Fabio Antonini Joshua Faber Alessia Gualandris and David

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. 2005 reported the discovery of the first HVS: SDSS J 090745. 0+024507, a 3 Mo 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.

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 rt<rper<rbt Vin 2000 AU Mb=3 or 6 Mo a 0=0. 2 or 0. 05 AU Escape velocity (Hills 1988)

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. 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 smaller the binary separation and the larger the stellar masses the larger the vej

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) a 0 , M => e Example: assuming M an initial distance of 4000 AU and a transverse speed of 5 km/s for a 3+10 Mo binary with a 0 we got that the 10 Mo companion remains bound to the BH with an orbital semi-major axis of 1430 AU and e=0. 99… S 14?

COLL MERG HVSs - 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 “blue straggle” formation and mass transfer

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. 05 AU 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 rt : the cases in which only one star is tidally disrupted by the SMBH are very rare and occur only at ~rt. 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 rt and rbt: there are many cases in wich one star of the binary is ejected at hypervelocity. The ejection velocity can reach values of 6500 km/s. The larger the stellar mass and the smaller the binary separation the larger the ejection velocity. For a 0=0. 05 AU and pericenters within the tidal disruption radius of the binary the hypervelocity ejection probability is ~70%. For pericenters between rbt and 60 AU: 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. 05 AU the probability that the stars merge is ~40% … SPH SIMULATIONS