Ages of the Bulge globular clusters NGC 6522 and NGC 6626 (M28) from HST Proper-motion-cleaned Color-Magnitude Diagrams

TL;DR

This study determines the ages and properties of bulge globular clusters NGC 6522 and NGC 6626 using HST data, finding they are approximately 12.5-13 billion years old and contain multiple stellar populations.

Contribution

It provides new age estimates and evidence of multiple populations in NGC 6522 and NGC 6626 using advanced statistical analysis of HST color-magnitude diagrams.

Findings

NGC 6522 and NGC 6626 are approximately 12.5-13 Gyr old.

NGC 6522 hosts at least two stellar populations.

Helium enhancement is unlikely based on RR Lyrae magnitudes.

Abstract

Bulge globular clusters (GCs) with metallicities [Fe/H] <~ -1.0 and blue horizontal branches are candidates to harbor the oldest populations in the Galaxy. Based on the analysis of HST proper-motion-cleaned color-magnitude diagrams in filters F435W and F625W, we determine physical parameters for the old bulge globular clusters NGC 6522 and NGC 6626 (M28), both with well-defined blue horizontal branches. We compare these results with similar data for the inner halo cluster NGC 6362. These clusters have similar metallicities (-1.3 <= [Fe/H] <= -1.0) obtained from high resolution spectroscopy. We derive ages, distance moduli, and reddening values by means of statistical comparisons between observed and synthetic fiducial lines employing likelihood statistics and the Markov chain Monte Carlo method. The synthetic fiducial lines were generated using alpha-enhanced BaSTI and Dartmouth stellar evolutionary models, adopting both canonical (Y ~ 0.25) and enhanced (Y~ 0.30-0.33) helium abundances. RR Lyrae stars were employed to determine the HB magnitude level, providing an independent indicator to constrain the apparent distance modulus and the helium enhancement. The shape of the observed fiducial line could be compatible with some helium enhancement for NGC 6522 and NGC 6626, but the average magnitudes of RR Lyrae stars tend to rule out this hypothesis. Assuming canonical helium abundances, BaSTI and Dartmouth models indicate that all three clusters are coeval, with ages between ~ 12.5 and 13.0 Gyr. The present study also reveals that NGC 6522 has at least two stellar populations, since its CMD shows a significantly wide subgiant branch compatible with 14% +/- 2% and 86% +/- 5% for first and second generations, respectively.