Origin of the Most Recently Ejected OB Runaway Star from the R136 Cluster

The ∼60 000 solar-mass (M_{⊙}) star cluster R136 (NGC 2070) in the Tarantula Nebula in the Large Magellanic Cloud is the host of at least 55 massive stars (M≳10M_{⊙}) which move away from the cluster at projected velocities ≥27.5 km/s. The origin of the high velocities of such runaway stars have been debated since the 1960s, resulting either from dynamical ejections or from supernova explosions. Because of the Gaia satellite's outstanding precision, we can now retrace the most recently ejected binary star, Mel 34, back to the center of R136 and reconstruct the events that 52 000 years ago led to its removal from R136; i.e., we establish its dynamical interaction and ejection history. We find that this ejection requires the participation of five stars in a strong interaction between a triple composed of the tight massive binary Mel 39, orbited by the star VFTS 590, and the binary star Mel 34. The participation of five stars is unexpected because runaway stars were not expected to result from triple interactions. The deterministic nature of the Newtonian dynamics in the scattering enables us to reconstruct the encounter that ejected Mel 34. We then predict that Mel 39 is a binary star with an 80M_{⊙} companion star that orbits within ∼1° in the same plane as Mel 34 and escapes the cluster with a velocity of ∼64 km/s. The five stars will undergo supernova explosions in the coming 5 Myr at a distance of ∼180-332 pc from their birth location (R136). The resulting black hole binaries, however, are not expected to merge within a Hubble time.