Multi-wavelength study of hot stellar population in Galactic Globular Clusters

Abstract

We present the multi-wavelength study of the hot stellar population in Galactic Globular clusters (GGCs). The population of interest in GCs is Blue straggler stars (BSSs) and the extreme end of the hor izontal branch (HB) sequence like Extreme HB (EHB), blue-hook, etc. The formation mechanism of these hot stellar populations can not be explained using the canonical stellar evolutionary theory. To study these hot stellar populations, we targeted six GCs in this the sis i.e., NGC 6656, NGC 1851, NGC 4590, NGC 6218, NGC 6838, and NGC 7078. The photometric data is largely contaminated by field stars that lie along the region of the clusters. Therefore, it is very important to decontaminate the field stars from the cluster stars. To select the genuine cluster members, we used HST and Gaia DR2/EDR3 astro-photometric catalogs. This allows us to select the hot stellar population from the center to the outskirts of the GCs. We deter mine the geometrical center, Cgrav of these six GCs and obtained the structural parameter of the clusters by fitting their projected den sity profile with the King’s (1962) model. We obtained the dynamical status of these six GCs using high-resolution and multi-wavelength data by using the radial distribution of BSS with respect to the reference population and determine the A + rh parameter. Based on the BSS radial distribution and A + rh parameter, our targeted GCs can be classified into three different families. We found that NGC 4590 is a dynamically young age cluster with Family I classification, NGC 6656, NGC 1851 and NGC 6218 are intermediate dynamical age cluster of Family II classification, and NGC 6838 & NGC 7078 are dynamical old age cluster of Family III classification. We study the SEDs of the FUV bright stars of the cluster NGC 1851, using various data sets including UVIT/AstroSat, Swift/U VOT, Gaia DR2, HST, and ground-based telescopes to identify the peculiarities in the HB sequence stars. The SEDs reveal that the HB stars near the “Grundahl jump” show a decrease in the FUV flux when atmospheric models of cluster metallicity are used, but a better fit is found with higher-metallicity models, as expected due to atmospheric diffusion. We found four EHB stars which are clas sified as blue-hook, sub-luminous EHB, BHB/EHB, and one with a blue straggler companion to an EHB. The discovery of the candidate binary BSS+EHB system is important since it can explain to the ex treme mass loss during the RGB phase of the populations located at the end of the HB sequence. The dependency of HB morphology on the cluster parameters is still debatable. There is a well known dependency of metallicity with HB morphology, which is regarded as “first parameter”. To parametrize the extent of HB morphology and to relate with the possible “second parameters”, we estimated the τHB parameter for these six clusters. We choose clusters with different pairs of metallicities, i.e., high, intermediate, and low. The τHB parameter follows a quadratic anti correlation with [Fe/H], indicating that metallicity indeed is the “first parameter”. We found a linear correlation between age and τHB parameter and anti-correlation with the mass-loss parameter, which suggests that age and mass-loss along RGB could be the possible “second parameter”.