hypervelocity stars as tools for galactic astrophysics · introduction hvss and star clusters milky...

Introduction HVSs and Star Clusters Milky Way Mass Exoplanets Conclusions

Hypervelocity Stars as Toolsfor Galactic Astrophysics

Giacomo Fragione

Sapienza University of Rome

collaborators: Roberto Capuzzo-Dolcetta (Sapienza University of Rome),Avi Loeb (Harvard University), Idan Ginsburg (Harvard University),

Pavel Kroupa (University of Bonn - University of Prague)

Giacomo Fragione Stellar aggregates over mass and spatial scales - 09/12/2016

Hypervelocity Stars as Tools for Galactic Astrophysics


Outline

1 Introduction

2 HVSs and Star Clusters

3 Milky Way Mass

4 Exoplanets

5 Conclusions




Hypervelocity Stars

Hypervelocity Stars (HVSs): unbound stars escaping the host Galaxy(Brown 2015)

Theoretical prediction:Hills 1988

Hills, Nature, 331, 687, 1988

Observational evidence:Brown+ 2005

Brown, Geller, Kenyon. ApJ, 622, L33, 2005




Hills Mechanism

Binary disruption by a super massive black hole (Hills 1988; Yu &Tremaine 03; Kenyon+ 2008; Kenyon+ 2014; Sari+ 2010)

Brown, Annu. Rev. Astron. Astrophys., 53, 157,2015

Mean ejection velocity(Bromley+ 2006)

vej ≈ 1.8× 103 km s−1 ×

×(

0.1 AU

a

)1/2 (mb

2 M�

)1/3

×

×(

MBH

4× 106 M�

)1/6

a = binary separationmb = total binary massMBH = black hole mass




Multiple Mirror Telescope Survey

21 HVSs (Brown+ 2005, 2007, 2010, 2014)+ US708 Helium-rich subdwarf O star(Hirsch+ 2005)+ HE0437-5439 MS 9 M�(Edelmann+ 2005)

• spectroscopic survey of starswith colors of 2.5÷ 4 M� lateB-type stars

• only radial velocity

• outer halo (& 20 kpc) and|b| & 30◦

• ejection rate 10−5-10−4 yr−1

• low density

n(r) ≈ 8

(r/kpc)2kpc−3

• Gaia satellite expected to find≈ 100 HVSs




Production Mechanisms

HVSs have both slow and rapid rotations suggesting differentacceleration mechanisms (Hansen 2007; Lopez-Morales & Bonanos 2008)

Tutukov & Fedorova. Ast. Rep. 53-9, 839, 2009

• Composition of the Galactic Centre(Gould & Quillen 2003; Gualandris+2005; Antonini+ 2010, 2011)

• Supernovae (Zubovas+ 2013)

• Galactic potential and Dark Matter(Gnedin+ 2005; Yu & Madau 2007)

• Planetary dynamics (Ginsburg+ 2012)

• Disks around Sgr A∗ (Lockmann+ 2008;Subr & Haas 2016)

• Star clusters (Capuzzo-Dolcetta &Fragione 2015; Fragione &Capuzzo-Dolcetta 2016; Arca-Sedda+ 16)




Young Star Clusters Infall

Infall of a young cluster (MYC = 103 M�) toward the Galactic Centre inelliptical orbit around the Milky Way’s BH (Fragione, Capuzzo-Dolcetta& Kroupa, submit. to MNRAS, arXiv:1609.05305, 2016)

• Plummer distribution

• Kroupa initial mass function (Kroupa 2001)

• mmax = 40.54 M� (Weidner & Kroupa 2004)

• rh = 0.1× (Mcl/M�)0.13 pc = 0.25 pc (Marks & Kroupa 2012)

• Canonical period distribution (Kroupa 1995)

• e drawn from thermal distribution (Kroupa 2008)

• Black Hole of MBH = 4 · 106 M� (Gillessen+ 2009, 2016)

