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

Sonika Piridi, Ranjan Kumar, Divya Pandey, Ananta C. Pradhan

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.

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

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 FUVV 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 \% to the total FUV emission of GCs, while the contribution of blue straggler stars is only 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 FUVoptical colors, consistent with a stronger contribution from hot evolved stars.

Paper Structure

This paper contains 20 sections, 2 equations, 10 figures.

Table of Contents

  1. Introduction
  2. UVIT observations of globular clusters
  3. Data Analysis
  4. Completeness
  5. Membership Probability
  6. CMDs and identification of sources
  7. Globular cluster parameters
  8. Measurement of integrated magnitudes of GCs
  9. Integrated FUV magnitudes
  10. Integrated optical magnitudes
  11. Results and Discussion
  12. Contribution of various evolved stars to the total integrated FUV emission
  13. Variation of HB Fractions with Metallicity
  14. Relative distribution of FUV emission across the globular clusters
  15. UV$-$Optical color$-$color diagram
  16. ...and 5 more sections

Figures (10)

  • Figure 1: Filter response curves of the two UVIT FUV filters, F148W and F154W. We used a sample of a GCs observed with these filters.
  • Figure 2: Left panel: image of the GC NGC 6723 observed by the F148W filter of UVIT. The red, blue, and black circles delineate the core radius, half-light radius, and tidal radius of the cluster, respectively. Right panel: zoomed-in view of the cluster's inner region, highlighting the core radius (red circle) and half-light radius (blue circle).
  • Figure 3: Recovery fraction of the artificial stars injected at each magnitude bin in the intermediate (solid downward triangles) and outer (solid circles) regions.
  • Figure 4: F148W$-$G versus F148W CMD for one of the GC, NGC 6723. The post-HB, HB, and BS stars are shown as orange, magenta, and cyan circles, respectively. The model isochrones for ZAHB and TAHB are overplotted as solid and dashed lines. The dotted line is the ZAMS isochrones of 0.5 Gyr.
  • Figure 5: Variation of total integrated FUV emission with fraction of HB (top left panel), post-HB (top right panel), and BS (bottom left panel) emissions. The bottom right panel shows the variation of the fractional contribution of HB stars with metallicity. The dashed line represents the straight-line fitting.
  • ...and 5 more figures