Yutaka Komiya
(National Astronomical Observatory of Japan)
Takuma Suda (NAOJ),
Masayuki Y. Fujimoto (Hokkai Gakuen Univ.)

Extremely metal-poor (EMP) stars
= “living fossils” in the local group

Observation :




2nd
~ 1,000 stars with [Fe/H]<-2.5 is identified in the Milky Way (MW) halo
Database: SAGA (Stellar Abundance for Galactic Archaeology, see Suda-san’s poater)
generation stars
chemical signature of Pop. III supernovae (SN)
Were low mass Pop. III stars formed ?


Pop. III star cluster : Clark+ (2008, 2011), Greif+ (2011), Susa+ (2012)
Pop. III binary : Machida+ (2008), Turk+ (2009), Stacy+ (2010)
Pop. III survivors
⇒

Pop. III survivors
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Where are they ?
What they looks like ?
How can we observe them ?
Method

Hierarchical chemical evolution model
based on the concordance cosmology
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

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Pop. III survivors
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In the MW halo
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Surface abundance
Outside the MW
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Merging history of the Milky Way (semi-analytic)
Gas outflow, circumgalactic matter
Surface pollution of stars by the accretion of interstellar matter.
Escape fraction
Spatial distribution, Detection probability
（2nd generation stars）


Metallicity distribution
Chemical signature of Pop. III stars (PISN)
First star
First supernova
Mini-halo
~106M☉
Milky
Way
Proto-galaxy





Merger tree: Somerville & K (1999)
MMW=1012 M☉, Mmin=M(Tvir=103K)
Gas infall (merger tree), outflow (SN)
All the individual EMP stars are registered in
computations
Constant star formation efficiency : 1×10-10/yr
Instantaneous mixing inside mini-halos.
mass

Yield : Kobayashi et al.(2006, Type II SN)
Nomoto et al. (1984, Type Ia SN)
Umeda & Nomoto (2002, PISN)
redshift

Lognormal IMF
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

Mmd=10Mʘ, σ=0.4 (Pop. II)
(Komiya et al. 2007)
Binary
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
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ξ(log m) = exp( -log(m/Mmd)2/σ２)
Primary
Secondary
Binary fraction: 50%
Mass ratio distribution: n(q) = 1
Pop. III IMF

Fiducial model:
III.2),


Mmd = 200Mʘ (Pop. III.1), Mmd = 40Mʘ (Pop.
Zcr = 10-6Zʘ
A little low mass Pop. III stars are formed.
Parameter dependence

 2 Gm
m    2
2
v

c
s





2
v  cs 
2
2
EMP star
「
[Mg/Fe]
[Ba/Fe]
Gray histogram: HES survey (Schöerck+ 2009)
Black line : SAGA sample
Data from SAGA
(Suda et al. 2008, 2010)
http://saga.sci.hokudai.ac.jp
Rp-rpcess source: 8 – 10 Mʘ
~ 800 Poop. III survivors

In the Milky Way halo



Their surface abundance is changed by the accretion of interstellar
medium (ISM)
⇒Observed as Z ≠ 0
How much are they polluted ?
Outside the Milky Way

Some Pop. III stars are escaped from mini-halo


when their primary companion explode
(3 body interaction in star cluster )
binary

SN explosion
Remains with Z=0
Secondary star
go away

In the Milky Way halo

Metallicity, chemical abundance
object
HE0107-5240:
HE1327-2326:
HE0557-4840:
[Fe/H]
-5.4
-5.7
-4.8
SDSSJ102915+172927: -4.89
[Fe/H] ~－５
⇒ Observed as Hyper Metal Poor stars.
（C, N, s-process: binary mass transfer）
~ 800 Poop. III survivors.

In the Milky Way halo



Their surface is polluted by the accretion of interstellar medium (ISM)
⇒Observed as Z ≠ 0
How much are they polluted ?
Outside the Milky Way

Some Pop. III stars are escaped from mini-halo


when their primary companion explode
(3 body interaction in star cluster )
binary
SN explosion

Remains with Z=0
Secondary star
go away

Outside the Milky Way

Escape frequency



(We assume that the distribution of the orbital parameters of
Pop. III binaries is the same as the solar vicinity )
From mini-halos with
106Mʘ, 20 % of low-mass
Pop. III stars go out.

