The low-mass starformation in the early
Universe
銀河系ハロー超金属欠乏星から探る
初期宇宙の星・連星系形成史
Masayuki Y. Fujimoto, Shimako Yamada (Hokkaido U)
Yutaka Komiya, Takuma Suda (Open U/Rescue U-Tokyo)，Masahiro N. Machida (Kyushu U)

Galactic Archeology
Extremely Metal-Poor (EMP) Stars in the Galactic Halo
〇 Large-scaled survey of metal-poor stars in the Galactic halo;
HK Survey (1992~), HES Survey(2001~), SEGUE(2004~), SMSS(2007~) ….

〇 High dispersion spectroscopy with the large telescopes (Subaru, VLT,…)
30kpc

Spatial distribution

Metallicity distribution[Fe/H]<-2.5: 682[Fe/H]<-3: 348[Fe/H]<-4: 180

SAGA database:www.sagadatabase.jp(Suda et al. 2008,2011,2017)-30kpc[Fe/H][ = log(NFe/NH) - log(NFe/NH) ☉][Fe/H]

EMP stars = mini-halos の低質量星の残滓 ⇒ 宇宙最初の星形成領域の特性
Evolution and Elemental Abundances⇒ Star formation process and the nucleosynthesis in the mini halos⇒ Star/binary formation history and the early evolution before the galaxy formation

1. Characteristics
of EMP starsStars with the peculiarabundances

Carbon Enhanced Metal-Poor(CEMP) Stars (Rossi+1999, Aoki+2002)
An extraordinary large fractionof CEMP stars among EMP stars
CEMP stars([C/Fe]≥0.7)

Fraction:
CEMP / EMP stars[Fe/H]<-2 : 30% (163/544)[Fe/H]<-3 : 27% (57/211)[Fe/H]<-4 : 60% (9/15)
SAGA database (2015)

Cf. CH stars, Ba stars of[Fe/H]>-2 : ~1%[Fe/H]

Variations of Neutron-Capture
Ryan , Aoki + 2005,
Elements
Aoki, Beers + 2007
[C/Fe]>0.7[Ba/Fe]CEMP-s(96)CEMP-no(51)CEMP-s : [Ba/Fe] ≥ 0.5
Ba enhanced ⇒s-process yields from AGBstars in the binary systems

CEMP-no : [Ba/Fe] < 0.5
EMP
blue: -2.5 <[Fe/H]<-2
gray: -3<[Fe/H]<-2.5
green:[Fe/H]<-3[C/Fe]

Ba = normal abundancesor with no s-process
yieldsMildly carbon-rich without
excess of neutron capture
elements (Aoki et al. 2002)

[Ba/C]

Distributions of Neutron-Capture
Elements among CEMP-no stars[Ba/Fe]-[C/H]
≃3.1

EMP

[Ba/Fe]-[C/H]≃ 0.8

Carbon enhancement accompany[Fe/H]
the neutron-capture elements,Ba ∝ Fe ⇒ s-process
different from those ofwith Fe as seednormal EMP stars (r-process elements+ the yields of massive stars)

Re-classification of
CEMP-no Stars○ The efficiency of s-process:[Ba/Fe/C] ≡ [Ba/Fe]-[C/H]= [Ba/C]-[Fe/H]
Large variations with(1) the mass of stars and(2) the neutron sources, 13C and 22Ne

○ Different grouping

EMP星[Ba/Fe]=0.5

Mixing track

CEMP-s: 3.1 ≥ [Ba/Fe/C] ≥ 0.8

CEMP-no ⇒Lo-CEMP-no: [Ba/Fe/C] ≥ 0.8 (40)
Hi-CEMP-no: [Ba/Fe/C] < 0.8 (11)
Carbon abundance [C(+N)/H]

Bimodal Distributions
Metallicity Norris+ 2013, etc.
frequencyCarbon abundance(1) CEMP-s:[C/H] ≃ -2 ~ 0: [Fe/H] > -3.5
(2) Hi-CEMP-no:[C/H] ≈ -1.3 ~ 0: [Fe/H] ≳ -3.5
(3) Lo-CEMP-no:[C/H] ≲-1.1:[Fe/H] ≈ -4.5 ~ -2.5

CEMP-s
CEMP-no2. Binary mass transfer
scenario for CEMP stars
1. s-process nucleosynthesis in extremely-metalpoor AGB stars
2. Binary characteristics among extremely metalpoor stars

Binary Origin of CEMP stars

Suda et al. (2004), Komiya et al. (2007)

 s-Process Yields: nucleosynthesis in the helium flashconvective zone and dredge-up to the surface by the surface
convection in TP-AGB stars
 Surface Pollution: mass transfer from AGB primary tolow-mass secondary for both carbon and s-process elements
accretion
Low-mass
EMP star(M~0.8M◉)

windmass loss

C,Odredge-up

HHeH:

CEMP starPrimary AGB stars ⇒ white dwarfs(1≲M/M◉ ≲ 8)
(0.6≲M/M◉<1)

