The binary mass transfer origin of the red blue straggler sequence in M30

Abstract

Two separated sequences of blue straggler stars (BSSs) have been revealed by Ferraro et al. (2009) in the color-magnitude diagram (CMD) of the Milky Way globular cluster M30. Their presence has been suggested to be related to the two BSS formation channels (namely, collisions and mass-transfer in close binaries) operating within the same stellar system. The blue sequence was indeed found to be well reproduced by collisional BSS models. In contrast, no specific models for mass transfer BSSs were available for an old stellar system like M30. Here we present binary evolution models, including case-B mass transfer and binary merging, specifically calculated for this cluster. We discuss in detail the evolutionary track of a $0.9+0.5 M_\odot$ binary, which spends approximately 4 Gyr in the BSS region of the CMD of a 13 Gyr old cluster. We also run Monte-Carlo simulations to study the distribution of mass transfer BSSs in the CMD and to compare it with the observational data. Our results show that: (1) the color and magnitude distribution of synthetic mass transfer BSSs defines a strip in the CMD that nicely matches the observed red BSS sequence, thus providing strong support to the mass transfer origin for these stars; (2) the CMD distribution of synthetic BSSs never attains the observed location of the blue BSS sequence, thus reinforcing the hypothesis that the latter formed through a different channel (likely collisions); (3) most ($\sim 60\%$) of the synthetic BSSs are produced by mass-transfer models, while the remaining $< 40\%$ requires the contribution from merger models.