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The Dynamical Ejection of High Velocity Stars from Dense Stellar Regions

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The Dynamical Ejection of High Velocity Stars from Dense Stellar Regions The dynamical ejection of high velocity stars from dense stellar regions Alessia Gualandris Astronomical Institute & Section Computer Science University of Amsterdam Runaway stars D efinition: massive stars of spectral type O-B peculiar velocities exceeding 30 km/s often away from known clusters or associations Interpretation: born in stellar clusters or associations and later ejected with high velocity Statistics: 40% O stars and 10% of B stars are runaways 10% of OB runaways are binaries Runaways: two competing scenarios 1. The binary supernova scenario Primary component of a massive binary explodes as supernova system is unbound: kick velocity to the secondary star of the order of orbital velocity system remains bound: HMXRB This scenario alone fails to explain the total fraction of single runaway stars Runaways: two competing scenarios 2. The dynamical ejection scenario Stellar dynamical encounters in dense clusters: binary-single star encounters binary-binary encounters OB stars likely participate in dynamical encounters Dynamical ejection occurs in young and dense star clusters, where a large number of single and binary stars are concentrated in a small volume. i m Orionis, AE Aurigae, Columbae ] c p [ ⊙ D Hoogerwerf et al. 2001 AE Aurigae m Columbae O9.5 V O9.5 V/ B0V M = 16-22 M⊙ M = 16-22 M⊙ V = 115 km/s V = 103 km/s no evidence of binary evolution same kinematical age ( t = 2.5 Myr) same parent stellar cluster i Orionis O-type spectroscopic binary Porb = 29 days, a = 0.741 AU High eccentricity e = 0.76 Mass ratio q = 0.5 Spectral Mass Radius Age Orion nebula type (M⊙) (R⊙) (Myr) P O9 III 38.9 ± 9.7 15.8 ± 3.2 3.5 S B1 III-IV 18.9 ± 4.7 9.6 ± 1.9 7 ι Ori Different ages ⇩ the system is not co-evolved A dynamical encounter involving the 4 stars occurred 2.5 Myr ago in the Trapezium cluster Opposite velocities of AE Aur and m Col ¯ binary-binary encounter Age difference i Ori P and S ¯ exchange-ionization encounter { i Ori P + AE Aur} + { i Ori S + m Col} ⇓ { i Ori P + i Ori S} + AE Aur + m Col Ejection of runaway stars: numerical simulations Direct N-body code to simulate dynamical encounters between 2 binaries: scatter in the STARLAB package (McMillan, Hut, Makino, Portegies Zwart) http://www.manybody.org Choice of initial conditions (masses, velocities, radii, orbital separations, eccentricities, orientation and inclination, impact parameter) Ejection of runaway stars: numerical simulations Preservation Ejection of runaway stars: numerical simulations Exchange Ejection of runaway stars: numerical simulations Ionization Ejection of runaway stars: numerical simulations Triple A model for i Orionis, AE Aurigae, m Columbae: stellar and binary evolution calculations {i Ori S, m Col} t = 7 Myr {i Ori P, AE Aur} t = 3.5 Myr 22 M 18 M 44 M 18 M ⊙ ⊙ ⊙ ⊙ Ionization + Exchange interaction 2.5 Myr ago {i Ori P + AE Aur} + {i Ori S + m Col} 42 M 18 M 21.5 M 18 M ⊙ ⊙ ⊙ ⊙ ⇓ {i Ori P + i Ori S} + AE Aur + m Col 42 M 21.5 M 18 M 18 M ⊙ ⊙ ⊙ ⊙ Gualandris, Portegies Zwart, Eggleton 2004 Ejection of runaway stars: the velocity distributions Observed ejection velocity V= 18 km/s Observed ejection velocity V= 115 km/s Observed ejection velocity V= 103 km/s Gualandris, Portegies Zwart, Eggleton 2004 Ejection of runaway stars: the semi-major axis distribution Observed semi-major axis a=0.74 AU <a> = 0.54 0.01 AU apeak= 0.72 AU Gualandris, Portegies Zwart, Eggleton 2004 The runaway stars i Ori, AE Aur, m Col were ejected from the Trapezium cluster by a binary-binary encounter. Hyper-velocity stars: the first discovery HVS Vrad = 850 km/s Vspace ~ 1000 km/s m = 3 M⊙ B9 MS d = 71 kpc tMS = 350 Myr tD = 80 Myr Brown et al. (2005) Hyper-velocity stars: the ejection mechanism SN explosion Dynamical encounter with ªnormalº stars Vmax = 300-400 km/s ⇒ need a more massive target: a SMBH 6 1/2 1/2 Vej ~ 3000 km/s (M/10 M⊙) (1mpc/r) The Galactic Centre Sagittarius A The hyper-velocity star: the trajectory in the Galaxy Vr = 850 km/s m ~ 2 mas/yr Vt = 675 km/s V ~ 1085 km/s Hyper-velocity stars: the theoretical prediction Dynamical encounters with the SMBH stellar binary + SMBH tidal breakup Hyper-velocity stars: numerical simulations 6 Mbh = 3.5x10 M⊙ m = 3 M⊙ 0.05 AU < a < 1 AU Vrel = 100 km/s Hyper-velocity stars: cross-sections Hyper-velocity stars: velocity distribution Hyper-velocity stars: the theoretical prediction Dynamical encounters with the SMBH BBH + single star flyby Hyper-velocity stars: numerical simulations 6 Msmbh = 3.5x10 M⊙ 3 Mimbh = 3.0x10 M⊙ m = 3 M⊙ 2 AU < a < 1000 AU Vrel = 100 km/s Hyper-velocity stars: velocity distribution The velocity of the hypervelocity star is consistent with a dynamical encounter with the SMBH in the Galactic Centre. We predict a proper motion of about 2 mas/yr..
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