Binary Evolution and Blue Stragglers in Different Regions of the Color–Magnitude Diagrams of Globular Clusters with Different Ages

Abstract

Blue stragglers (BSs) in different regions (e.g., two sequences) of the color–magnitude diagram (CMD) have been suggested to provide a strong constraint on the origin of BSs in globular clusters (GCs). However, our understanding of the locations of binary-induced BSs is incomplete, e.g., its low-luminosity boundary. By employing Eggleton’s stellar evolution code, the locations of mass-transferring (MT) BS binaries and post-mass-transfer (post-MT) BS binaries are investigated in the CMDs of GCs with different ages. The results show that post-MT BS binaries always populate the region of the CMD below the ZAMS+0.75 for GCs older than 10 Gyr and can also populate the region above the ZAMS+0.75 (the region of MT BS binaries). The low-luminosity boundary of the binary-induced BSs is located near the ZAMS in a way that is independent of the age of old GCs. The BS binaries draw a narrow sequence below the ZAMS+0.75 and cover most of the BSs in NGC 362. These results support that binary evolution can explain the appearance of BSs in different regions of the CMDs of GCs with different ages, and at least a fraction of BSs along the blue sequence could be post-MT BSs. However, the theoretical gap does not exactly correspond to the observed gap in NGC 362, and the models predict an excess of BSs below ZAMS+0.75 with respect to the observations in NGC 362. Therefore, more theoretical and observational work is needed to clarify the origin of these discrepancies.