High-velocity stars in the cores of globular clusters: the illustrative case of NGC 2808

Context. We report the detection of five high-velocity stars in the core of the globular cluster NGC 2808. The stars lie on the red giant branch and show total velocities between 40 and 45 km s-1. For a core velocity dispersion σc = 13.4 km s-1, this corresponds to up to 3.4σc. These velocities are close to the estimated escape velocity (~50 km s-1) and suggest an ejection from the core. Two of these stars have been confirmed in our recent integral field spectroscopy data and we will discuss them in more detail here. These two red giants are located at a projected distance of ~0.3 pc from the center. According to their positions on the color magnitude diagram, both stars are cluster members. Aims. We investigate several possible origins for the high velocities of the stars and conceivable ejection mechanisms. Since the velocities are close to the escape velocity, it is not obvious whether the stars are bound or unbound to the cluster. We therefore consider both cases in our analysis. Methods. We perform numerical simulations of three-body dynamical encounters between binaries and single stars and compare the resulting velocity distributions of escapers with the velocities of our stars. If the stars are bound, the encounters must have taken place when the stars were still on the main sequence. We compare the predictions for a single dynamical encounter with a compact object with those of a sequence of two-body encounters due to relaxation. If the stars are unbound, the encounter must have taken place recently, when the stars were already in the giant phase. Results. After including binary fractions and black-hole retention fractions, projection effects, and detection probabilities from Monte-Carlo simulations, we estimate the expected numbers of detections for all the different scenarios. Based on these numbers, we conclude that the most likely scenario is that the stars are bound and were accelerated by a single encounter between a binary of main-sequence stars and a ~10 M⊙ black hole. Finally, we discuss the origin of previously discovered fast stars in globular clusters, and conclude that the case of NGC 2808 is most likely a representative case for most other detections of fast stars in globular clusters. We show that with the present analysis we are able to explain high-velocity stars in the clusters M3 and 47 Tucanae with simple dynamical encounters