Fabio Antonini Joshua Faber Alessia Gualandris and David Merritt Rochester Institute of Technology

Unbound hypervelocity stars (HVS) were predicted by Hills(1988) as the natural consequence of the interaction of the massive black hole at the galactic center with binary stars Brown et al reported the discovery of the first HVS: SDSS J , a 3M o main-sequence star travelling with a Galactic rest-frame velocity of at least 700 ± 12 km/s Now, we know about 20 stars with velocities larger than the escape velocity from the Galaxy Other ejection mechanisms can be: 1) Encounters between stars and a binary BH 2) Close encounters between two single stars 3) Stellar massive black hole kicks

Each star ejected during a binary-BH encounter is associated with a captured companion which loses energy in the process and becomes more tightly bound to the BH on a high eccentricity (e>0.9) orbit This model is able to reproduce the main orbital properties of the S-stars observed at the galactic center (Perets et al. 2009)‏ Our simulations show that the interaction between a SMBH and binaries can, in many cases, end up in the coalescence and/or strong mass transfer of the binary components. This can lead to the formation of a rejuvenated star (Vanbeveren et al. 1998, Dray & Tout 2007). If the binary is initially bound to the SMBH the remnant will be part of the bound population.

2000AU V in r t <r per <r bt All the simulations were carried out by means of the N-body code ARCHAIN (Mikkola and Merritt 2008): - algorithmically regularized chain structure - PN terms up to order 2.5 Escape velocity (Hills1988)‏ M b =3 or 6 M o a 0 =0.2 or 0.05 AU

In the considered range of initial velocities (50 km/s < v < 310 km/s), on the 7200 integrated orbits we got 331 HVSs with a probability of ejection close to the 50% for pericenters smaller than the tidal disruption radius of the binary due to the black hole. The smaller the binary separation and the larger the stellar masses the larger the v ej A direct comparison between Newtonian and post-Newtonian simulations shows that relativistic effects do not have any strong systematic influences on the mean properties of the sample at all.

THE POPULATION OF BOUND STARS 1) Bound stars whose the companion also orbits the SMBH (grey points) 2) Bound stars whose the companion has been ejected by the SMBH, but remains bound in the Galaxy (red points) 3) Bound stars whose the companion is a HVS unbound from the Galaxy (blue points) M, M=> e a0a0 Example: assuming an initial distance of 4000AU and a transverse speed of 5 km/s for a 3+10Mo binary with a 0 we got that the 10Mo companion remains bound to the BH with an orbital semi-major axis of 1430 AU and e=0.99… S14?

- After the first encounter, most of the binaries with small pericenter are broken a part depositing one star on a tight orbit around the BH while ejecting the companion as a HVS. -The larger the pericenter the larger the time required for the perturbations to become important, and the collision and meger frequency increases strongly with time The BH perturbations on the internal binary orbit induce binary merger, binary collision and mass transfer “blue straggle” formation HVSs COLL MERG

Kozai Cycles The Kozai mechanism can lead to a large periodic oscillations in the eccentricity and inclination of highly inclined binary orbits. This process can lead to stellar coalescence and serve as an important source of young stars at the galactic center. The apsidal precession due to the inclusion of PN terms reduces only partially the number of observed mergers. For a 0 =0.05AU ad 6 solar mass stars the merger frequency decreases from 53% to 45%

We study by means of post-Newtonian N-body simulations the evolution of main sequence binaries as they make close passages by a SMBH, determining the conditions under which the gravitational interaction produces a hypervelocity star, and the properties of distribution of both bound and unbound stars. For small pericenters that take the binary inside or just above r t : the cases in which only one star is tidally disrupted by the SMBH are very rare and occur only at ~r t. This show that the distance between the stars does not change much until the pericenter passage. The relativistic terms increase the probability that the stars fall inside their tidal disruption radius for a given initial velocity. For pericenters between r t and r bt : there are many cases in wich one star of the binary is ejected at hypervelocity. The ejection velocity can reach values of 6500km/s. The larger the stellar mass and the smaller the binary separation the larger the ejection velocity. For a 0 =0.05AU and pericenters within the tidal disruption radius of the binary the hypervelocity ejection probability is ~70%. For pericenters between r bt and 60AU: The gravitational perturbations induced by the SMBH on the internal orbital paramiters of the binary can produce stellar collision, binary merger and binary mass transfer. For our choise of initial conditions and for a 0 =0.05AU the probability that the stars merge is ~40% … SPH SIMULATIONS