Bianchi type-II space-times with constant deceleration parameter in self creation cosmology

The properties of locally rotationally symmetric Bianchi type-II perfect fluid space-times are analyzed in Barber’s second self-creation theory by using a special law of variation for Hubble’s parameter that yields a constant value of deceleration parameter. By assuming the equation of state p=γ ρ, many new solutions are obtained for different era—Zel’dovich, radiation, vacuum and vacuum energy dominated. The solutions with power-law and exponential expansion are discussed. A detailed study of geometrical and physical parameters is carried out. The nature of singularity is also clarified in each case.     

Turbulent compressible convection with rotation–penetration below a convection zone

We examine the behaviour of penetrative turbulent compressible convection under the influence of rotation by means of three dimensional numerical simulations. We estimate the extent of penetration below a stellar-type rotating convection zone in an f-plane configuration. Several models have been computed with a stable-unstable-stable configuration by varying the rotation rate (Ω), the inclination of the rotation vector and the stability of the lower stable layer. The spatial and temporal average of kinetic energy flux (F_k) is computed for several turnover times after the fluid has thermally relaxed and is used to estimate the amount of penetration below the convectively unstable layer. Our numerical experiments show that with the increase in rotational velocity, the downward penetration decreases. A similar behaviour is observed when the stability of the lower stable layer is increased in a rotating configuration. Furthermore, the relative stability parameter S shows an S ^−1/4 dependence on the penetration distance implying the existence of a thermal adjustment region in the lower stable layer rather than a nearly adiabatic penetration region.     

Slowly rotating cosmological perfect fluids in Brans-Dicke theory

The Brans-Dicke field equations for a perfect fluid distribution representing slowly rotating fluid spheres are investigated. Exact solutions are obtained for differential rotation and perfect dragging by imposing some physical restrictions on the matter rotation (r,t). The physical properties are discussed fork=±1.     

Scanning sky monitor (SSM) onboard AstroSat

Scanning Sky Monitor (SSM) onboard AstroSat is an Xray sky monitor in the soft X-ray band designed with a large field of view to detect and locate transient X-ray sources and alert the astronomical community about interesting phenomena in the X-ray sky. SSM comprises position sensitive proportional counters with 1D coded mask for imaging. There are three detector units mounted on a platform capable of rotation which helps covering about 50% of the sky in one full rotation. This paper discusses the elaborate details of the instrument and few immediate results from the instrument after launch.     

On the stability of twisted magnetic fields

The stability of helical magnetic fields is investigated when fluid motions are present along the lines of force. The general dispersion relation is obtained and some limiting cases are examined. It is established that the configuration can be unstable when the velocity field exceeds a certain critical value. This result is to be compared with the case when the helical fields confined by a rigid boundary are stable when the energy density in the velocity field is at least equal to that of the magnetic field.     

Equilibrium structure of differentially rotating polytropic models of the stars

In this paper we propose a method for computing the equilibrium structure of differentially rotating polytropic models of the stars. A general law of differential rotation of the type ²=b ₀+b ₁ s ²+b ₂ s ⁴, which can account for a reasonably large variety of possible differential rotations in the stars has been used. The distortional effects have been incorporated in the structure equations up to second order of smallness in distortion parametersb ₀,b ₁, andb ₂ using Kippenhahn and Thomas' averaging approach in conjunction with Kopal's results on Roche equipotentials in manner similar to the one earlier used by Mohan and Saxena for computing the equilibrium structure of polytropes having solid body rotation. Numerical results have been obtained for various types of differentially rotating polytropic models of stars of polytropic indices 1.5, 3, and 4. Certain differentially rotating models of the Sun which are possible with such a type of law of differential rotation, have also been computed.     

Propagation of magneto-radiative shock waves,II

Self-similar motion of a perfect gas behind a cylindrical shock wave with radiation heat flux in the presence of an azimuthal magnetic field have been discussed. The shock is assumed to be propagating in a medium at rest with non-uniform density. The conductivity of the gas is infinite and magnetic permeability is one everywhere. Also, the shock is assumed to be transparent and isothermal.     

Haser model CN,C2 and C3 production rates in some comets

Emission fluxes of CN, C₂ and C₃ species observed in the coma of some comets are analysed in the framework of Haser model. CN, C₂ and C₃ production rates are determined using recently derived fluorescence efficiencies and dependence of CN, C₂ and C₃ production rates on the heliocentric distance is studied. Evidence for a burst type activity around January 15, 1974 in the post-perihelion period of comet Kohoutek (1973f) is observed.     

Self-similar power-driven isothermal flows behind cylindrical shock in magnetohydrodynamics

The present paper deals with a self-similar power-driven isothermal expansion headed by magnetohydrodynamic cylindrical shock surface in a non-homogeneous medium. It has been found that the region, behind the discontinuity surface, widens when interaction of magnetic field with other gasdynamic variables is perceived.Calculation has been performed for different Alfvén Mach numbers.     

An analysis of spectrophotometric observations of comets Austin (1982g) and Bradfield (1980t)

Post-perihelion observed emission fluxes at 388 nm (CN) and 516 nm (C₂) of the coma of comets Austin (1982g) and Bradfield (1980t) are analysed in the framework of the Haser model. Ratios of Haser model CN and C₂ parent production rates with expansion velocity show that each comet behaves normally. For comet Austin (1982g), the Q _CN/v and Q _c2/v values decrease with increase of heliocentric distance of comet. For an assumed %; activity of the total spherical surface area of the nucleus, the water vaporization theory coupled with derived water production rates from the International Ultraviolet Explorer H and OH flux data yields a nuclear diameter of about 6 km for comet Austin (1982g). For comet Bradfield (1980t), the derived nuclear diameter is expected to be of about 1 km. In each comet, the dust mass production rates as well as ratio of dust-to-gas mass production rates decrease with increase of heliocentric distance of comet.