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Dynamical evolution of the young stars in the Galactic center

Dynamical evolution of the young stars in the Galactic center. Hagai B. Perets Harvard-Smithsonian Center for Astrophysics. Outline. Observations of young stars in the GC Possible origins Dynamical processes The S-stars The disk (?) stars Summary. Observations: overview.

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Dynamical evolution of the young stars in the Galactic center

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  1. Dynamical evolution of the young stars in the Galactic center Hagai B. Perets Harvard-Smithsonian Center for Astrophysics

  2. Outline • Observations of young stars in the GC • Possible origins • Dynamical processes • The S-stars • The disk (?) stars • Summary

  3. Observations: overview • The MBH: ~4x106 Msun • Main sequence B-stars (isotropic) • Tens in <0.04 pc • more beyond – smooth distribution? • Regular mass function • Young OB stars (coherent structures + isotropic) • scale: 0.05-0.5 pc • mass:103-104 Msun age: ~5 Myrs • Coherent structures (warped disk/s, streams) • intermediate eccentricities • Top heavy IMF (~m-0.5 ) • Older stellar cusp (possibly with an inner core/hole). • Hypervelocity B stars (HVSs) in the Galactic halo • few x 10 kpc away, anisotropic distribution HVS

  4. Originssetting the initial conditions • In situ formation • Formation in gaseous disk(s)/rings/streams • Impostors • Collision products • Stripped old stars • Tidally heated stars • Migration • Stellar cluster in-fall • Disk planetary like migration • Capture • Exchanges with stellar black holes • Binary disruptions • From >1 pc • Disk->inner region

  5. Dynamical evolutionImportant processes and components • Important processes • Loss cone refill • 2-body regular relaxation • Massive perturbers • Eccentric disk instability • Resonant relaxation • Mass segregation • Binary heating • Continuous star formation • Continuous captures • GR effects • Important components • MBH • Stars • Stellar black holes • Stellar disk(s) • Massive perturbers • IMBH (?)

  6. The S-stars are young Lifetimes of 107-108 yrs Can not form in-situ: tidal forces are too strong Can not form far away migration time is longer than their lifetime Paradox of youth The s-stars

  7. Exotic stars (e.g. Hansen & Milosavljevic 2003, Alexander & Morris 2003) Fast migration Cluster in-spiral (e.g. Gerhard 2001) Planetary like disk migration (Levin 2005) Capture scenario Binary disruption (Hills 1988, Gould & Quillen 2003, Perets et al. 2007, Löckmann et al. 2009, Madigan et al. 2009) S-stars seems regular Collisions are not efficient enough The s-stars: origins • Cluster stars do not arrive close enough to the MBH • low eccentricity stars • Similar population as in stellar disk • Requires efficient scattering processes (e.g. massive perturbers, eccentric disk instability)

  8. Giant Molecular cloud The Capture Scenario MBH Binary

  9. Giant Molecular Cloud The CaptureScenario MBH Binary Eccentric disk instability

  10. abin (Hills, 1988; Quillen & Gould, 2003; Yu & Tremaine, 2003). Capture scenario Binary Binary disruption MBH

  11. afinal abin Mapping binaries to S-stars Binary disruption Runaway high velocity star MBH Captured star Hills (1991,1992)

  12. Capture origin: implications • Initial eccentric orbits • Spatial distribution reflects binary progenitor separation distribution (should extend much beyond 0.04 pc) • MF reflects progenitor MF (but note binary distribution effects) • Hypervelocity stars be consistent with S-stars (numbers, mass function, spatial and temporal distribution)

  13. Dynamical evolution of the S-stars • Scattering by stellar black holes • Resonant Relaxation • Tidal disruption by the MBH • Stellar Collisions • Tidal friction by the MBH • GR effects • Scattering by an IMBH (?) (Merritt & Gualandris 2009)

  14. Resonant Relaxation Rauch & Temaine 1996 Effective torque (coherent) and at late time (diffusion) Alexander 2007

  15. Dynamical evolution of the S-stars:N-body simulations Captured stars Migrating disk stars 30 % are destroyed Simulations on the special purpose GRAPE 6 computer in RIT (Collaborators Allesia Gualandris, David Merritt and Tal Alexander) Perets et al. 2009

  16. Dynamical evolution of the S-stars:N-body simulations 30 % are destroyed Simulations on the special purpose GRAPE 6 computer in RIT (In collaboration with Allesia Gualandris and David Merritt) Perets et al. 2009

