The Ages of Globular Clusters

Abstract

ABSTRACTWe examine the luminosity levels of the main‐sequence turnoffs, MTOv, and horizontal branches, Mv(HB), in 16 globular clusters. An entirely new approach of inferring the luminosity levels by utilizing high‐amplitude δ Scuti variables (HADS) is introduced. When the MTOv values are compared with theoretical values inferred from models, we find all 16 clusters (metal‐strong to metal‐poor) are coeval with an average age of ∼11.3 Gyr. A considerable scatter of Mv(HB) values of clusters at similar [Fe/H] values is found. A trend for clusters with blue horizontal branches to have brighter Mv(HB) than clusters with blue‐red horizontal branches is suggested by the data. The Mv(HB) values appear to depend on another or other parameters in addition to the [Fe/H] values. In spite of this problem, we derive an equation relating Mv(HB) values of globular clusters to their [Fe/H] values. We also derive an equation relating the MTOv values of clusters to their [Fe/H] values. Both of these equations can be utilized to find cluster distances. The distance modulus of the LMC is found to be 18.66 from the VTO values of three LMC globular clusters; RR Lyrae stars in seven globular clusters yield 18.61, and RR Lyrae stars in the LMC bar yield 18.64.