• Kenyon+ 2008, 2014 Milky Way potential




Single and Binary HVSs

V (km/s)

P

1000 1500 2000 2500 3000 3500 4000

0e

+0

02

e−

04

4e

−0

46

e−

04

8e

−0

41

e−

03

Gamma

Weibull

Lognormal

Normal

HVSs ejection velocity aredistributed as a Gaussiandistribution with mean ≈ 1200km s−1

0

0.2

0.4

0.6

0.8

1

0 0.2 0.4 0.6 0.8 1

Fra

ctio

n

B

SingleBinary

The relative number of ejectedsingle and binary HVSsdepends on the primordialbinary fraction B: as Bincreases, the number ofejected binary HVSs increases




Ejection Geometry

0

0.2

0.4

0.6

0.8

1

-100 -50 0 50 100

P

latitude (degree)

0

0.2

0.4

0.6

0.8

1

0 50 100 150 200 250 300 350 400

P

longitude (degree)

• stars ejected nearly in thecluster orbital plane

• stars ejected preferentiallywhen the cluster passesnear the pericentre

• HVSs in burst-like eventsnearly in the clusterorbital plane and in thedirection of the clusterorbital motion, whosebinary fraction dependson the initial binaryfraction in the progenitorcluster




Role of the Initial Mass Segregation

The initial degree of mass segregation S affects the mass distribution ofHVSs

0

0.2

0.4

0.6

0.8

1

0 0.5 1 1.5 2 2.5 3 3.5

P

mass (MO•)

Single, IMS=0

Unsegregated young cluster (S = 0):≈ 20% of HVSs have m∗ & 0.5 M�

0

0.2

0.4

0.6

0.8

1

0 0.5 1 1.5 2 2.5 3 3.5

Pmass (M

O•)

Single, IMS=1

Segregated young cluster (S = 1):≈ 97% of HVSs have m∗ . 0.5 M�




Role of the Initial Mass Function

The initial mass function affects the mass distribution of HVSs

0

0.2

0.4

0.6

0.8

1

0 0.5 1 1.5 2 2.5 3 3.5

P

mass (MO•)

Kroupa IMF

Segregated young cluster withKroupa IMF:≈ 97% of HVSs have m∗ . 0.5 M�

0

0.2

0.4

0.6

0.8

1

0 0.5 1 1.5 2 2.5 3 3.5

P

mass (MO•)

Top-heavy IMF

Segregated young cluster withTop-heavy IMF:≈ 18% of HVSs have m∗ & 0.5 M�




Role of the Initial Total Mass

The initial total mass affects the mass distribution of HVSs

0

0.2

0.4

0.6

0.8

1

0 1 2 3 4 5 6

P

mass (MO•)

Mcl

=104 M

O•

104 M� young cluster:≈ 15% of HVSs have m∗ & 0.5 M�

0

0.2

0.4

0.6

0.8

1

0 0.5 1 1.5 2 2.5 3 3.5

P

mass (MO•)

Single, IMS=1

103 M� young cluster:≈ 97% of HVSs have m∗ . 0.5 M�




Blue Stragglers HVSs

0

0.2

0.4

0.6

0.8

1

0 50 100 150 200 250 300

P

a(AU)

0

0.2

0.4

0.6

0.8

1

0 0.2 0.4 0.6 0.8 1

P

e

Binary eigenevolution (Kroupa 95)

e

e= −ρ

ρ =

(λR�

Rper

)χqfin = qin + (1 − qin)ρ∗

ρ∗ =

{ρ ρ ≤ 1

1 ρ > 1

m2,fin = qfinm1,in ; m1,fin = m1,in

Pb,fin = Pb,in

(mtot,in

mtot,fin

)1/2 (1 − ein

1 − efin

)3/2

λ = 28 , χ = 0.75 ; λms = 24.7 , χms = 8

• ≈ 7% of binary HVSs mergebecoming blue stragglers HVSs




Observations vs Theory

Observations

• HVSs mass2.5 M� . MHVS . 4 M�(Brown+ 14)

