Isochrone Fitting of Galactic Globular Clusters -- VIII. Homogeneous estimates of parameters for 27 clusters

Abstract

We use the Stetson, HST, and Gaia DR3 data sets for 22 Galactic globular clusters to select their members and fit their CMDs with isochrones from DSED and BaSTI for $[α/$Fe$]=+0.4$ and an adopted helium mass fraction $Y$. As a result, we estimate the metallicity [Fe/H], age, distance from the Sun, and reddening E(B-V) for these clusters. Special attention is paid to identify variable stars among the cluster members. We combine these results with our earlier estimates for five other clusters into a homogeneous set of parameters for 27 clusters and investigate relationships among these parameters and their statistical properties. In particular, we count the giants on the red (RGB), horizontal (HB), and asymptotic branches using the clean data sets cross-identified to cover the entire cluster fields and count each giant only ones. This allows us to calculate the $R$-parameter, the ratio between the numbers of the HB and RGB, with unprecedented accuracy and to examine its relations with other parameters. These relations are much stronger for accreted clusters rejecting a constant $R$ value for them, possibly because of their heterogeneous origins and environments. For all in-situ clusters, a constant value of $R=1.31^{+0.06}_{-0.05}$ is consistent with the data. According to previous calculations, this value may imply anomalous energy losses in HB stars corresponding to an axion--photon coupling of $g_{aγ}=(0.66^{+0.11}_{-0.13})\times10^{-10}\,\mathrm{GeV}^{-1}$. However, the observed dependencies of $R$ on cluster parameters require improved statistics and a theoretical understanding of these dependencies. By comparing $R$-parameter estimates from HST and Gaia in cluster centres and peripheries, respectively, we suggest that the peripheral HB population is depleted by about half when the cluster crosses the Galactic disk, then the HB recovers over 60-80 Myr.