Study of Integrated Far-ultraviolet Emissions from Galactic Globular Clusters using AstroSat/UVIT observations

TL;DR

This study used AstroSat/UVIT observations to analyze the FUV emissions of 30 Galactic globular clusters, revealing the dominant role of HB stars and the influence of cluster parameters on UV properties.

Contribution

First comprehensive analysis of integrated FUV emissions from Galactic GCs using AstroSat/UVIT, linking stellar populations and cluster parameters to UV characteristics.

Findings

HB and post-HB stars contribute ~40-45% to FUV emission.

Blue straggler stars contribute only ~3% to FUV emission.

Metal-poor GCs have significantly bluer FUV-optical colors.

Abstract

We used observations obtained with the Ultraviolet Imaging Telescope on board the AstroSat satellite to measure the integrated far-ultraviolet (FUV) and optical (V) magnitudes of 30 Galactic globular clusters (GCs). We classified the UV-bright evolved stellar populations of the GCs using FUV$-$V versus FUV color-magnitude diagrams (CMDs) and BaSTI-IAC isochrones and subsequently quantified their contributions to the total integrated FUV emissions. We found that the horizontal branch (HB) and post-HB (post-HB) stars contribute $\sim 40\%-45$\% to the total FUV emission of GCs, while the contribution of blue straggler stars is only $\sim$3\%. The HB stars especially dominate the UV budget of the metal-poor clusters. The observed spread in FUV-optical color in the color-color diagram supports the phenomenon that the UV upturn of early-type galaxies is due to the evolved stars. We studied for the first time the variation of integrated FUV magnitudes and colors with several cluster parameters in the core, intermediate, outer, and tidal regions, such as the fraction of second-generation stars, helium mass fraction, HB morphology, and mass of the GCs. We found that the GCs with a higher second-generation star fraction, helium mass fraction, and cluster mass are brighter in all the regions. The GCs with bluer HB morphologies also have brighter and bluer FUV magnitudes in the core and intermediate regions. Metal-poor GCs show significantly bluer FUV$-$optical colors, consistent with a stronger contribution from hot evolved stars.