1. Takuma Suda, Asao Habe, Masayui Fujimoto (Hokkaido Univ.)
OMEG07 Hokkaido University Dec. 04, 2007
Near Field Cosmology with Binary ― among extremely metal-poor stars in the Galactic halo　―
Yutaka Komiya
(Tohoku Univ.)
Takuma Suda, Asao Habe, Masayui Fujimoto (Hokkaido Univ.)
I’m Yutaka Komiya. from Tohoku University.
Today, I'd like to talk to you about “Near Field Cosmology with Binary”

2. Near Field Cosmology
EMP (Extremely Metal-Poor) stars ([Fe/H]≦-2.5) =“Messengers from the early universe”
Stars formed in the proto-galaxy in the early universe. And low mass stars still alive near to us.
I also would like to talk about extremely metal-poor stars and the cosmology with these stars as a probe.
These EMP stars were formed in the proto-galaxy in the early universe, and low mass stars still survive and are alive in the Milky Way.
So, we can call them “messengers from the early universe”.
From them,
we can investigate the
Formation and evolution of stars in the early universe,
Nature of first stars and first supernovae,
& Galaxy formation process. ?
Formation and evolution of stars in the early universe
Nature of first stars and first supernovae
Galaxy formation process
WMAP
SUBARU

3. Near Field Cosmology Observations of EMP stars Early universe
Chemical evolution
Supernova nucleosynthesis
Dispersion of supernova yields
Changes of the surface abundances of EMP stars
Evolution of EMP star
Binary mass transfer
Accretion of interstellar matter
Early universe
We may investigate the physical condition in the early universe by analyzing the abundances of EMP stars,
For that purpose, we have to study not only chemical evolution in the early universe
but also the changes of the surface abundances of EMP stars during their long lives from early universe to now.
The changes are caused by 3 processes.
Evolution of EMP star
Binary mass transfer
Accretion of interstellar matter

4. Binary Low mass EMP survivor Massive primaryH
C,O He
Massive primary evolved to produce C and transfer them onto the low mass secondary through the wind accretion.
Low mass EMP survivor
Massive primary
Especially, binarity play an important role.
Observationally, 25% of EMP stars show large C enhancement.
& they thought to be formed in a binary system.
Massive primary evolved to produce C and transfer them onto the low mass secondary through the wind accretion.
In other word, EMP survivor formed as secondary component of binary can be a probe into more massive primary star
~25% of EMP stars show large C enhancement. (CEMP) They are formed in a binary system. (Komiya et al. 2007)
An EMP survivor formed as a secondary component of a binary can be a probe into more massive primary star.

5. Initial Mass Function Abundance pattern of CEMP stars
Mass of the primary stars
IMF of EMP stars
Typical mass of EMP stars is ~10 M☉.
(EMP survivors are 1/10 low mass component)
Most of EMP survivors are secondary companion of binary systems.
IMF of primary star
IMF of secondary companion
mξ(m)
Actually,
we can estimate the mass of primary stars of them
from the abundance patterns of C-rich EMP stars.
This figure shows the resultant IMF of EMP stars.
Red line is mass distribution of primary stars. &
Green line shows mass distribution of secondary companions.
Typical mass of EMP stars formed in early universe is 10Mo.
The stars of mass <0.8Mo can survive to date
and most of them are the secondary companions of binary systems.
AGB ⇒WD
EMP survivors
Supernova
(M☉ )

6. Ultra/Hyper metal-poor stars
Norris+ (2007)
3 stars with [Fe/H]<-4.5
All 3 stars show C enhancement.
Formation scenarios
Pop.III stars with AGB mass transfer + ISM accretion (Suda et al.2004)
Peculiar supernovae (Umeda & Nomoto 2003)
Massive stellar wind (Maynet 2006)
EMP
UMP
HMP
Observationally,
most interesting objects for NFC are UMP or HMP stars.
3 stars observed and all of them show C enhancement.
One probable hypothesis to explain their abundance peculiarities is that
these stars are Pop.III stars affected by mass transfer from AGB star and accretion of ISM.
This study suggest that surface pollution play an important role in NFC.

7. Early Chemical Evolution & Structure Formation
To realize the formation and evolution history of EMP stars
Early chemical evolution with
Structure formation
Top heavy IMF
Binary
Accretion of ISM
First star
Mini-halos
~106 M☉
With these effects peculiar to the early universe taken into account,
now, we are trying to realize the total formation & evolution history of EMP stars,
based on the current theory of structure formation, that the galaxies are formed through the merging of the smaller scaled cloud,
We calculate the MDF of the EMP stars and our ingredients are early chemical evolution with
Effect of structure formation
Top heavy IMF
Binarity,
Change of element abundance by surface pollution.
First stars were produces from out of the primordial gas and hence should be totally lack of the C and heavier elements.
Galaxies are formed through the merging of the smaller scaled cloud
HERMES program
(Hierarchical Evolution Researcher by MEtal-poor Stars) 7

8. Assumptions Merging history : Somerville & Kollat (1999)
Mini halos are well mixed, closed boxes.
Supernovae
Fe yield: 0.07 M☉, (stars with 12~50M☉ become Type II SN)
10 M☉(Pair instability supernova )
Star formation
Constant star formation efficiency : 10-10/yr
IMF
High mass IMF
Equal numbers of binaries and single stars. Mass ratio distribution of binary components n(q)=const.
Here are our assumptions.
Merging history of the mini-halos is realized by method proposed by SK.
We assumed metallicity in mini-halos are homogeneous and there is no outflow.
Type II SN eject 0.07 Mo of iron, and PISN eject 10Mo of iron.
SFE is constant.
We use high mass IMF derived from study of CEMP stars.

