Abstract
We report the detection of five high-velocity stars in the core of the
globular cluster NGC 2808. The stars lie on the the red giant branch and show
total velocities between 40 and 45 km/s. For a core velocity dispersion sigma_c
= 13.4 km/s, this corresponds to up to 3.4 sigma_c. These velocities are close
to the estimated escape velocity (~ 50 km/s) 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. 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. 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. 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. 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_sun black hole.