Photometric, kinematic, and variability study in the galactic open clusters

Abstract

Open stars clusters are excellent laboratories to test stellar and dynamical evolution theories as they host coeval stars situated at the approximately same distance. The physical parameters like kinematic motions, age, distance, and chemical composition are important in studies of the formation and evolution of stars as well as their hosting open cluster. Recently, open clusters have been interesting targets for the investigations of the possibilities of multiple populations inferred from the presence of extended main sequence turn-off (eMSTO). The stars in the open clusters show various types of variability at different stages of their evolution. The study of these variable stars in the star clusters offer to understand stellar evolution scenarios at various stages. The objective of the thesis is to explore the stellar and dynamical evolution of the open clusters through photometric, kinematic, and variability studies. We carried out photometric and kinematic study of ten open clusters using UBVRcIc data, near-IR data from 2MASS, Gaia and Pan-STARRS data to accomplish our objectives. We also carried out long-term observation for variability analysis in NGC 1960, NGC 559, and NGC 381. We found a rich variety of variable stars including pulsating, rotational, and eclipsing binaries. The member stars in the clusters are identified through membership probability calculation using proper motions and parallaxes from Gaia DR2 & eDR3 kinematic data. The extinction values for the clusters are calculated with the help of colour-colour diagram (CMD) constructed using optical and near-IR data. The anomalous reddening laws are found in the direction of open clusters NGC 381, IC 1442, and King 21. This anomaly in the law indicates larger dust grain size in the directions of these clusters than the dust grain size in the diffused interstellar medium. The ages of the clusters are estimated by fitting isochrones on their colour-magnitude diagrams. The mass functions slopes are determined for the clusters and a two-step mass function slope is found for the open cluster NGC 381. The mass function slopes are found to be generally steeper in the outer regions of the clusters indicating the presence of mass segregation. It has been found that the structure and dynamics of the clusters are affected by tidal interactions. The ratios of half mass radii to tidal radii are found to be positively correlated with the Galactocentric distances suggesting that the clusters located at the larger Galactocentric distances are subjected to weaker tidal field. The CMDs of the open clusters NGC 2360 and SAI 45 are found to exhibit eMSTOs. The presence of eMSTOs primarily seems to suggest age spreads in the cluster stars as we estimated apparent age spreads of 357 and 493 Myr for NGC 2360 and SAI 45, respectively. However, due to fact that the detection of extended stars formations is elusive till today we explored other likely reasons behind the presence of the eMSTOs. The other factors like the presence of variable stars, binary stars, blue stragglers, and uncertainty in colour and metallicity are found to be unlikely for the large apparent spreads of 300-400 Myr in these clusters. However, such a large apparent age spread can be produced by stellar rotations of eMSTO stars in the clusters. The fast-rotating stars are found to be preferentially located in the red parts of the CMDs of the synthetic cluster populations. This suggests stellar rotations might be responsible for the existence of the eMSTOs. The stars in the red parts of the eMSTOs are found to be preferentially concentrated in the inner region of the clusters which also indicates towards possibility of stellar rotations producing the eMSTOs. Thus, we conclude that the eMSTO are mainly caused by stellar rotations of the stars. We identified a total of 72, 67, and 57 periodic variable stars after light curve inspection of the stars in the region of clusters NGC 1960, NGC 559, and NGC 381, respectively. These periodic variables are characterized based on period, amplitude, the shape of the light curves, and the location on the H-R diagram. These variables comprise pulsating stars such as - Scuti stars, -Doradus stars, and slowly pulsating B type stars (SPBs). One blue straggler star is identified in the intermediate-age cluster NGC 559. The rotational variables and chromospheric active stars such as RS CVn and FKCOM are also detected in these clusters. The physical parameters of the eclipsing binaries found in the cluster regions NGC 559 and NGC 381 are estimated through model fittings. The work presented in the thesis has helped in improving membership identification and estimation of the physical parameters of the studied clusters. The analysis carried out, here, is helpful in understanding the dynamical evolution and the mass functions of the clusters. The effects of the tidal interactions on the structure and shapes of the Galactic open clusters is also probed in the thesis. The thorough study of the eMSTOs in the clusters has enhance our understanding of the various factors possibly responsible for the presence of the eMSTOs in the Galactic open clusters. The variable stars identified in the three open clusters are characterized in the thesis which is helpful in understanding the various stages of the stellar evolution.