Dynamical evolution of the young stars in the Galactic center

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 lt0.04 pc
• more beyond smooth distribution?
• Regular mass function
• Young OB stars
• (coherent structures isotropic)
• scale 0.05-0.5 pc
• mass103-104 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 gt1 pc
• Disk-gtinner 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

• Paradox of youth

• 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

7
The s-stars origins

• 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

• 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
The Capture Scenario
MBH
Binary
9
The CaptureScenario
MBH
Binary
Eccentric disk instability
10
Capture scenario
Binary
Binary disruption
MBH
11
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-starsN-body
simulations
Captured stars
Migrating disk stars
30 are destroyed
Perets et al. 2009
Simulations on the special purpose GRAPE 6
computer in RIT (Collaborators Allesia
Gualandris, David Merritt and Tal Alexander)
16
Dynamical evolution of the S-starsN-body
simulations
30 are destroyed
Perets et al. 2009
Simulations on the special purpose GRAPE 6
computer in RIT (In collaboration with Allesia
Gualandris and David Merritt)
17
Capture origin dynamical evolution implications

• Distribution of orbits is distance dependent
  isotropic and thermal in inner region (lt0.04)
• for initially eccentric orbits, but not so for
  initially low eccentricity orbits (planetary
  migration scenario).
• gt Look for eccentric orbits in outer regions !
• Weak bias towards younger stars
• Possible correlation between age and eccentricity

18
S-stars current status

• Theory
• v 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)
• v Consistent with capture, not
• with planetary like migration
• v but note that comparison
• is indirect (different
• masses probed)
• v Temporal continuous
• ? Spatial - (Ann-Maries talk).

• 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

19
The disk (?) stars
Hobbs Nayakshin 2009
Bonnel Rice 2008
Initial conditions single/multiple
circular/eccectric disks/rings/streams Choose
your favorite
20
Dynamical evolution of a stellar diskA two
temperature system

• 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.)

21
Evolution of a single diskCusp heating and
torque
With The Cusp
Without The Cusp
ß14 0
ß
Perets et al., in prep.
A stellar nuclear disks after 6 Myrs of dynamical
evolution
22
Evolution of a single diskLong 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 starsDynamical 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
The s-stars theories

• 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

• 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
• s velocity dispersion
• For µ2gtgt1, 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
The Capture Scenario
MBH
Binary
34
The CaptureScenario
MBH
Binary
35
Captured Young Stars and Hypervelocity stars

Perets et al. 2007

• 5-35 captured young B-stars (gt 4 Msun)
• 10-65 observable young hypervelocity B-stars (gt
  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-starsN-body
simulations
Captured stars
Migrating disk stars
60 are destroyed
Perets et al. 2008a, in prep.
Simulations on the special purpose GRAPE 6
computer in RIT (In collaboration with Allesia
Gualandris and David Merritt)
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
Laser Interferometer Space Antenna (LISA)
Gravitational Waves Sources
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 10ltrlt100 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,
  Oleary 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
51
Constraints on the origin of HVSs The number of
hyper-runaway stars

• The ejection rate of hyper-runaway stars
• is very low
• lt 10-8 from supernovae binary disruption
• The rate of binary disruption through scattering
  is even lower in regular clusters
• Super star clusters (e.g. Westerlund I) may
• contribute 0.25 HVSs scattered hyper-runaways
  per cluster favors massive HVSs
• gt another few HVSs, but can not explain most of
  the HVSs observations

(Brown et al. 2008)
(Perets Subr 2008, in prep.)
52
Hyper-runaway stars

• Super star clusters (e.g. Westerlund I) may
  contribute 0.25
• hyper-runaways per cluster. For an optimistic
  estimate of 40
• such clusters existing in the last 4x108 yrs we
  get 10 HVSs.
• At most explains A
• small fraction of
• the observed HVSs.

(Perets Subr 2008, in prep.)
53
Constraints on the origin of HVSs The S-stars
(Perets 2007)

• IMBH inspiral - HVSs ejection efficiency f0.05
  (0.07) at 0.001 (0.01) pc for IMBH mass of 5x103
  (1.5x104) MSun
• 100 HVSs/f gt 2000 (1400) B-stars required
• SBHs scattering - HVSs ejection efficiency
• f0.005 at 0.01 pc, but in a longer period
• 100 HVSs/f gt 2x104 B-stars required
• Obsevations lt1 B-star in 0.001 pc
• 10 B-stars in 0.01 pc
• Incomplete gt could be a few times higher
• Binary disruption by MBH good agreement, but
  note potential problem with S2

(Sesana et al. 2008)
(Oleary Loeb 2007)
(Perets, Hopman Alexander 2007)
54
Constraints on the origin of HVSsSpatial-tempora
l distribution
Brown et al. 2008
55
Constraints on the origin of HVSs Binarity
(Perets 2007)

• IMBH can eject binaries while other mechanisms
  can not ! (Lu, Yu Lin 2007)
• But
• Survival of binaries in the galactic center was
  not taken into account
• Disruption of massive triples can produce binary
  HVSs

56
Constraints on the origin of HVSs Binarity
Survival of binaries in the Galactic center
0.1 AU
1 AU
10 AU
Perets 2007
57
Triple disruption
Triple
MBH
58
Triple disruption
MBH
Hypervelocity Binary/star
Captured star/binary

