理論懇シンポジウム@国立天文台 16th Dec, 2008

Stellar Evolution at LowMetallicity
Takuma Suda
Keele University,
Marie Curie Fellow (Oct.08-Apr.10)
collaborator

Masayuki Fujimoto (Hokkaido Univ.)Chemical Evolution in
Molecular Clouds
our Galaxy
Proto Stars

Main Sequence
Stars

Metal-Deficient Stars
First Stars(Metal-Poor Stars)
stars consisting ofelements lighter than = Ancient Stars
berylium

Supernovae
Supernova Remnant

Red GiantStars

Planetary Nebulae
White Dwarfs
NGC2440 : from HST
Crab nebula : from Subaru

Metallicity Distribution Function
1000

HE0107-5240 : -5.3: Christlieb et al.
(2002)

1205+356HE1327-2326 : -5.4: Frebel et al.(2005),
Aoki et al.(2006)

100
NumberHE0557-4940 :-4.8 : Norris et al.
(2007)

470 stars

10

Data from SAGA database(Suda et al. 2008)1
http://saga.sci.hokudai.ac.jp-6-5-4-3[Fe/H]-2-1

01

Characteristics of EMP Stars• Large fraction of carbon rich stars ([C/Fe] >~ 0.5) (CEMP stars)

compared with the Population I & II stars• Two groups of CEMP stars : with/without enhancement of s-process elements (CEMP-s and CEMP-no(s), respectively)“C-rich” for [C/Fe] > 0.5“EMP” for [Fe/H] < -2.5
“Giants” for Teff < 6000K and log g < 3.5[C/Fe][Ba/Fe]CEMP-s

CEMP-no[Fe/H][C/Fe]

Figures created with SAGA-DB

Stellar Structure and Evolution - H-R Diagram
Assymptotic Giant Branch

lifetime

Main Sequence~1-10Myr

HMasslog L>12M

~10Myr

Red Giant Branch

H
8-11M
H

Horizontal Branch
Hayashi line

0.6-8M
0.1-0.6M
White Dwarf~0.1-1GyrH<0.08Mlog Teff~10Gyr

H>10Gyr5

Neutron Stars

Evolution of Massive Stars
Black Holelog L

Type II SupernovaeO,NeO,NeO,Ne
Si
He
H
FeHe
H C,O
He
H C,O
He
HC,OSiMg
MgMg
α-element,Fe,r-process

H
H
H
H

log Teff6Evolution of Low- andC,OIntermediate-Mass Stars

C,Olog L

HeC,OH

He
H
H

C,O

log Teff7Evolution of Low-Mass Stars
Helog L

He

He

H
H

He

log Teff8

3rd dredge-up
C, s-process rich

log L

C,OC,O

2nd dredge-up
He, N enrichment

Why Low- andIntermediate-mass stars?
Complicated nuclear
reactions (flashes)
Observed (halo) stars
are low-mass stars

HeC,OH

1st dredge-up
He, CNOHe enrichment(slightly)

C,Olog Teff9

Comparisons of Theorywith Observations• 低、中質量の恒星進化理論で観測をどこまで説明できるか。
• 連星起源説の追求、低金属量星の進化
低金属量星の進化 (Sudaet al. 2004, 2007, 2009)• 低、中質量AGB星における元素合成(Nishimura et al. 2009)• 連星シナリオの枠組みで何が解明できるか。
• 初期質量関数の推定、化学進化(Komiya et al.
2007, 2009ab, poster session)• 大質量星連星の影響(Yamada et al. 2009)

Stellar Evolution at
Low-Metallicity•

初期のZ=0、低質量星の進化計算では特に注目すべき振る舞いは発見されなかった。
•~0.8 Msunの計算例: Wagner 74, D’Antona 82, Guenther+83•

ZCNO<10-7の低質量星モデルではHe-burning時に水素がHe-burning対流層に混入する。(Hollowell+90, Fujimoto+90)•

水素混入の結果、爆発的水素燃焼(flash)により表面対流層が内部の物質を汲み上げ、表面のCN組成が劇的に変化する (HeliumFlash Driven Deep Mixing, He-FDDM)。
•0.8-1.0 Msunの進化計算例: Hollowell+90, Fujimoto+90, Cassisi+96, Fujimoto+00, Schlattl+00, Weiss+04, Straniero+04, Iwamoto+04, Picardi+04, Campbell+08•

2-8 Msunの進化計算例: Fujimoto+00, Chieffi+01, Herwig+03,Iwamoto+04, Campbell+08•

水素混入が起こる計算例: Schlattl+01, Suda+04, Suda+07•

水素混入が起こらない、あるいは発見できなかった計算例: Weiss+00, Marigo+01, Siess+02, Gil-Pons+05, Gil-Pons+07

