A Triple Origin for Twin Blue Stragglers in Close Binaries

We propose that twin blue stragglers (BSs) in compact binaries evolve through mass transfer from a giant outer tertiary companion on to the inner binary. We apply this scenario to the twin BS binary WOCS ID 7782 in the old open cluster NGC 188. This binary has two comparable-mass main-sequence stars in a ≲10 days almost circular (e ≲ 0.1) orbit. Our theoretical arguments are supported by simulations of an inner binary that accretes from an outer Roche-lobe overfilling star using the Astrophysical Multipurpose Software Environment. At least 80% of the tertiary's liberated mass accretes onto the inner binary via a circumbinary disk, turning both stars into BSs. Relatively stable mass transfer occurs for donors with ∼1.4 M⊙ that overfill their Roche lobe before ascending the asymptotic giant branch. The system is best reproduced if this tertiary is in an 220–1100 days orbit around an inner binary composed of an 1.1 M⊙ primary and a m2 = 0.7–0.9 M⊙ secondary in an 8.6–24 days orbit. The tertiary eventually turns into a 0.43–0.54 M⊙ white dwarf in a relatively wide ≳5.8 yr orbit. The scenario is generic, but requires some fine-tuning to achieve parameters comparable to WOCS ID 7782. We predict that twin BSs formed through mass transfer from a Roche-lobe overfilling tertiary are generally comparable in mass with aligned spins, which are in turn aligned with the tertiary white dwarf's orbit. If the two inner stars were initially unequal in mass the less massive star will accrete more, becoming more enhanced in CNO-processed material.