The Distribution of Blue Straggler Stars in the Color–Magnitude Diagrams of Old Open Clusters*

Abstract We examine the blue straggler star (BSS) populations of six old (≥4 Gyr) open clusters: M67, NGC 188, NGC 6791, Berkeley 32, Berkeley 39, and Trumpler 19. We find that 50% of BSSs have color–magnitude diagram locations corresponding to single stars in the final third of their main-sequence lifetimes. This buildup of BSSs near the terminal-age main sequence (TAMS) is primarily, but not solely, driven by more-massive BSSs. Eleven of the BSSs have white dwarf companions with measured cooling ages; their evolution age distributions indicate that more-massive BSSs typically form far from the zero-age main sequence, whereas lower-mass BSSs can form at every evolutionary age. We show that inferred core helium amounts (above primordial) of late-evolution-age BSSs correspond to the core helium fused by cluster main-sequence stars near the turnoffs. We also find that the masses of asymptotic giant branch (AGB) mass-transfer BSSs require evolved main-sequence accretors and conservative mass transfer. These findings indicate that helium enrichment of progenitor accretors leads to the prevalence of BSSs near the TAMS. We further classify the evolutionary stages of the progenitor donors in M67 and NGC 188 and find mass transfer during the AGB accounts for at least half of the BSSs. We trace how the main-sequence binary population of NGC 188 evolves, and we find that only 30%–40% of interacting binaries create BSSs and that progenitor orbits must change to match current BSS periods.