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