Inquiry into Low-Mass Star Survivors from

the Early Universe and the Quest of First Generation Stars

Masayuki Y. Fujimoto,

Takuma Suda,

Takanori Nishimura (Hokkaido University)

Masayuki Aikawa (Universite Libre de Bruxelles)

Nobuyuki Iwamoto (Japan Atomic Energy Research Institute)

Icko Iben, Jr. (University of Illinois at Urbana-Champaign)

Extremely Metal-Poor (EMP) Stars 1. Introduction 2. External pollution 3. Internal Pollution 4. Binary nature 5. Summary

1. Introduction

・ first stars formation transition from the dark age to luminous world re-ionization (z ~ 15) from WMAP metal-production (z > 5) from quasers

・ low-mass stars, if born, survive today as nuclear burning, luminous stars

⇔ Top-heavy mass function, especially for the first stars

・ Extremely Metal-Poor (EMP) stars may serve as probe into the evolution of early universe

Recent Increase in number of known EMP stars

Deep survey of metal-poor stars in the Galactic Halo:

HK survey （ Beers et al. 1992 ） + Hamburg/ESO (Christlieb et al. 2000)・ more than 100 stars for –3.5 < [Fe/H] < －３・ ～ 10 stars for – 4.0 < [Fe/H] < －３ .5 ･ no stars for – 5 < [Fe/H] < －４

・ ２ stars at [Fe/H] = －５ . ３ and － 5.4 (2002 、 2005)

Metallicity distribution from HK survey (1998)

－５1

10

100

1000

"- 2 - 3"～ "- 3 - 4"～ "- 4 "～[Fe/ H]

N

History of search for Metal-Poor stars

1950’s: discovery of stars of [Fe/H] = -1~ -2 (Chamberlain & Aller 1951)

1970’s: survey up to B < 11.5 and no star found below [Fe/H]=-3 → idea of no low-mass stars formed with smaller metallicity (Bond 1980)

1980’s: a high-velocity dwarf G64-12 with [Fe/H]=-3.5 at B=11.8 (Carney & Peterson 1981) a blue subdwarf CD-38○245 with [Fe/H]=-4.5 at B=12.0 (Bessell & Norris1984; later revis

ed as [Fe/H]=-3.9, Ryan et al. 1996)

a high velocity carbon dwarf G77-61 with [Fe/H]=-5.6 at B=12.0 (V=13.9) (Gass et al. 1988; later revised as [Fe/H]=-4.0, Plez & Cohen 2005)

1990's: HK survey of B=15.5 (Beers et al. 1992)

uncovered over 100 stars with [Fe/H]<-3, yet, no star with [Fe/H]<-4

2000’s: Hamburg/ESO survey of B=17.5 (Christlieb et al 1999)

HE0107-5240 giant with [Fe/H]=-5.3 at B=15.89 (Christlieb et al. 2002)

HE1327-2327 dwarf with [Fe/H]=-5.4 at V=13.5 (Frebel et al. 2005)

Probe into Early Universe with Low-Mass Survivors as Tool

Numbers of EMP stars will increase and makes such study promising

In actuality, EMP stars are known to display various peculiar abundance patterns, different from the stars of younger populations

It is true, however, that during their long lives, these low-mass survivors may experience modifications of their surface characteristics, e.g., through accreting metal from interstellar gas and/or from evolved companion, and also, through internal nucleosynthesis and material mixing.

Our purpose:

Based on the characteristics of evolutions of low-mass, extremely metal-poor stars,

to separate these external and/or internal pollution and disclose their pristine nature,

in particular, to identify the survivors of first stars, if exist, and to inquire into dawn of our Universe the first stars were born

Bondi Accretion

First star survivors: [Fe/H] ~ - 3 for Dwarfs with Msurf,conv~0.001Mo [Fe/H] ~ - 5 for giants with Msurf,conv ~ 0.1Mo

2. External Pollution --- Surface Pollution through accreting metal-rich gas

110 10

acc

314

-4

24 10 ( / ) ,

hence, accretion of

10

is if they could stay for more than 1 Gr yrs

in the they were low-mass born. mothe

inevi

r c

tabl

s

e

loud

M M M M yr

M M

n v

Overpopulation of carbon stars ・ 25% of Extremely Metal Poor Stars [Fe/H] <-2.5

・ A few % for Pop. I (N- R, J-type carbon stars, Ba

Stars ) and Pop. II stars (CH stars)

S-process element enhancement ・ large star-star variations in ・ A small Pb/Ba ratios

3. Internal Pollution - Nucleosynthesis and material mixing - Mass transfer from evolved companion

Extremely Metal-Poor Stars (EMPS)

larger fraction of carbon stars （２５％） ⇔ a few ％ in Pop II. (Rossi et al. 1998)

large variations in the abundances of s - process elements

3.1. The Origin of Carbon Stars Structure of AGB

Stars

• TP-AGB phase during H and He double shell burning in stars of mass M > 1.5 M for Pop. I． II