59
Rejuvenated massive HVSs
(Perets 2008)
60
Constraints on the origin of HVSs Rotational
velocities

• Stars in binaries have different mean rotational
  velocities than single stars (in the field)
• Could be used to constrain HVSs scenarios
• But, mean (value) is dangerous
• Taking into account
• the whole distribution
• requires gt25 and more
• likely gt 100 HVSs
• to differentiate between
• ejection scenarios

(Hansen 2008)
Perets 2007
61
Constraints on the origin of HVSs Metallicity

• Different galactic regions may differ in mean
  metallicity - constrain HVSs origin
• ... but, again, the mean is dangerous !
• Taking into account the whole distribution
• make this method irrelevant in regard
• to single stars

(Przybla et al. 2008 Bonanos Lopez-Morrales
2008)
(Perets 2008)
62
Dynamical evolution of hypervelocity stars and
the Galactic potential
See also Gnedin 2005 Yu et al. 2007 Kenyon et
al. 2008
Perets et al. 2008
63
Estimate of the number of HVSs
Perets et al. 2008
Note Tidal streams can contaminate the sample
64
Extragalactic HVSs

• Luminous young stars in old galactic halos
• Contamination from Galactic white dwarfs
• Statistical approaches could be used
• Hypervelocity planetary nebula ?
• Hypervelocity Supernova ?

65
Extragalactic hypervelocity star (?)
SN 2005E
Perets et al. 2008b, in prep.
66
Hypervelocity stars summary

• Hundreds of HVSs may exist in the galaxy
• Most are likely to have been ejected from the
  Galactic center most are not hyper-runaways
  (but maybe ejected through an otherwise unknown
  process)
• Observations of stars in the Galactic center
  constrain HVSs ejection scenarios - favor the
  binary disruption scenario induced by scattering
  massive perturbers
• Some observational constraints are more
  complicated than they seem

67
Hypervelocity stars summary

• Possible existence of rejuvenated and binary HVSs
• HVSs asymmetric distribution a tool to probe
  the Galactic potential (dark matter, MOND) and
  estimate HVSs numbers
• Future observation of extragalactic HVSs -
  Hypervelocity Supernovae (?)

68
The Capture Scenario and Fast Relaxation Summary
Relaxation by stars
Relaxation by massive perturbers
Binary scattering into MBH
LOW rates
HIGH rates
Ejection of a hypervelocity star
Binary/triple Disruption
Binary MBHs coalescence
Capture of star Close to the MBH
Capture of a MS star close to the MBH
Capture of compact object close to the MBH
The S-stars
Probes of GR effects MBH properties
Gravitational wave in-spiral
Gravitational wave burst
69
The Capture Scenario and Fast Relaxation Summary
Relaxation by stars
Relaxation by massive perturbers
Binary scattering into MBH
LOW rates
HIGH rates
Ejection of a hypervelocity star
Binary/triple Disruption
Binary MBHs coalescence
Probes of the Galactic potential
Capture of star Close to the MBH
Capture of a MS star close to the MBH
Capture of compact object close to the MBH
The S-stars
Probes of GR effects MBH properties
Gravitational wave in-spiral
Gravitational wave burst
70
The Capture Scenario and Fast Relaxation Summary
Relaxation by stars
Relaxation by massive perturbers
Binary scattering into MBH
LOW rates
HIGH rates
Ejection of a hypervelocity star
Binary/triple Disruption
Binary MBHs coalescence
Capture of star Close to the MBH
Capture of a MS star close to the MBH
Capture of compact object close to the MBH
Gravitational wave in-spiral
The S-stars
Probes of GR effects MBH properties
Gravitational wave burst
71
The Capture Scenario and Fast Relaxation Summary
Relaxation by stars
Relaxation by massive perturbers
Binary scattering into MBH
LOW rates
HIGH rates
Ejection of a hypervelocity star
Binary/triple Disruption
Binary MBHs coalescence
Capture of star Close to the MBH
Capture of a MS star close to the MBH
Capture of compact object close to the MBH
Gravitational wave in-spiral
The S-stars
Probes of GR effects MBH properties
Gravitational wave burst
72

• Thank you !
• Some bibliography

In prep Perets, Hopman Alexander Perets
Alexander Perets et al. a,b,c Perets Subr
Perets 2007, 2008 Perets, Hopman Alexander
2007 Perets Alexander 2008 Perets et al. 2008
73
Two disks interaction Torques, Kozai and cusp
stabilization
With The Cusp
Without The Cusp
Perets et al. 2008d, in prep.
Two perpendicular disks after 6 Myrs of dynamical
evolution
74
Two disks interaction Torques, Kozai and cusp
stabilization
Eccentricity Distribution
Edge-on view
Perets et al. 2008d, in prep.
Two perpendicular disks after 6 Myrs of dynamical
evolution
75
Binary MBH coalescence
76
Binary MBH coalescence
77
The Final Parsec Problem
78
Binary-MBH Coalescence
79
Semi-major axis evolution
80
Semi-major axis evolution
109
108
107
106
81
Coalescence time
Stars
Clusters
GMCs