Advances in Astrophysics

6

Authors: Raj Bali, Parmit Kumari

Inflationary universe scenario with constant deceleration parameter in the presence of massless scalar field and flat potential taking Bianchi Type VI0 space time as a source is discussed. We find that the rate of expansion slows down with increase of time. It is also observed that the ratio of shear and expansion is non-zero for all values of T where at + b = T, t being cosmic time, a and b being constants. Thus the universe remains anisotropic throughout the evolution. The Higgs field is constant for large values of T when α < 2 and the Higgs field evolves slowly but the universe expands for α > 2 where α is a constant. It may be positive and negative both. The model represents decelerating and accelerating phases of universe and has Point Type singularity at T=0 (MacCallum [25]). In special case i.e. if N=0 and α > 0 then the model isotropizes at late time, N being constant of integration.

9

Authors: M.V. Dawande, K. S. Adhav, S. S. Nerkar

We have studied the spatially homogeneous and anisotropic Locally Rotationally Symmetric (LRS) Bianchi type-I universe in F(T) theory of gravity. By using a conservation equation, we have discussed some well known F(T) models. It is interesting to observe that these F(T) gravity models represent the different phases (matter, radiation and dark energy eras) of the universe. An attempt has been made to retain Sharif and Rani’s [1] forms of the various quantities. Our results are analogous to the results obtained by Sharif and Rani.

10

Authors: Smibi M.J., Harishkumar Sellamuthu, Ram Krishan Sharma

A new non-singular, analytical theory with respect to the Earth’s zonal harmonic term J2 has been developed for short-periodic motion, by analytically integrating the uniformly regular KS canonical equations of motion using generalized eccentric anomaly ‘E’ as the independent variable. Only one of the eight equations needs to be integrated analytically to generate the state vector, as a result of symmetry in the equations of motion, and the computation for the other equations is by changing the initial conditions. The integrals are much simpler than earlier obtained in [20] in terms of the independent variable ‘s’. Numerical results indicate that the solution is reasonably accurate for a wide range of orbital parameters during a revolution. The error in computing the most important orbital parameter ‘semi-major axis’ which is the measure of energy is less than five percentage during a revolution. The analytical solution can have number of applications. It can be used for studying the short-term relative motion of two or more space objects. It can also be useful in collision avoidance studies of space objects. It can be used for onboard computation in the navigation and guidance packages, where the modeling of J2 effect becomes necessary.

14

Authors: Cristiana Dumitrache

We have performed numerical magnetohydrodynamic (MHD) simulations of two closed active regions (AR). The input magnetic field values were the coronal magnetic field computed as extrapolation coronal from observations of the photospheric magnetic field. The studied active regions, NOAA AR12565 and AR12567, were registered as different bipolar region. Our investigation, the 3D coronal extrapolations, as well as the numerical MHD experiments, revealed that actually they evolved together as a quadrupolar active region. The second region emerged later under the loops system of AR12565 and separated from this one. A natural current sheet formed between then and it plays an important role in the explosive events (flares and coronal mass ejections) occurrence.