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
Introduction HVSs and Star Clusters Milky Way Mass Exoplanets Conclusions
Outline
1 Introduction
2 HVSs and Star Clusters
3 Milky Way Mass
4 Exoplanets
5 Conclusions
Giacomo Fragione Stellar aggregates over mass and spatial scales - 09/12/2016
Hypervelocity Stars as Tools for Galactic Astrophysics
Introduction HVSs and Star Clusters Milky Way Mass Exoplanets 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
Giacomo Fragione Stellar aggregates over mass and spatial scales - 09/12/2016
Hypervelocity Stars as Tools for Galactic Astrophysics
Introduction HVSs and Star Clusters Milky Way Mass Exoplanets Conclusions
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
Giacomo Fragione Stellar aggregates over mass and spatial scales - 09/12/2016
Hypervelocity Stars as Tools for Galactic Astrophysics
Introduction HVSs and Star Clusters Milky Way Mass Exoplanets Conclusions
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
Giacomo Fragione Stellar aggregates over mass and spatial scales - 09/12/2016
Hypervelocity Stars as Tools for Galactic Astrophysics
Introduction HVSs and Star Clusters Milky Way Mass Exoplanets Conclusions
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)
Giacomo Fragione Stellar aggregates over mass and spatial scales - 09/12/2016
Hypervelocity Stars as Tools for Galactic Astrophysics
Introduction HVSs and Star Clusters Milky Way Mass Exoplanets Conclusions
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
Giacomo Fragione Stellar aggregates over mass and spatial scales - 09/12/2016
Hypervelocity Stars as Tools for Galactic Astrophysics
Introduction HVSs and Star Clusters Milky Way Mass Exoplanets Conclusions
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
Giacomo Fragione Stellar aggregates over mass and spatial scales - 09/12/2016
Hypervelocity Stars as Tools for Galactic Astrophysics
Introduction HVSs and Star Clusters Milky Way Mass Exoplanets Conclusions
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
Giacomo Fragione Stellar aggregates over mass and spatial scales - 09/12/2016
Hypervelocity Stars as Tools for Galactic Astrophysics
Introduction HVSs and Star Clusters Milky Way Mass Exoplanets Conclusions
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�
Giacomo Fragione Stellar aggregates over mass and spatial scales - 09/12/2016
Hypervelocity Stars as Tools for Galactic Astrophysics
Introduction HVSs and Star Clusters Milky Way Mass Exoplanets Conclusions
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�
Giacomo Fragione Stellar aggregates over mass and spatial scales - 09/12/2016
Hypervelocity Stars as Tools for Galactic Astrophysics
Introduction HVSs and Star Clusters Milky Way Mass Exoplanets Conclusions
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�
Giacomo Fragione Stellar aggregates over mass and spatial scales - 09/12/2016
Hypervelocity Stars as Tools for Galactic Astrophysics
Introduction HVSs and Star Clusters Milky Way Mass Exoplanets Conclusions
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
Giacomo Fragione Stellar aggregates over mass and spatial scales - 09/12/2016
Hypervelocity Stars as Tools for Galactic Astrophysics
Introduction HVSs and Star Clusters Milky Way Mass Exoplanets Conclusions
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
Giacomo Fragione Stellar aggregates over mass and spatial scales - 09/12/2016
Hypervelocity Stars as Tools for Galactic Astrophysics
Introduction HVSs and Star Clusters Milky Way Mass Exoplanets Conclusions
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
Giacomo Fragione Stellar aggregates over mass and spatial scales - 09/12/2016
Hypervelocity Stars as Tools for Galactic Astrophysics
Introduction HVSs and Star Clusters Milky Way Mass Exoplanets Conclusions
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
Giacomo Fragione Stellar aggregates over mass and spatial scales - 09/12/2016
Hypervelocity Stars as Tools for Galactic Astrophysics
Introduction HVSs and Star Clusters Milky Way Mass Exoplanets Conclusions
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�
Giacomo Fragione Stellar aggregates over mass and spatial scales - 09/12/2016
Hypervelocity Stars as Tools for Galactic Astrophysics
Introduction HVSs and Star Clusters Milky Way Mass Exoplanets Conclusions
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)
Giacomo Fragione Stellar aggregates over mass and spatial scales - 09/12/2016
Hypervelocity Stars as Tools for Galactic Astrophysics
Introduction HVSs and Star Clusters Milky Way Mass Exoplanets Conclusions
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
Giacomo Fragione Stellar aggregates over mass and spatial scales - 09/12/2016
Hypervelocity Stars as Tools for Galactic Astrophysics
Introduction HVSs and Star Clusters Milky Way Mass Exoplanets Conclusions
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
Giacomo Fragione Stellar aggregates over mass and spatial scales - 09/12/2016
Hypervelocity Stars as Tools for Galactic Astrophysics
Introduction HVSs and Star Clusters Milky Way Mass Exoplanets Conclusions
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
Giacomo Fragione Stellar aggregates over mass and spatial scales - 09/12/2016
Hypervelocity Stars as Tools for Galactic Astrophysics
Introduction HVSs and Star Clusters Milky Way Mass Exoplanets Conclusions
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)
Giacomo Fragione Stellar aggregates over mass and spatial scales - 09/12/2016
Hypervelocity Stars as Tools for Galactic Astrophysics
Introduction HVSs and Star Clusters Milky Way Mass Exoplanets Conclusions
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.
Giacomo Fragione Stellar aggregates over mass and spatial scales - 09/12/2016
Hypervelocity Stars as Tools for Galactic Astrophysics