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 O­B 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: binary­single star encounters binary­binary 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.  Orionis, AE Aurigae,  Columbae

] c p [

⊙ D

Hoogerwerf et al. 2001 AE Aurigae  Columbae

O9.5 V O9.5 V/ B0V

M = 16­22 M⊙ M = 16­22 M⊙ V = 115 km/s V = 103 km/s

no evidence of binary evolution same kinematical age (  = 2.5 Myr) same parent stellar cluster  Orionis O­type 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 co­evolved A dynamical encounter involving the 4 stars occurred 2.5 Myr ago in the Trapezium cluster

Opposite velocities of AE Aur and  Col  binary­binary encounter

Age difference  Ori P and S  exchange­ionization encounter

{  Ori P + AE Aur} + { Ori S +  Col} ⇓ {  Ori P +  Ori S} + AE Aur +  Col Ejection of runaway stars: numerical simulations

Direct N­body 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  Orionis, AE Aurigae, Columbae: stellar and binary evolution calculations

{ Ori S,  Col}  = 7 Myr { Ori P, AE Aur}  = 3.5 Myr 22 M 18 M 44 M 18 M ⊙ ⊙ ⊙ ⊙

Ionization + Exchange interaction 2.5 Myr ago

{ Ori P + AE Aur} + { Ori S +  Col} 42 M 18 M 21.5 M 18 M ⊙ ⊙ ⊙ ⊙ ⇓ { Ori P +  Ori S} + AE Aur +  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 semi­major axis distribution

Observed semi­major axis a=0.74 AU

= 0.54 0.01 AU

apeak= 0.72 AU

Gualandris, Portegies Zwart, Eggleton 2004 The runaway stars  Ori, AE Aur,  Col were ejected from the Trapezium cluster by a binary­binary encounter. Hyper­velocity stars: the first discovery HVS Vrad = 850 km/s Vspace ~ 1000 km/s m = 3 M⊙ B9 MS d = 71 kpc MS = 350 Myr D = 80 Myr Brown et al. (2005) Hyper­velocity stars: the ejection mechanism

SN explosion Dynamical encounter with “normal” stars Vmax = 300­400 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 hyper­velocity star: the trajectory in the Galaxy

Vr = 850 km/s

~ 2 mas/yr

Vt = 675 km/s  V ~ 1085 km/s Hyper­velocity stars: the theoretical prediction

Dynamical encounters with the SMBH stellar binary + SMBH

tidal breakup Hyper­velocity stars: numerical simulations

6 Mbh = 3.5x10 M⊙ m = 3 M⊙ 0.05 AU < a < 1 AU Vrel = 100 km/s Hyper­velocity stars: cross­sections Hyper­velocity stars: velocity distribution Hyper­velocity stars: the theoretical prediction

Dynamical encounters with the SMBH BBH + single star

flyby Hyper­velocity 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 Hyper­velocity 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.