AR_final file_2018-19

reconstructed potential and scalar field of tachyon based on the standard Chaplygin gas. It has been observed that the equation of state parameter due to the reconstructed torsion contribution to the density and pressure behaves like quintom, and is consistent with the observational value of the equation of state parameter for the current uni- verse. The reconstructed f ( T ) has then been tested for gravitational perturbation by deriving the fric- tional term, the effective mass and the sound speed parameter for the gravitational potential, and it has been found to be stable against gravitational perturbations through positive value of the squared speed of sound. Finally, statefinder diagnostics has been carried out and the reconstructed f ( T ) grav- ity is found to interpolate between dust and ΛCDM phases of the universe. Tanuka Chattopadhyay Star formation under explosion mechanism in a magnetized medium The model of star formation has been developed as a result of explosive phenomena in the central region of our Galaxy. The shock wave generated as a result of such explosion, during its propagation, cools and compresses the ambient medium in a thin layer, which subsequently fragments into molecular clouds. These clouds finally form star clusters or field stars under gravitational collapse. We primar- ily consider the central region of our Galaxy un- der the influence of a magnetic field for modelling such explosive phenomena and derive the minimum Jeans mass for gravitational collapse. It is found that under an inverse variation of temperature with density, a wide range of fragments can be formed. The mass range is enhanced in the presence of a constant as well as varying magnetic field. Un- der suitable physical conditions a burst of star for- mation is possible. It is also found that rotation of such fragmented clouds of the order of few km s − 1 kpc − 1 , might lead to a stable structure. This work has been done in collaboration with Debasish Mondal. Fragmentation of molecular cloud in a polytropic medium In search of the scenario of the star formation pro- cess, fragmentation of molecular clouds has been modelled under two different conditions. In the first case, thermal instability along with an equa- tion of state with negative polytropic indices (n) has been considered, which give rise to minimal fragment masses in the range 0 . 001 M ⊙ to 8 . 5 M ⊙ for − 10 < n < 0 , 6 = − 1. In the second case, an opacity limited fragmentation along with positive polytropic indices is considered, which leads to a wide range of minimal fragment masses resulting in field stars, massive stars, and star bursts. This might be due to the effect of opacity, which is re- sponsible for slow dissipation of heat in a com- pressed medium. This work has been done in col- laboration with Ashok Mondal. Raghavendra Chaubey Dynamical analysis of anisotropic cosmological model with quadratic dark sector coupling The present work deals with the dynamical evolu- tion of LRS Bianchi type I (LRS BI) cosmological model with quadratic dark sector coupling. We in- vestigate the phase-plane analysis of LRS BI model with dark energy, when it is modelled as exponen- tial quintessence, and is coupled to dark matter via energy exchange. The evolution of cosmolog- ical solutions is studied by using dynamical sys- tems method. Stability and viability of the models are discussed for four different choices of quadratic dark sector coupling parameter. In each model, we have obtained a late-time accelerating fixed point (future attractor), which is not a scaling solution. This work has been done in collaboration with Rakesh Raushan. Aspects of non-flat FRW bouncing models with quadratic equation of state In this work, we investigate the possibility of a non-singular model of universe in the framework of general relativity in non-flat FRW geometries with quadratic equation of state and bulk viscos- ity. We study whether a non-singular bounce re- quires violation of energy conditions. We discuss the thermodynamical aspects of the resulting mod- els with equilibrium description. In particular, we discuss the validity of the generalized second law of thermodynamics for resulting cosmologies. This work has been done in collaboration with Ashutosh Singh, Rakesh Raushan, and T. Singh. ( 182 )