Basic Equations forStellar Structure• 質量保存（連続の式） dr1=dMr
4πr2 ρ• 静水圧平衡

GMrdP
=−dMr
4πr4

• 輻射輸送3κLdT=dMr
64π 2 acr4 T 3• エネルギー保存• 元素合成dL= !n + !g − !ν
dMr

ρdXi
ρ
NA !σv"ji − NA !σv"ik=dtµeµe

Input Physics
One-dimensional adaptive mesh
Thermal convection by Schwartzschild Criterion
Mixing parameter α=1.5No overshoot and No semi-convection

Most up-to-date input physics
9 elements with reaction rate : NACREradiative and conductive opacities : OPAL, Alexander & Ferguson(1994), Itoh et al. (1983)

neutrino cooling processes : Itoh et al. (1996)

No inclusion of mass loss and extra mixing
Proper assumption to follow He-FDDM
Mixing timescale = Nuclear timescaleとなる層まで混ぜる。
see T.S., Fujimoto, Itoh (2007, ApJ)

Evolution of EMP Models
5.0

4.0

8.8

8Msun 9Msun

8.6
8.4
8.2

7.0

8.0

5.0

7.0clog Tc log T (K)sunlog L (Lsun)log L (L )

3.0

2.0

[Fe/H]=-3=-4=-5
Z=0

1.00.04.6

4.5

4.4

4.3

0.8Msun

3.0
1.5

7.8
7.6

7.2

4.1
4.0
log Teff (K)log Teff

3.9

3.8

3.7

3.6

3.0

2.0
1.5
1.2
1.0
0.8

0.8

7.0

4.2

1.58

8.06.0
7.4 4.0

2.0

6.8
1.0

2.0

3.0

log ρc4.0
5.0
log !c (g/cm3)

6.0

7.0

8.0

T.S. and Fujimoto (2009)Model Computations:
M=2 Msun, [Fe/H]=-4
0.63
0.625

Center of
H-burning shell

He-FDDM

H-burning
convection

Bottom ofH-burning shell[C/Fe]=3.2[N/Fe]=2.9 10-6[O/Fe]=1.3X

0.61

He-burning
convection

0.605
0.6
86

10-4
10-5

0.615

log(LHe/Lsun)

Mr(Msun)0.62

Surfaceconvection

10-3

10121416182022
22Time (year)

10-7

Border ofC+O core
2426
28Time (year)30

32 35 40 45 50 55 60

10-8[Thermal Pulse AGB phase]

42

Helium burning
luminosity

0
2.59e+16

2.591e+162.592e+162.593e+162.594e+16Time (sec)

2.595e+162.596e+162.597e+16

Stellar Evolution at[Fe/H] < -3-1
Case IVC richs-process rich

- [Fe/H]<-2.5でHe-FDDMが-2起こる。
- 一度He-FDDMが起こるとのmixingは起こらない。

[Fe/H][CNO/H]>-2.5となり二度目- AGBと低質量星(0.8Msun)-3-4

FDDMの証拠を検出可能- 連星間距離（連星周期）の違いによる組成のばらつき

Case II
CN richs-process rich

He-FDDM
0.1との連星系があればHe-10-2

0.1

10-5

0.8

1.0

computedmodels

1.2

C burning

Case II’
C-rich (N-rich)s-process rich

Mup(SNe)Oﬀ-centre [central]
He-burning

Z=010-7 0

Case IV’
C richNot s-process rich

-5Case I
CN rich10-6

He-FDDM

0-810-5 1010-3

1.5

2.02.5 3.0
MiniCase IIINo mixing
No dredge-up

Third dredge-up
4.05.06.0 7.0 8.0 9.0

Suda & Fujimoto (2009, submitted to ApJ)

Implications for Observation Blue Horizontal Branch Stars0• Bluer horizontal
branch for metal-poorstars
1• due to the decrease ofentropy in H-burning
shell

He-FDDM-R, Z=0
ZAHB, Z=0
ZAHB, [Fe/H]=-52logg• ~30 Myr Lifetime atblue HBHe-FDDM-R, [Fe/H]=-5ZAHB, [Fe/H]=-2.33• 1/30 of RGB having
comparable luminosity
1 BHB star with [Fe/H] < -3 4should appear per 30 RGB

ZAHB, [Fe/H]=-4

ZAHB, [Fe/H]=-3[Fe/H]=-1[Fe/H]=-2.3[Fe/H]=-3[Fe/H]=-4[Fe/H]=-5Z=0

Metal-poor dSph (e.g., segue 1,[Fe/H]~-3) consits only of BHB (?) 510000 9000

8000

7000 6500 6000 5500 5000

Teff4500

4000

3500

Conclusion
EMP starsのモデルを計算した。
0.8-9 Msun, Z=0, -5 < [Fe/H] < -2.3

He-Flash Driven Deep Mixingによる表面のCN
enhancement
Third dredge-up efficiencyのmass, metallicity依存性
Massive, Metal-poorほどTDUが効かなくなる。

観測との比較[Fe/H] < -3のBlue Horizontal Branch starsがRGB stars 30個に1個ぐらい見つかるはず。
連星輸送を仮定すると、表面対流による希釈効果が期待されるが、観測では見えない。主系列でmetal diffusionが効いている可能性がある。