  17. Capture origin: dynamical evolution implications • Distribution of orbits is distance dependent; isotropic and thermal in inner region (<0.04) for initially eccentric orbits, but not so for initially low eccentricity orbits (planetary migration scenario). => Look for eccentric orbits in outer regions ! • Weak bias towards younger stars • Possible correlation between age and eccentricity

  18. S-stars: current status • Observables: • Numbers: few tens • MF: regular with continuous SFR over 60 Myrs • Spatial distribution: smooth up to large distance • ~Thermal eccentricity distribution, isotropic orbits • HVSs numbers and MF • HVSs temporal and spatial distribution • Theory: √ Consistent with MPs scenario Possible inconsistency with a a disk origin (requires older disk with regular MF; smooth distribution of B-stars not expected to continue to outer parts) √ Consistent with capture, not with planetary like migration √ but note that comparison is indirect (different masses probed) √ Temporal – continuous ? Spatial - (Ann-Marie’s talk).

  19. The disk (?) stars Hobbs & Nayakshin 2009 Bonnel & Rice 2008 Initial conditions: single/multiple circular/eccectric disks/rings/streams… Choose your favorite…

  20. A cold stellar disk embedded in a hot stellar cusp Important components Stellar black holes Massive stars Binaries Disk heating: Self interactions (Alexander et al. 2007) Disk-cusp coupling (Perets et al. 2008, Löckmann et al. 2009, Madigan et al. 2009) Resonant relaxation (Hopman & Alexander 2006, Perets et al. In prep.) Interaction with 2nd disk/CND/IMBH (Löckmann et al. 2009, Yu et al. 2008 , Subr 2007, Gualandris et al., in prep.) Dynamical evolution of a stellar disk:A two temperature system

  21. Evolution of a single disk:Cusp heating and torque Without The Cusp With The Cusp β=14 0 β Perets et al., in prep. A stellar nuclear disks after 6 Myrs of dynamical evolution

  22. Evolution of a single disk:Long term evolution • After only a few tens of Myrs the disk puffs and expands to become almost spherical • An isotropic distribution of older B-stars could, in principle, originate from an older stellar disk • Eccentricities distribution may still hold some memory of a disk origin (if originally on ~circular orbits)

  23. Two disks interaction: Torques, Kozai and cusp stabilization With The Cusp Without The Cusp Gualandris, Perets et al., in prep. Two perpendicular disks after 6 Myrs of dynamical evolution

  24. Two disks interaction: Torques, Kozai and cusp stabilization Eccentricity Distribution Edge-on view Gualandris, Perets et al., in prep. Two perpendicular disks after 6 Myrs of dynamical evolution

  25. Summary • S-stars population is consistent with a captured population of stars, which dynamically evolved through resonant relaxation due to the cusp component • Disk stars population likely to have formed in situ through instabilities in gaseous streams then dynamically evolved, driven by the cusp component • Additional isotropic distribution of stars is either related to the initial conditions (streams, eccentric disk etc.), or due to additional massive components (e.g. 2nd stellar disk, CND, IMBH…)

  26. Future theoretical directions • Planetary like migration • Evolution of an eccentric disk • Evolution of streams • Binaries in a stellar disk • Realistic cusps • Long term evolution • Continuous/cycled star formation • Continuous capture • GR effects • Resonances

  27. Evolution of the S-stars

  28. The disk stars:Dynamical evolution of a stellar disk • A cold system (disk) embedded in a hot system (cusp) • Fast expansion • Evolution dominated by the stellar cusp, both through resonant relaxation (coherent and stochastic) and regular 2-body relaxation • Self heating by massive stars and through binary heating

  29. Exotic stars (e.g. Hansen & Milosavljevic 2003, Alexander & Morris 2003) Fast migration Cluster in-spiral (e.g. Gerhard 2001) Planetary like disk migration (Levin 2005) Capture scenario Binary disruption (Gould & Quillen 2003) S-stars seems regular Collisions are not efficient enough The s-stars: theories • Cluster stars do not arrive close enough to the MBH • Maybe • low eccentricity stars • Same population as disk stars • High eccentricity stars

  30. Fast Relaxation by Massive Perturbers • Relaxation time: • n: number density of stars/MPs • σ: velocity dispersion • For μ2>>1, MPs dominate relaxation • Fast relaxation induces high rates of scattering stars into the MBH Spitzer & Schwartzchild 1953 Zhao et al. 2002 Perets et al. 2007 Perets & Alexander, 2008