[Eu/Ba], intermediate
between r- and s-processr-process: [Eu/Ba]≃ 0.7 (= 0.61 ± 0.17),
Solar system s-process: [Eu/Ba} ≃ -1.5

r-processs-processr-process

Lo-CEMP-noCEMP-ssolar s-process

CEMP-r/s: -0.5 ≤ [Eu/Ba] ≤ 0.1

EMP

Comparison with the observations

s-process in metal-poor (Yamadastarset al. 2019)hydrogen mixing into He flash convective zone inAGB stars of M ≤ 3M◉
high neutron density:
CEMP-r/snn (cm-3) ≃ 1011 -1014
CEMP-nothe solar s-processnn (cm-3) ≃ 106 - 109

independent of
the metallicity
for [Fe/H]≲-2

neutron absorber:
16O(n,γ)17O(n,α)14C
instead of 56Fe

1×10143×1013
1× 10135×1012 2×1011

CEMP-sC-normalized enrichment

Bi-modal Distribution ofCarbon Abundances
Differs between CEMP-sand Lo-CEMP-no stars[C/H] = -1.5
Lo-CEMP-noCarbon abundance =amount of accreted mass
⇒ binary separation

CEMP-sgiants[C(+N)/H]
Surface carbon abundances

Assumptions:(1) Log-normal distribution oforbital periods(2) Bondi accretion rate(3) Wind: [C/H] = 0, 𝒗𝐰𝐰𝐰𝐰 = 𝟑𝟎
km s-1(4) Limited to Giants with the surface
convection of ～ 0.2M☉

Mother binary systems
Surface carbon abundances

Period distributions
Rochelobe
overflow

104 days
C-normal EMPstars: [C/Fe]<0.7
depending on themetallicity

Bi-modal distributions
CEMP-s: short periods
log P (day) = 2.8 ± 0.6
Lo-CEMP-no: long periods
log P (day) = 4.6 ± 0.3(M1=2M⊙ , M2=0.8M⊙ )Solar neighborhood (主星=G-type stars)(Duquennoy & Mayer 1991)

Low-mass
populations
low-massstar
populations
Binarity: ＣＥＭＰ-s ⇔ short periods (~200 - 104 days) withPrimary stars; M ≤ 3M◉
 Hi-CEMP-no ⇐ short periods with Primary stars;
to be observed
3M◉ < M ≲~4.5M◉ (hot bottom burning) Lo-CEMP-no ⇐ ｌong periods (≃104 ~105 days)
with Primary stars; 3M◉ ≤ M ≲~4.5M◉• Large fractions > 20%• Metallicity dependence

CEMP-s only for [Fe/H] ≳ -3.5
Lo-CEMP-no for [Fe/H] ≲ -2

Top Heavy IMF for [Fe/H]≲-2.5
despite the dust cooling• Lognormal IMF(1)• Binary fraction fb=0.5,• Mass ratio; n(q) = const, q=m2/m1(1) CEMP星の fraction (Komiya+2007)(2) 観測されたEMP星数からの求めたSN1個当たりのFe 生成率（Komiya + 2009）(1) High-mass IMF: Mmd ≃10M◉(2) Low-mass stars born as asecondary member of binaries:⇒ C-normal EMP stars from binaries withlonger periods and/or massive star primaries

CEMP-sM≤3 M◉

Hi-CEMP-no3<M/M<81

Mmd (M◉)

Komiya et al. 2007

(2)
Woosely & Weaver(1995)
Umeda &Nomoto(2002)
Mmd (M◉)

Komiya et al. 2009

(I) Short-period binaries with
Low-mass companion(Machida & Nakamura 2015)
Mass growth of Protostars

Formation of stable thin- disk

Fe/H]≤-4
massive stars[Fe/H]≥-3
Low accretion
rates

Fragment at the outer edge of stable disk (r ≳ 10 au) + migration⇒ low-mass companion of short periods

Mass of central stars: small accretion rates (cs3/G) but longer KelvinHelmholtz timescale:Low temperatures ⇒ small mass fragmentation in the disk

(II) Long-period binaries[Fe/H] ≲ -4: hot, high accretion rate
(heating by 3-body H2 formation; Tsuribe& Omukai2008)

Omukai + (2005)⇒ thick disk + massive star binaries
Secondary sinks
merger + ejection
Stellar cluster +Binary formation
Low-massbinaries formedin outer parts =long periods
Stacy et al. (2016)

IMF averaged
number ratio oflowmass/massivestars≃ 0.1 for EMPstars(≃20 for theSalpeter)

(II) When full-scaled low-massstar formation starts?
IMF: high-mass → Salpeter(Komiya et al. 2010)

HES: Schroeck + 2009
Kinematically selected
Ryan + Norris 1991

decrease inthe carbon
stars fraction(Yamada + 2013)

Break at [Fe/H]≃-2.2

The fragmentation of star
forming clouds
Fragmentation ⇔ d ln P/ d ln ρ ＝ 1

Metallicity ofgalactic halo GCs

Thermal Jeans mass

(Schneider et al. 2006)

The effect of disk formation
Z < 1kpc:
Low-mass IMF
components
for [Fe/H]<-2.2

Z > 1.5kpc:
Absence ofLow-mass IMF
components

Summary
Low-Mass Formation History
1. A small number as the byproducts of massive starformation in mini-halos = most as the secondary ofbinaries
o [Fe/H] ≲ -4: truncation of accretion in the secondary clumps,⇒ long period binaries in the stellar clusters
o [Fe/H] ≳ -3: the fragment of cold, thin circum-stellar disks⇒ short periods (P < 104d), in addition to the long period binaries

2. Transition from the high-mass IMF to low-mass (Saltpeter)
IMF for [Fe/H] ] ≳ -2.5 = single and/or primaries(1)(2)

Predominance of the dust cooling over the line cooling
Formation of pre-galactic gas disk

Full scaled low-mass formation ⇒ galaxy formation