HD 271791: dynamical versus binary-supernova ejection scenario

The atmosphere of the extremely high-velocity (530 − 920 km s) early B-type star HD271791 is enriched in α-process elements, which suggests that this star is a former secondary component of a massive tight binary system and that its surface was polluted by the nucleosynthetic products after the primary star exploded in a supernova. It was proposed that the (asymmetric) supernova explosion unbind the system and that the secondary star (HD 271791) was released at its orbital velocity in the direction of Galactic rotation. In this Letter, we show that to explain the Galactic rest-frame velocity of HD 271791 within the framework of the binary-supernova scenario, the stellar remnant of the supernova explosion (a . 10 M⊙ black hole) should receive an unrealistically large kick velocity of > 750 − 1200 km s. We therefore consider the binary-supernova scenario as highly unlikely and instead propose that HD271791 attained its peculiar velocity in the course of a strong dynamical threeor four-body encounter in the dense core of the parent star cluster. Our proposal implies that by the moment of encounter HD271791 was a member of a massive post-supernova binary.