  31. Massive Perturbers in the GC Perets et al. 2007 Similar conditions are likely to exist in other galactic nuclei (Perets & Alexander, 2008)

  32. Relaxation Time Perets et al. 2007 Major importance for coalescence of MBHs (the last parsec problem) (Perets & Alexander 2008) Loss rate:

  33. Giant Molecular cloud The Capture Scenario MBH Binary

  34. Giant Molecular Cloud The CaptureScenario MBH Binary

  35. Captured Young Stars and Hypervelocity stars Perets et al. 2007 • 5-35 captured young B-stars (> 4 Msun) • 10-65 observable young hypervelocity B-stars (> 3 Msun) in 20-120 kpc.

  36. Dynamical evolution of the S-stars • Scattering by stellar black holes • Stellar Collisions • Tidal friction by the MBH • Tidal disruption by the MBH • GR effects

  37. Dynamical evolution of the S-stars:N-body simulations Captured stars Migrating disk stars 60 % are destroyed Simulations on the special purpose GRAPE 6 computer in RIT (In collaboration with Allesia Gualandris and David Merritt) Perets et al. 2008a, in prep.

  38. The origin of the S-stars • Both the existence of the S-stars and their observed orbital properties can be explained by the binary disruption and massive perturbers scenario • What about other populations of captured stars – older, fainter or compact objects ? 60 % are destroyed

  39. The closest stars to the MBH: Probes of general relativity Monte-Carlo simulations S,J,Q2 for 5 μas yr-1 (Will 2008) • Scattering by stellar black holes • Stellar Collisions • Tidal friction by the MBH • Tidal disruption by the MBH • GR effects Perets & Alexander 2008, in prep.

  40. Gravitational Waves Sources Laser Interferometer Space Antenna (LISA)

  41. Gravitational wave in-spirals and bursts MBH LISA

  42. Gravitational wave sources Extreme Mass Ratio Inspirals GW bursts Perets, Hopman and Alexander 2008, in prep.

  43. The capture scenario: What have we learned ? • The capture scenario could have a major role in the dynamics near MBHs if relaxation is fast enough • Massive perturbers induce fast relaxation: • Help resolve the last parsec problem • Explain the origin of the S-stars

  44. The capture scenario: What have we learned ? • Additional implications: • Capture of compact objects could enhance the production rate of GW sources by orders of magnitudes • The closest captured stars near the MBH could serve as direct probes of GR effects, and MBH properties such as its spin • Captured stars could change the structure of nuclear clusters • Production of hypervelocity stars

  45. The capture scenario: What have we learned ? • Additional implications: • Capture of compact objects could enhance the production rate of GW sources by orders of magnitudes • The closest captured stars near the MBH could serve as direct probes of GR effects, and MBH properties such as its spin • Captured stars could change the structure of nuclear clusters • Production of hypervelocity stars

  46. Hypervelocity stars (HVSs): Observations ~100 3-4 MSUN HVSs at 10<r<100 kpc

  47. Hypervelocity stars: Theories • Binary disruption by a MBH (Hills 1988, Yu & Tremaine 2003, Perets et al. 2007) • Scattering by an in-spiraling intermediate mass black hole in the Galactic center (Hansen 2003; Yu & Tremaine 2003, Levin 2005) • Scattering by SBHs/stars in the Galactic center (Miralda-Escude & Gould 2000, Yu & Tremine 2003, O’leary & Loeb 2007) • Hyper-runaway stars (binary disruption by stellar scattering or supernova) (Leonard 1991; Heber et al 2008, Brown et al. 2008; Perets & Subr 2008, in prep.)

  48. Observational constraints on the origin of HVSs • Velocity vector and distribution • Spatial (and temporal) distribution • Lifetime vs. trajectory • Metallicity • Binarity • Rotational velocity • Total number • Galactic center S-stars

  49. Observational constraints on the origin of HVSs • Velocity vector and distribution • Spatial (and temporal) distribution • Lifetime vs. trajectory • Metallicity • Binarity • Rotational velocity • Total number • Galactic center S-stars (Too) large statistics are required (see Perets 2007, 2008)

  50. Observational constraints on the origin of HVSs • Velocity vector and distribution • Spatial (and temporal) distribution • Lifetime vs. trajectory • Metallicity • Binarity • Rotational velocity • Total number • Galactic center S-stars (Too) large statistics are required

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