Multi-Wavelength Study of Globular Cluster NGC 6218 and Understanding The Evolution of Low Mass Stars

Adarsh, K (2018) Multi-Wavelength Study of Globular Cluster NGC 6218 and Understanding The Evolution of Low Mass Stars. Masters thesis, Indian Institute of Science Education and Research Kolkata.

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Abstract

A multi-wavelength study of globular star cluster NGC 6218 is presented. This study uses Ultraviolet (UV) to optical photometric data from space and ground observations. The UV photometry was performed on FUV and NUV images obtained from Galaxy Evolution Explorer(GALEX) and the Optical data were adopted from the literature. These data were used to create UV and optical color-magnitude diagrams (CMDs). We identify the Main-sequence stars, red giant stars, horizontal branch (HB) stars and blue straggler stars (BSS) of the cluster. Isochrones generated for the relevant filter systems were used to fit the UV and optical CMDs. A comparison of the observed UV colors with the models was used to estimate the temperatures of HB stars, BSS, and white dwarf stars. The estimated temperatures of HB stars were found to have a range with the peak of the distribution in the 8000-10000 K. An asymmetry in the spatial distribution of HB stars are also observed, suggesting that they are not well mixed spatially. The temperature distribution of HB stars is generally known to be a function of the metallicity of the cluster. We used the pipeline GALEX data in the UV for 8 clusters with similar metallicity and generated CMDs. We compared the HB morphology as well as the temperature distribution of the HB stars. We find differences in the HB morphology amongst these clusters. Some clusters have BHB stars with a large temperature range, whereas the others have a small temperature range. Based on the mean temperature of HB stars, we have classified the clusters into two groups. Clusters in the first group have a mean temperature in between 9000 to 10000K and the second group have a mean temperature in between 8000 to 9000K. We thus classify the GCs based on their BHB morphology and speculate possible reasons. This is the well-known second parameter problem in Globular clusters, which is known to create differences in the HB morphology. Age and Helium abundance are the two favorite candidates for this parameter. We find a few clusters to show a significant spread in the HB temperature range, suggesting significant impact from the second parameter. The significantly affected clusters do not have similar temperature profile for the HB stars, which suggests that a single second parameter is not sufficient to explain the observed diversity.

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