AR-2019-2020

generalization of the Vaidya and Tikekar super- dense stellar model. In this treatment, the Ein- stein field equations have been solved in terms of hypergeometric functions. Subsequently, for par- ticular parameter choices, the solutions have been expressed in terms of elementary functions. In this treatment, stellar systems with is tropic pres- sure can be generated in the limit of vanishing anisotropy. Consequently, some of the well known superdense star models, which are isotropic and have specific spheroidal geometries have been re- gained. The impact of anisotropy on the gross physical properties of a compact star has been an- alyzed. This work is done in collaboration with S. Thirukkanesh, and Sunil D. Maharaj. Gyan Prakash Singh Variable Chaplygin gas cosmologies in f ( R, T ) gravity with particle creation A flat FLRW cosmological model with perfect fluid comprising of variable Chaplygin gas has been studied in the context of f ( R, T ) gravity with par- ticle creation. The solutions of the modified field equations are obtained through three different con- sidered form of scale factors. The effective pres- sure is negative throughout the evolution of uni- verse, which leads to accelerated expansion of the universe. In addition to that, we have also dis- cussed the importance of particle creation pres- sure on the cosmological parameters, energy con- ditions and state-finder diagnostic parameters. It is noticed that, the time evolution of source func- tion yields almost constant particle production at late times.This work has been done in collaboration with Nikhil Hulke, Binaya K. Bishi, and Ashutosh Singh. Harinder Pal Singh The tale of the Milky Way Globular Cluster NGC 6362 – I. The orbit and its possible extended star debris features as revealed by Gaia DR2 We report the identification of possible extended star debris candidates beyond the cluster tidal radius of NGC 6362 based on the second Gaia data release ( Gaia DR2). We found 259 objects possibly associated with the cluster lying in the vicinity of the giant branch and 1–2 magnitudes fainter/brighter than the main-sequence turn-off in the cluster colour-magnitude diagram, and which cover an area on the sky of ∼ 4.1 deg 2 centered on the cluster. We traced back the orbit of NGC 6362 in a realistic Milky-Way potential, using the GravPot16 package, for 3 Gyrs. The orbit shows that the cluster shares similar orbital properties as the inner disk, having peri-/apo-galactic distances, and maximum vertical excursion from the Galactic plane inside the corotation radius (CR), moving in- wards from CR radius to visit the inner regions of the Milky Way. The dynamical history of the clus- ter reveals that it has crossed the Galactic disk sev- eral times in its lifetime and has recently undergone a gravitational shock, ∼ 15 . 9 Myr ago, suggesting that less than 0.1% of its mass has been lost during the current disk-shocking event. Based on the clus- ter’s orbit and position in the Galaxy, we conclude that the possible extended star debris candidates are a combined effect of the shocks from the Galac- tic disk and evaporation from the cluster. Lastly, the evolution of the vertical component of the an- gular momentum shows that the cluster is strongly affected dynamically by the Galactic bar poten- tial. This work has been done in collaboration with Richa Kundu, Jose G. Fernndez-Trincado, Dante Minniti, Edmundo Moreno, Cline Reyl et, al. Unveiling Vela - variability of interstellar lines in the direction of the Vela supernova remnant - III. Na D and Ca II K High-resolution optical spectra were obtained in 2017-2019 with the Southern African Large Tele- scope of fifteen stars in the direction of the Vela supernova remnant. Interstellar Ca ii H and K and Na i D lines are discussed. In particular, the line profiles are compared with profiles at a comparable spectral resolution obtained in 1993- 1996 by Cha and Sembach. Ten of the lines of sight show changes to one or more of the compo- nents in that line of sight. Changes include small changes (1-2 km s − 1 ) in radial velocity and/or increases/decreases in equivalent width over the two decades between the periods of observation. Changes are more obvious in the Ca K line than in the Na D lines. These changes are attributed to gas disturbed by interactions between the supernova ejecta and the surrounding interstellar medium. A representative timescale may be 20-50 years. Small-scale variations in line profiles across the face of the remnant suggest, as previously remarked that a linear scale for interactions is a small fraction of the 40 pc size of the present remnant.