youjun lu - university of california, los angeles

YoujunLuNa*onalAstronomicalObservatoryofChina

2016.02.08@Aspen

2/11/16 GCconference@Aspen 1

Collaborators:FupengZHANG(SYSU)QingjuanYU(KIAA)

Ø ConstrainingthespinofthemassiveblackholeattheGalac*ccenterviathemo*onofasurroundingstar(QingjuanYU)

Ø Ontes*ngtheKerrmetricintheGCviastarorbits:theeffectsofstellarperturba*ons(FupengZHANG)


Outline

Ø Background:GCS-stars,andhypervelocitystars

Ø Dynamicalmodels:TidalbreakupofbinarystarsinthevicinityoftheGCMBH

Ø Confron*ngobserva*onswithmodelsØ Modelpredic*onsØ Summary


Introduc*on:evidenceforanMBHintheGC

Velocities & Orbits of Stars ! Mass M • = rv

2 /G M • = 4 ×106M⊙

2/11/16 GCconference@Aspen 4Ghezetal;Genzel,etal.

Introduc*on:Galac*ccenterAuniquelaboratoryforstellardynamicsandblackholephysics

v  StellarstructuresintheGalac*cCenter(GC)

Ø Youngstellardisk(s):0.04-0.5pcclockwiserota*ngstellardisk(CWS)+acounterclockwisedisk?

Ø S-stars:<4000AU(isotropic)theclosestonetotheMBH(<1000AU)(S0-2/S2,S0-102)

InterplaybetweenstarsandMBH


Introduc*on:forma*onofS-stars

2/11/16 GCconference@Aspen 6Alexander+Antonini+Chen+Perets+manyothertalks

v  Stellardisks:starswereformedinagaseousdisk(insitu)

v  S-stars:<4000AU(Sdalforce)Ø Youthparadox:(Ghezetal.2003)

²  RejuvenaSonofoldstars?²  MigraSonofstarsfromthestellardisk?

(Madigan+ 2009; Baruteau+ 2011, Chen+ 2014)²  Exchangecaptures?(Gould & Quillen 2003) Hills mechanism (tidal breakup of binaries)

v WhataretheoriginsoftheS-stars?ArethereanybrightstarsclosertothecentralMBHthantheknownS-stars?v CantheclosestonebeusedtoprobethemetricoftheGCMBHorthedistribuSonofbackgroundstars(stellarremnants)?

Hypervelocitystars(HVSs)intheGalac*chalo


Brownetal.2014,2015

Ø  EscapingawayfromtheMilkywayhalo(hypervelocity)Ø  Distancefrom~40kpcto~120kpcfromtheGCØ  MainlytypeBstars(3-4M")

Hypervelocitystars(HVSs)

HVSsasapredic*onofHillsmechanism(andothers)aredetectedbylaterobserva*ons!


21EscapingawayfromMilkyWay

Brownetal.2014,2015

Tidalbreakupofbinarystars v  TidalbreakupofbinarystarsinthevicinityoftheMBH

Ø capturedcomponent:S-star(<4000AU)

Ø ejectedcomponent:hypervelocitystar(>700km/s)

Injec*ngbinary

Captured“S-star”

Ejected“HVS”

(Gould&Quillen2003)

(Hills1988;Yu&Tremaine2003)


HVSs as a prediction of the Hills mechanism (and others) are confirmed by later observations!

Tidalbreakupofbinarystars

180°-Θ≈10°

Weaklyboundbinaries

HVSscanwellmemorizetheinjec*ngdirec*onsoftheirprogenitors,i.e.,theejec*ngdirec*onofanHVSisalmostan*-paralleltotheinjec*ngdirec*onofitsprogenitor. Luetal.2010

Zhangetal.20102/11/16 10S-stars,HVSsandtheMBHintheGC

Ini*allyunboundbinaries

Summaryontheobserva*onalsta*s*csforHVSsandGCS-stars

HVSs: Ø  Mass ~3-4M"; B-type

Ø  Number: ~21(24) (~100)

Ø  Spatial distribution: anisotropic

Ø  Distance distribution

Ø  Velocity distribution

S-Stars

Ø  Mass ~7-15M"; B-type

Ø  Number: ~17 (<4000AU)

Ø  Spatial distribution: isotropic

Ø  Semi-major axis distribution

Ø  Eccentricity distribution

Consistent?


DynamicsofHVSsandS-stars:assumingHillsmechanism

Modelingredient HVSs S-Stars

Injec*ngstellarbinaries

Fromthestellardiskorfrominfinity;(slowly)scaqertothelosscone;1-150M";semimajoraxisandmassra*odistribu*onsofthebinaries;

IMF:intermediatetop-heavyorothersTidalbreakup Ejected Captured(full3-body)

Laterdynamicalevolu*on

TransportfromtheGCtotheGalac*chalo

² Non-andResonantrelaxa*ons;²  GRprecessionandGravita*onalwavedecay.

Stellarevolu*on Mainsequenceorpulsar,SNkickLu(withZhangandYu:2010a;2010b;2013;2014)


ARMAmodelbyMadiganetal.

Confron*ngobserva*onswithmodels:velocitydistribu*onofHVSs

v  ReproducingmostHVSsta*s*cs:Ø  theory: memory of injecting direction; Ø  observation: consistent with being on two disk planes, one of which is the same as that of the CWS disk in the GC; Ø  conclusion: HVS probably originated from the GC disk(s).

