Abstract:
We report an extremely rare X-ray active blue straggler star (BSS) in Collinder 261, undergoing ongoing Rochelobe overflow in a semidetached binary in an Algol-type configuration with an orbital period of Porb ∼ 2.112 days, providing direct observational evidence for the ongoing binary mass-transfer formation channel of BSS. Lightcurve modeling with PHOEBE reveals a low mass ratio of q ∼ 0.19, corresponding to a mass of 1.67 M⊙for the BSS primary and a mass of 0.32 M⊙ for the Roche-lobe-filling companion. Reflection effects and a hotspot near the inner Lagrangian point (L1) reproduce the observed light-curve asymmetries, while the observed X-ray luminosity is consistent with direct-impact accretion-driven emission, supporting ongoing mass transfer. Radial velocities from the Very Large Telescope/GIRAFFE are also consistent with the photometric solution and reveal enhanced primary rotation of v sin i 69.55 km s −1, consistent with spin-up through direct-impact accretion. Evolutionary modeling with MESA reproduces the observed configuration for an initially detached system with Md ∼ 1.12 M⊙, Ma ∼ 0.92 M⊙, and Pini ∼ 0.83 day. Roche-lobe overflow and mass transfer begin at ∼5.46 Gyr with mass ratio reversal at ∼5.58 Gyr. The present configuration at ∼7 Gyr, consistent with the cluster age, corresponds to stable mass transfer of M2 × 10−10 M⊙ yr−1. These results indicate that the system represents a very rare case of a BSS currently forming through binary mass transfer.
