AR_final file_2018-19

work has been done in collaboration with Nikhil Hulke and Ashutosh Singh. Harinder Pal Singh On the variation of light curve parameters of RR Lyrae variables at multiple wavelengths We present a detailed light curve analysis of RR Lyrae variables at multiple wavelengths us- ing Fourier decomposition method. The time-series data for RR Lyrae variables in the galactic bulge and the Magellanic Clouds are taken from the Opti- cal Gravitational Lensing Experiment survey, while the infrared light curves are compiled from the lit- erature. We also analyse the multiband theoretical light curves that are generated from the stellar pul- sation models of RR Lyrae stars for a wide range of metal-abundances. We find that the theoretical light curve parameters with different metal abun- dances are consistent with observed parameters in most period bins at both optical and infrared wave- lengths. The theoretical and observed Fourier am- plitude parameters decrease with increase in wave- length, while the Fourier phase parameters increase with wavelength at a given period. We use absolute magnitudes for a subset of theoretical models that fit the observed optical RR Lyrae light curves in the Large Magellanic Cloud to estimate a distance modulus, µ LMC = 18 . 51 ± 0 . 07, independent of the metallicity. We also use Fourier analysis to study the period-colour and amplitude-colour relations for RR Lyrae stars in the Magellanic Clouds us- ing optical data, and find that the slope of period- colour relation at minimum light is very shallow or flat and becomes increasingly significant at the maximum light for RRab stars. We also find that the metallicity dependence of the period-colour re- lations increases as we go from minimum to max- imum light, suggesting that the mean light results are indeed an average of the various pulsational phases. We summarize that the average variation in these relations is consistent between theory and observations, and supports the theory of the inter- action of the stellar photosphere and the hydrogen ionization front. This analysis has been done in col- laboration with Susmita Das, Anupam Bhardwaj, Shashi M. Kanbur, and Marcella Marconi. Search for extra-tidal RR Lyrae stars in Milky Way globular clusters from Gaia DR2 We used extra-tidal RR Lyrae stars to study the dynamics of galactic globular clusters, and know how effects like dynamical friction and tidal dis- ruption affect these clusters. The Gaia DR2 cata- logue for RR Lyrae stars (Clementini, et al. 2018) is used along with the proper motions and tidal radii data for the globular clusters compiled from literature. A sample of 56 galactic globular clusters is analysed. Out of these 56, only 11 have extra- tidal RR Lyrae stars. However, only two clusters, namely, NGC 3201 and NGC 5024, have enough extra-tidal RR Lyrae stars to draw interesting con- clusions. NGC 3201, has 13 extra-tidal RR Lyrae stars, which are asymmetrically distributed around its centre with more number of stars in its trailing zone than its leading part. We conclude that these asymmetrical tidal tails are due to the combined effect of tidal disruption and the stripped debris from the cluster. On the other hand, NGC 5024 has five extra-tidal RR Lyrae stars, four of them are concentrated in a region which, is at a distance of about three times the tidal radius from its cen- tre. These may be the stars that are being ripped apart from the cluster due to tidal disruption. The presence of these extra-tidal RR Lyrae stars in the clusters can be an indication that more cluster stars are present outside their tidal radii, which may be revealed by deep wide field colour-magnitude dia- grams of the clusters. This work has been done in collaboration with Richa Kundu, and Dante Min- niti. Vikram Soni A new solution for city water: Quality drinking wa- ter from the river floodplains Cities world over are facing drinking water prob- lem. The planners are often over emphasizing on sourcing surface water from far off places. This will involve exorbitant cost, and many a times di- version of river flow beyond the permissible limit. Obviously such river flow diversions will have ad- verse ecological consequences. In this context, the study reinvents traditional knowledge with sound scientific rigour. It argues for ecologically sustain- able local solutions for meeting the drinking water need of cities from floodplain of rivers. A case of river Yamuna in Delhi has been discussed to high- light the potential of floodplain aquifer as a drink- ( 215 )