9. Result 1. Metallicity Distribution Function
Model reproduce cut-off around [Fe/H]~-4.
⇒hierarchical structure formation
Result of our calculation.
Pop.III stars
>100 stars predicted
⇒a fraction of low mass star is much smaller for Pop.III.
Obserbed number districution
This is the results.
X-axis shows metallicity and Y-axis shows number of EMP stars.
Red histogram shows observed number of stars as a function of metallicity &
Blue line shows the result of our theoretical calculation.
We can reproduce the observed distribution except for UMP and HMP stars.
Cut-off around [Fe/H]~-4 is due to the effect of hierarchical formation.
That is,
In the low mass mini-halo,
only 1 SN enrich gas with metal beyond [Fe/H]=-4.
So, second or later generation stars distribute above [Fe/H]>-4
However,
Our model predicts hundreds of Pop.III stars.
But observationally, only 3 UMP/HMP stars detected.
It means that fraction of low mass star is much smaller for small metallicity.
Z=0

10. Result 2. Supernova progenitors
Distribution of stars formed in gas polluted by 1, 2, 5, 10 SNe.
This graph shows the number of SN progenitors of EMP stars..
Red line shows the distribution of stars formed in the gas which is polluted by only 1 SN ejecta.
Abundance of these stars represent the yield of individual first supernovae.
Green, purple, blue line shows distribution of stars with 2, 5, 10 progenitors, respectively.
Stars with [Fe/H]~-2.5 is formed from the mixture of 10 SN ejecta.
Z=0

11. Results 3. Pair-Instability Super Novae
Case I
Assumptions
PISNe blow out all gas and metal in the minihalo.
Case I: PISNe are formed in all minihalos
Case II: [Fe/H]<-5.5 : PISN, [Fe/H]>-5.5 : Type II SN
Case II
Results
Case I:
⇒ metal overproduction
Case II: ⇒Many HMP stars
In the previous cases, we did not take account of PISNe.
Now We investigate the effects of PISNe.
Here We assume that if PISN takes place,
all gas and metal in the minihalo are blown off and are distributed uniformly in all the galactic matter.
In the Case I, we assumed PISN is formed in all mini-halos.
In this case, too much metal is ejected by PISNe and only few EMP stars are formed.
In the Case II, we assumed that PISN is formed only in the cloud with [Fe/H]<-5.5.
This model predicts hundreds of stars with critical metallicity,
These stars should have the abundance pattern of the yields of PISNe.
But this is in contrast with the observed abundance patterns of UMP/HMP stars.
It means that most of Pop.III stars not become PISN.

12. Surface pollution
Accretion of interstellar matter change surface abundance of Pop.III or UMP star.
Assumptions
Bondi accretion
(Mini-halo ⇒ low V ⇒high accretion rate)
Case I: Stars move with virial velocity
ρ=ρvir= ρc(z)ΩbΔc, V=Vvir (minihalo virial velosity）
m=m2 (Bondi radius of secondary star)
Case II． Gas & stars are concentrated to center of mini-halos
ρ=ρvir×(Tvir/200K), V=Vvir ×(200K/Tvir)1/2,
Halo merge after
m=m1+m2 (Bondi radius of bianary system)
Finally, I talk about surface pollution by the accretion of ISM.
We assumed Bondi accretion. &
We calculate under 2 assumptions.
For case I, we assume that stars move at the virial velocity of the mini-halo.
For case II, we assume that stars & gas concentrated to center of the mini-halo and have negligible relative velocities.

13. Result 4.1 ISM accretion (case I)
Accretion rate: <10-14~-15M☉/yr Total: ~10-7~-8 M☉
Main sequence (Mscz=0.003 M☉ ) ⇒ [Fe/H]~-7
Giant (Mscz=0.3M☉ ) ⇒[Fe/H]~-9
(Mscz: mass of surface convection zone)
dwarfs
giants
This is the result for case I.
Total mass of accreted ISM to each stars become 10^-7~-8.
By accretion,
metallicity of main sequence Pop.III star becomes [Fe/H]=-7.
If the accreted metals are mixed in the deep surface convection of giants, the metallicity decreases down to –9.
Z=0

14. Result 4.1 ISM accretion (case II)
Accretion rate : ~10-9~-12M☉/yr
For the case II, the accretion rate increases.
In this case, metallicity of giant Pop.III star become [Fe/H]=-5.
This result show the possibility that UMP & HMP stars are polluted Pop.III stars.
giants
dwarfs
Pop.III star become [Fe/H]~-5 (giants)
⇒HMP, UMP star?
Z=0

15. Summary EMP stars have undergone some changes in the surface abundance
Many EMP survivors affected by binary mass transfer.
IMF of EMP star ([Fe/H]≦-2.5) is high mass.
Interstellar pollution is important for HMP/UMP stars.
Chemical evolution + structure formation
Formation process of our Galaxy affect metallicity distribution of EMP stars.
Most of Pop.III stars may not become pair-instability supernovae.
UMP/HMP stars are possibly Pop.III survivors
Summary.
I showed that binary play an important role in NFC.
IMF of EMP star, not only Pop3 star, but also EMP star with metallicity up to -2.5, is high mass.
From the calculation of CE+SF, we showed that
Formation process of our Galaxy effect to metallicity distribution.
Most of Pop.III stars may not become Pair Instability Super Nova.
UMP/HMP stars are possibly Pop.III survivors
Thank you very much for kind attention.