• HVSs clumped in thedirection of Leo constellation(Brown+ 14)

• Binary HVS(Nemeth+ 16)

• Blue straggler HVS(Edelmann+ 05)

Theory

• Low initial degree of masssegregation/top-heavyIMF/massive cluster

• HVSs ejected in the clusterorbital plane and in thedirection of its motion

• Binary HVSs produced bybinary-rich clusters

• Blue stragglers HVSs frommerged binary HVSs




Milky Way Dark Matter Halo

Wang et al, MNRAS, 377, 687, 2015

Cosmological problems of the MilkyWay (Taylor+ 16)

• Efficiency conversion ofbaryons into stars

• Large Magellanic Cloud andthe Leo I orbit

• Too-big-to-fail problem

• Missing satellite problem

A light halo mass (≤ 1012 M�) cansave ΛCDM small-scales predictions




HVSs Asymmetric Distribution

-400

-200

0

200

400

10 20 30 40 50 60 70 80

V (

km

/s)

r (kpc)

MDM

=0.6•1012

MO•

MDM

=1.0•1012

MO•

MDM

=1.4•1012

MO•

MDM

=1.8•1012

MO•

-400

-200

0

200

400

20 40 60 80 100 120

V (

km

/s)

r (kpc)

t*=300 Myrt*=410 Myrt*=510 Myrt*=640 Myr

HVSs asymmetric distributiondepends on the Milky Way potentialand HVSs propagation time (Perets+2009)

• Unbound HVSs are onlyoutgoing

• Bound HVSs can be outgoingand ingoing

• Ingoing stars have velocity up to−vesc (r)

• Ingoing stars can evolve to adifferent stellar type because offinite lifetime

Fragione & Loeb, submitted,arXiv:1608.01517, 2016




Constraining Milky Way Mass with HVSs

20

30

40

50

60

70

3 3.5 4 4.5 5 5.5 6 6.5

∆

t* ( 100 Myr )

MDM

=0.6•1012

MO•

MDM

=0.8•1012

MO•

MDM

=1.0•1012

MO•

MDM

=1.2•1012

MO•

35

40

45

50

55

60

65

70

3 3.5 4 4.5 5 5.5 6 6.5

∆

t* ( 100 Myr )

MDM

=1.4•1012

MO•

MDM

=1.6•1012

MO•

MDM

=1.8•1012

MO•

The HVSs asymmetry ∆ mustbe compatible with the numberof stars (Γ = 37) that give theexcess in the velocitydistribution (Brown+ 2014)

ΦNFW (r) = −GMDM ln(1 + r/rs )

r

HVSs favour a light darkmatter halo with

0.6×1012 M� . MDM . 1.2×1012 M�

12.7 kpc . rs . 20.0 kpc

Total Milky Way mass

1.2×1012 M� ≤ MDM ≤ 1.9×1012 M�




Hypervelocity Stars and Exoplanets

HVSs can be ejected with a planetary system and binary disruption cangenerate Hypervelocity Planets (Ginsburg+ 12; Fragione in prep.;Fragione & Ginsburg in prep.)

−100 −50 0 50 100 150 200 250 300

0

500

1000

1500

2000

2500

3000

X (AU)

Y (

AU

)

Starting Location

Binary System

Star 1

HVS + 2 Planets

HVP 1

HVP 2

Ginsburg, Loeb & Wegner, MNRAS, 423, 948, 2012

• HVSs can retain compactplanets (Ginsburg+ 2012)

• Kepler mission (Borucki+2010; Lissauer+ 2011)

• Transiting Exoplanet SurveySatellite mission by NASA:expected launch 2017-2018(Sullivan+ 2015)