Outside the Milky Way

Spatial distribution
10 merger trees
100 – 170 Pop. III stars
1000 – 1800 EMP stars
([Fe/H]< -2.5)
2 – 3 Mpc
300kpc
1Mpc
3Mpc
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Detection probability

Giant




V ~ 26 mag @ 1Mpc
(Subaru Strategic Program, i<26 mag, u,g,r,I,z band, 1,400
deg^2 by 5 yrs, )
Discrimination
 Narrow band filter ?
 Spectroscopic follow-up
Main sequence, Turn-off star ⇒ very difficult
Evidence of the Hierarchical Galaxy Formation
 Constrain the Dark-halo Mass of the First Galaxy

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Hierarchical chemical evolution model
Surface pollution
 Metal enrichment of circum-galactic matter

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Pop. III survivors


In the Milky Way halo
⇒ observed as HMP stars by the surface pollution
Outside the Milky Way halo
remained with Z=0
~100 Pop. III survivors, 2 – 3 Mpc
 can be observed by Subaru Hyper Suprime-Cam (?)

IMF of Pop.III
Mmd=10Mʘ
Minimum halo mass
Tvir > 104 K

MDF

Chemical signature

Parameter dependence
Mmd(Pop.III.1) = 40Mʘ
Mmd(Pop.III.1) = 10Mʘ
Zcr = 10-4Zʘ
Greif+ (2011)

Low mass Pop. III stars
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Cluster :
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Binary (multiple system)：

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Clark+ (2008, 2011)
Greif+ (2011)
Susa+ (2012)
…
Machida+ (2008)
Turk+ (2009)
Stacy+ (2010)
…
How and where can we observe Pop. III
survivors ?
Machida+ (2008)
Mini halo
Gas blowout (SN driven wind)
•Energy injection :
•Mass loading :
First SN
Ek: SN kinetic energy = 0.1*Eexp
Ebin: Binding energy of a proto-glaxy
ε(=0.1): minimum outflow energy rate
Msw: mass swept up by a SN shell
•Metal loading :
Evolution of galactic wind in the CGM
•momentum conservation snowplow of th spherical shell
SN ejecta
Pre-enriched
mini halo
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IMF:
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Binary orbit

  log P  4.8  2
Period: Duquennoy & Mayer (1991) f  log P ( day )   exp  
2
2  2.3


Eccentricity: e=1
Remnant mass of massive stars

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Woosley (2002)
Mini-halo
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Lognrmal IMF, Mmd=200Mʘ (Pop. III.1), Mmd=40Mʘ (Pop.III.2)
Binary fraction: 50%
Mass ratio distribution: n(q)=1
NFW density profile
Stars are formed at the center of mini-halo
Escape criterion




Merger tree
tmerge
Main halo
Mass: Mmh(t)
Initial distance: estimated from merger tree.
We assume that,
distance of mini-halo which accrete to main halo with mass M at tmerge
= radius of a spherical shell with M which collapse at tmerge
d2r/dt2 = -GM/r2 + Λc2r/3
We computed distance and radial velocity of mini-halos as a function of
tmerge and Mmh(tmerge). Where tmerge is a age when the mini-halo accrete to the
main halo and Mmh(tmerge) is the mass of main halo at the merger.
Universe
d2r/dt2 = -GM/r2 + Λc2r/3
time
Angle Θ (random)
vinit
d2r/dt2 = -G(Mmain(t)+4πρavr(t)3/3)/r2 + Λc2r/3 + l2/r3
l = r(tform)vescsinθ
Mini halo
rinit
Main halo

In the Milky Way halo
Hyper metal poor stars = Pop. III survivors ?
object
HE0107-5240:
HE1327-2326:
HE0557-4840:
[Fe/H]
-5.4
-5.7
-4.8
SDSSJ102915+172927: -4.7
[C/Fe]
+3.7
+4.16
+1.65
<0.93
Fe: accretion of ISM
C, N. Mg.. : binary mass transfer
Umeda & Nomoto (2002)

PISN ? (~200 Mʘ)
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Type II ? (10 – 50 Mʘ)
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(typical abundance of the halo stars)
Hypernovae ? ( 20 – 50 Mʘ)
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Low [Zn/Fe]
High [Si/Fe], [Ca/Fe]
Odd even effect
Large [Zn/Fe]
(Fast rotating star ?)
(Supermassive star ?)

Mass ratio

Sana & Evans 2010
Raghavan et al. 2010

In the Milky Way halo

Formation epoch
Formation redshift of low mass EMP
stars (red) and Pop.III stars (green) .
Metal enrichment history
of the CGM