Hyper-velocity stars from globular clusters hosting intermediate-mass black holes

Sammanfattning: In 1988 Hills proposed that stars can gain velocities above 500 km s−1 if they are in a stellar binary which gets disrupted by a super massive black hole (SMBH). One of the stars would get trapped in an elliptical orbit around the SMBH while the other gets ejected and obtains a relatively high velocity. Normally stars within galaxies have low peculiar velocities but stars ejected with velocities over 500 km/s can potentially escape the galaxy. Stars of this nature are called hypervelocity stars (HVS) and in 2005 the first of such a star was discovered in the halo of our Galaxy. For this project we study the possibility of HVSs being generated by the less established intermediate-mass black hole (IMBH). There is some evidence, while not concrete, suggesting that IMBHs can exist in globular clusters (GCs) making this the environment we are considering. We set up and study the outcome of about 100,000 scattering encounters between a stellar binary and an IMBH for a range of different initial conditions. Not only do we analyze the interactions that result in the production of a high-velocity star due to the Hills mechanism but also the ones resulting in mergers and flybys. All of this is performed using the Tsunami code which is an N -body integrator with a lot of useful tools and features. We reach the conclusion that the semi-major axis of the stellar binary is the main factor in determining whether or not an HVS can be generated. Of significant importance is also the mass of the IMBH and the initial orientation of the interaction. In addition, mergers occur more frequently when the semi-major axis is tighter and flybys are more likely when the ratio between the pericenter distance of the encounter and the binary semi-major axis is larger. Finally, we obtain 6 HVSs out of 27 encounters between binary white dwarfs and an IMBH for which the initial conditions were taken from realistic GC models.