a boost transformation, one does not obtain a new solution. We find that the net electric charge of these traveling branes with one or more nonzero boost parameters is proportional to the magnitude of the velocity of the branes. We also use the counterterm method and calculate the conserved quantities of the solutions.

Rotating Solutions of Einstein-Maxwell-Dilaton Gravity with Unusual Asymptotics

We study electrically charged, dilaton black holes, which possess infinitesimal angular momentum in the presence of one or two Liouville type potentials. These solutions are neither asymptotically flat nor (anti)-de Sitter. Some properties of the solutions are discussed.

SOLITON DECAY IN A COUPLED SYSTEM OF SCALAR FIELDS

A system of coupled scalar fields is introduced which possesses a spectrum of massive single-soliton solutions. Some of these solutions are unstable and decay into lower mass stable solitons. Some properties of the solutions are obtained using general principles including conservations of energy and topological charges. Rest energies are calculated via a variational scheme, and the dynamics of the coupled fields are obtained by solving the field equations numerically.

Time-dependent wormholes in an expanding universedominated by traceless matter

Time-dependent wormhole solutions are found which evolve in a cosmological background. Solutions are presented both for GR and Brans-Dicke field equations. Conditions are derived for the supporting matter to be non-exotic. The traceless energy-momentum tensor needed to support the geometry is in the form of an anisotropic fluid. Far from the wormhole, the equation of state rapidly approaches that of an isotropic perfect fluid with p = 1/3 ρ. For the BD wormholes we obtain ρ = 0everywhere, except for the π = const. limit, in which case the GR results are reproduced.

(n+1)-dimensional Lorentzian wormholes in an expanding cosmological background

We discuss (n + 1)-dimensional dynamical wormholes in an evolving cosmological background with a throat expanding with time. These solutions are examined in the general relativity framework. A linear relation between diagonal elements of an anisotropic energymomentum tensor is used to obtain the solutions. The energy-momentum tensor elements approach the vacuum case when we are far from the central object for one class of solutions. Finally, we discuss the energy-momentum tensor which supports this geometry, taking into account the energy conditions .

A class of cosmological solutions of Brans-Dicke theory with cosmological constant

We present a class of exact cosmological solutions of Brans-Dicke (B-D) equations with cosmological constant in flat Robertson-Walker metric. These solutions are based on the relation øRn= constant between the B-D field and the scale factor of the universe. This relation turns out to be consistent with the equation of statep =mρ for the cosmic matter, provided thatn andm are suitably related to each other. Several special cases and asymptotic solutions are derived and discussed.

B and V photometry and analysis of the eclipsing binary RZ Cas

Photoelectric light curves of the eclipsing binary RZ Cas are presented forB andV filters. The light curves are analysed for light and geometrical elements, starting with a previously suggested preliminary method. The approximate results thus obtained are then optimised through the Wilson-Devinney computer programs.

Hα-wide and narrow-band photometry of R Canis Majoris

Hα wide and narrow-band photoelectric observations of the eclipsing binary R CMa were made at the Biruni Observatory between 18 February to 2 March, 1983. The light curves obtained from the observations are analysed according to Kopals method for the orbital elements by using frequency-domain techniques. A total of obvervations was secured and light curves were formed from these data. From the available evidence it appears likely that an exchange has taken place on the angle of the first contact and is probably continuing at the present time in the system. New geometric and photometric elements are derived and new value for the angle of the first contact is also given.

Equation of State of Newborn Neutron Star Matter with Untrapped Neutrinos

The equation of state of the newborn neutron star matter with untrapped neutrinos is calculated with the AV18 potential along isentropic paths. The same calculations are done with the AV14 potential for the sake of comparison. Temperature–density correlation, proton fraction, adiabatic index, and the velocity of sound are also obtained at different entropies. It is shown that the proton fraction (adiabatic index) increases (decreases) by increasing entropy. We have shown that our calculated equations of state obey the causality condition. The results are compared with those of others in the literature.