Geometric Transit Probabilities

10-2

10-1

100

0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5

Pro

ba

bili

ty

a(AU)

e=0.0;M*=3.0 MO•

e=0.4;M*=3.0 MO•

e=0.0;M*=1.0 MO•

e=0.4;M*=1.0 MO•

e=0.0;M*=0.3 MO•

e=0.4;M*=0.3 MO•

10-2

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4

Pro

ba

bili

ty

σe

M*=0.3 M

O•

M*=0.5 M

O•

M*=1.0 M

O•

M*=2.0 M

O•

M*=3.0 M

O•

Geometric probability for single planets(Murray & Correia 10; Winn 10)

pT ≈R∗

a(1− e2)

R∗ = star radiusa = planet semi-major axise = planet orbital eccentricity

corbits algorithm computes thecombined geometric probability of amulti-transit in an exoplanetary system(Brakensiek & Ragozzine 16)

Fragione & Ginsburg, accepted byMNRAS, arXiv:1609.03905, 2017




Observing Real Transits

Probability of observing a transitdetermined by physical parameters...

• Transit duration (Winn 10)

T ≈ 6P

1 day

R∗

a

√1− e2 hr

• Drop in brightness (Winn 10)

f (t) =F (t)

F∗∝(

Rp

R∗

)2

• Signal to Noise ratio (Burke+ 15)

S/N =√

Ntr∆

σ

...but also observational strategy

• Probability of detection (Fressin+13, Burke+ 15)

Pdet =1

2+

1

2erf

(S/N− 7.1√

2

)• Window function: probability that a

requisite number of transits requiredfor detection occurs in theobservational data as a function ofplanetary orbital period

10−3 . P . 10−1 geometrical probabilitiesimplies that at least one transit can bespotted with & 90% probability around alow-mass (. 1 M�) HVSs




Radial Velocities

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5

K(m

/s)

a(AU)

e=0.0;M*=3.0 MO•

e=0.4;M*=3.0 MO•

e=0.0;M*=1.0 MO•

e=0.4;M*=1.0 MO•

e=0.0;M*=0.3 MO•

e=0.4;M*=0.3 MO•

0

50

100

150

200

250

300

350

400

450

500

0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5

K(m

/s)

a(AU)

e=0.0;M*=3.0 MO•

e=0.4;M*=3.0 MO•

e=0.0;M*=1.0 MO•

e=0.4;M*=1.0 MO•

e=0.0;M*=0.3 MO•

e=0.4;M*=0.3 MO•

Stellar velocity amplitude induced bythe planet (Cummings+ 08)

K =28.4 m/s√

1− e2

Mp sin i

MJ×

×( a

1 AU

)−1/2(

M∗

M�

)−1/2

M∗ = star massMp = planet massa = planet semi-major axise = planet orbital eccentricity

Radial Velocity surveys are able toobserve massive planets andEarth-sized planets provided that theduration of the survey is large enough




Conclusions

• Physical and kinematic feature of HVSs can be used to constrainthe physical properties of star clusters infallen onto the massiveblack hole

• An unsegregated/top-heavy IMF/massive young stellar cluster mayhave originated part of present HVSs (Fragione, Capuzzo-Dolcetta& Kroupa 2016)

• HVSs data can constrain the dark content of the Milky Way

• HVSs data suggest a light dark matter halo . 1.2 × 1012 M�(Fragione & Loeb 2016)

• Possibility of studying planetary dynamics after strong gravitationalperturbations

• New-generation telescopes are expected to spot planets aroundHVSs (Fragione & Ginsburg 2016)




A Bright Future for Hypervelocity Stars

Brown 2015: We are at an exciting time in the study of HVSs. New HVScandidates will soon come from Southern Hemisphere imaging surveys.The distribution of HVSs in large, uniform surveys will test proposedHVS ejection mechanisms. Gaia proper motions may directly link HVSsto their MBH origin. The future of HVSs appears unbounded.



hypervelocity stars as tools for galactic astrophysics · introduction hvss and star clusters milky...

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