CWSstellardisk

Luetal.2010


HVSsfromLAMOST

AnHVSfoundintheLAMOSTGalac*csurvey:

Ø  Mass:~9M"(possiblecompanionofS-stars)Ø  Velocity:vrf=477km/s(~545km/s)Ø  Loca*on:~19kpc(Galactocentricdist.)Ø  GCorigin:consistentAnotheroneisrecentlyfoundinLAMOST.

2/11/16 GCconference@Aspen 14Zhengetal.2014

Confron*ngobserva*onswithmodels:velocitydistribu*onofHVSs


Mainfactors:Ø  Theini*aldistribu*onofthe

semimajoraxesofinjec*ngbinaries;

Ø  Theperturba*onthatcausestheinjec*onofbinaries.

Canbewellreproducedifbinarystarsperturbedontolow-Jorbitsbydiffusionprocesses/randomwalkorΔJ<<J.

Largeperturba*onsleadtotooflatvelocitydistribu*on.

Slowlydiffusedtothelosscone,binariesaredisruptedbymul*ple*mesofcloseencounter.

CWSdisk

AssumingseveralGalacScpotenSal

ElenaRossiandReemSari’stalksEmptylosscone:slowlydiffusedThroughtheboundaryofthelosscone

Zhangetal.2010

Confron*ngobserva*onswithmodels:S-stars

v  ReproducingmostoftheS-starsta*s*csØ semi-majoraxisdistribu*on(importanceofrelaxaSonresonanceandGRdynamics)Ø eccentricitydistribu*on LuwithZhang&Yu(2010;2013)

Semimajoraxis eccentricity


Stellarmassfunc*on:topheavy

Ø  Thenumberra*oofHVSs/S-starscanbere-producedifthestellarini*almassfunc*onistop-heavy,withMFslope∼−1.6(arequirement).–  AsteeperIMFleadstoatoolargenumberra*oofHVSstoGCS-stars;a

shallowerone->atoosmallnubmerra*o.

Ø  Recentobserva*onsuggeststhattheIMFslopeoftheyoungclusterintheGCis-1.7±0.2,consistentwithourmodelresult.

AfurthersupportforthecommonoriginoftheGCS-starsandtheHVSs.

(Lu,J.etal.,2013;Jessica’stalk)2/11/16 GCconference@Aspen 17

Zhang,Lu,&Yu2013

Modelpredic*ons v  Loca*onsofHVSsatthesouthernheimsphere

v  EjectedcompanionsoftheS-stars:Ø ∼20-60detectableHVSs(~7-15M")intheGalac*cbulgeandhaloØ locatedinadistance<30kpcfromtheGC,radialvelocity～−500-1500km/s,andpropermo*on~5-20mas/yr(thefirstHVSfoundbyLAMOST~9M")

v  CapturedcompanionsofthedetectedHVSsØ ∼20-30capturedstars(~3-7M")intheGC(≲4000AU)Ø theinnermostone:~300-1000AU

v  PulsarsintheGCØ ~100within4000AU;~10within1000AU(alsoPfahl&Loeb2004,Dexter&O’Leary2014)Ø theinnermostone:~120-460AU

v HyperfastpulsarsintheGalac*chalo(resulSngfromexplosionofmassiveHVSsandS-stars)

Ø  ~severaltotenhyperfastpulsars(>1500km/s)


Modelpredic*ons:theinnermostcapturedstarwithmass~3-7M"

(Zhang, Lu, & Yu, 2013, 2014)

Testing GR Probability distribution of the semimajor axis

Probability distribution of the pericenter distance

The innermost one is expected to have a semimajor axis ∼300–1500AU and a pericenter distance ∼10–200AU (depending on different injection models) with a significant probability of being closer to the MBH than S2.


Semimajor axis

Prob

abili

ty

Prob

abili

ty

pericenter distance

Modelpredic*ons:thecloseststartotheMBH

S0-102

S0-102

The predicted probability distribution of the semimajor axis and pericenter distance of the closest star is well consistent with the new observations.

Zhang et al. (2013, 2014)


Semimajor axis pericenter distance

Prob

abili

ty

Prob

abili

ty

Modelpredic*ons:theclosestpulsartotheMBH

The predicted probability distribution of the semimajor axis and pericenter distance of the closest pulsar to the central MBH. (even closer to the MBH than the captured S-stars)

Zhang, Lu, & Yu (2014)

Gravitational Wave decay í


Semimajor axis

Prob

abili

ty

Prob

abili

ty

pericenter distance

SummaryØ AlmostallthediscoveredHVSsarespa*allyconsistent

withbeinglocatedontwothindiskplanes,similartotheorienta*onsofsomeGCstructures(CWSdiskandNortharmofthemini-spiral?),whichsupportstheGCorigin.

Ø Weinves*gatethelinkbetweentheHVSsandtheGCS-starsunderthehypothesisthattheybotharetheproductsof*dalbreakupofthesamepopula*onofstellarbinariesbythecentralMBH.Mostofthesta*s*calproper*esofthedetectedHVSsandGCS-starscouldbereproducedundersomebinaryinjec*ngmodels.

Ø Wepredictthesta*s*caldistribu*onsoftheircompanions(ejectedcompanionsforGCS-starsandcapturedcompanionforHVSs)andthepulsarsresul*ngfromthoseejec*ngstarsandcapturedstars,whichmaybetestedbyfutureobserva*ons.


youjun lu - university of california, los angeles

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