⇒ large 12C/13C + s - process elements

(a) Third dredge-up (Iben 1975) for Pop.I & II

Helium flash convection

Helium flash convection

(b) H mixing into He convection below [Fe/H] ~ -2.5

Z=0

[Fe/H]＝－４

[Fe/H]＝－２

structure just before He core flash

dredge-up by surface convection

helium core flash

helium shell flash

helium shell flash

Ｘ

水素燃焼殻

s ＝ｃｏｎｓｔ

larger entropy

H mixing

H-mixing

lower metallicity lower entropy in the hyd

rogen burning shell lower barrier for mixing

Extremely Metal-Poor Carbon stars within the standard framework of stellar evolution

Initial mass

Init

ial

meta

llici

ty

Population I, II

Population III

ＥＭＰＳ

0

－ 1

(Fujimoto, Ikeda, Iben 2000)

grow Carbon star earlier than Pop. I & II stars for M < 3 M.

HE0107-5240 HE1327-2326

EMP Stars: Earlier Evolution to Carbon Stars

He-Flash Driven Deep Mixing for [Fe/H]<-2.5

Thermal Dredge-up for Pop. I and Pop. II

Up to thermal pulse for Z=0.001(Lattanzio 1986)

Z=0 (Suda et al. 2004)

Carbon star

3.2 s-Process Nucleosynthesis (Pop. I & II )

(a) radiative １３ C （ ,n) １６ O （ Straniero et al. 1995 ）

1) The mixing mechanism of proton into the carbon-rich radiative shell is assumed.

2) The number of neutrons per seed nuclei increases for lower metallicity

extra mixing - １３

C pocket-

prediction: larger Pb/Ba ratio for metal poorer stars

Observed Pb/Ba from EMPSs and Pop. II

EMPS (Aoki et al. 2001)

small Pb/Ba ratio Large Pb/Ba ratio

[Fe/H]=-2.71

[Fe/H]=-2.74

Pop.II ： CH stars (Van EcK et al. 2001)

[Fe/H]=-2.45

[Fe/H]=-1.67 =-1.7

(b) Convective １３ C（ ,n)16O below [Fe/H]~-2.5

In EMP 　 stars ， the　 engulfed 　 H 　 by the He-convection burns as:

１２Ｃ（ｐ，）１３Ｎ（ｅ＋）１３Ｃ（，ｎ）１６Ｏ

Smaller neutrons/seed nuclei ratio.

expected because of dilution mixed protons in the 　 He flash convection. 　

[Fe/H]＝－２．７ , 2Mo

Iwamoto et al． 2004

H mixing

He　 convection

H convection.

But the radiative burning requires the reduction of mixed protons by large factors.

Comparisons with EMPSs

(Ryan et al.2001)

The convective １３ C（ ,n) burning gives the neutron exposures consistent with the observations.

convectiveconvective radiativeradiative

Radiative vs. Convective 13C(,n)16O Burning　

Suda et al. 2004Suda et al. 2004

Why not the radiative 13C burning works for [Fe/H]

< -2.5? the inner edge of surface convecti

onHelium Layer Surface Conv.

He + C + Fe H + He + Fe Opacity < or >

C+N and s-process elements evolution ／ mass transfer in close binary

[C,N/H] ＝ 0 ~ -1 as a result of hydrogen mixing into the helium shell flash convection and the subsequent dredge-up by the surface convection.

EMPS grows carbon star at an earlier evolutionary stage than the star of younger population.

surface pollution with material transferred from the AGB companion in close binary.

s-process may occur with mixed proton but can be dredged up for M > 1.5 M.

(Suda et al． 2004）

2nd possibility: weak flashes of H-mixing without splitting the convection large 13C/12C ~ 0.01

4. Binary Nature of Carbon Stars --- over-populations of EMP carbon-rich stars ---

1au

100au

10au

He Flash-Driven Deep Mixing

Third Dredge-up

Hot Bottom Burning

of Primary

Bin

ary

se

para

tion

The mass of secondly 0.8 Mo

•[Fe/H] = -2 •O/C = 2.7

Lower mass limit

Surface C-abundance as a result of wind accretion for giants

Period distributions EMP Carbon stars vs. CH stars (Pop.II)

0

2

4

6

8

10

12

14

<-1 -1~-0.5 -0.5~0 0~0.5 0.5~1 >1

EMP stars

CH stars

log a (au) a: binary separation

Nu

mb

er

s

CH stars – concentrated at a = 1 ~ 10 au.EMP – smaller separation ~

SummarySo far so good between the observed EMP stars and the theory of low

mass star evolutionWhy so many carbon starsWhy different trend in s-process elements Why so few evidence of binary nature

By separating these effects, we may reveal the origin and pristine natur

e of EXP stars: EMP stars as the second stars, triggered by First Sne (Machida et al. 2005) No need for peculiar C-enhanced SNe proposed by Umeda & Nomoto (2003) Pair Instability SNe are incompatible with the abundance pattern of EMP stars

in particular, the origin of HE0107-5240, HE1327-2327 of [Fe/H] <-5

two stars of [Fe/H] < -5

as the candidates for the first star survivors (initially metal-free) → Key to understanding the nature of low mass star survivors from early Universe