chemical e volution of r- p rocess e lements in hierarchical g alaxy f ormation models
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Chemical Evolution of R-process Elements in Hierarchical Galaxy Formation Models
Yutaka Komiya(Tokyo Univ., RESCEU)Collaborators
Takuma Suda(Tokyo Univ., RESCEU)Shimako Yamada(Hokkaido Univ.)
Masayuki Y. Fujimoto(Hokkaido Univ.)
RIKEN-IPMU-RESCEU Joint Meeting
2014-7-4 理研 , 和光
Komiya et al. (2014) ApJ, 783, 132
Chemical evolutionGalaxy formation
Star formation
Stellar evolution
Metal-poor stars
Big Bang
2R-process ?
Neutron star merger
R-process ?
Supernova
Metal-poor stars Extremely metal-poor (EMP) stars
= Living fossils Galaxy formation, First stars, First supernova
Observation : Milky Way halo
More than 1000 stars with [Fe/H] < -2.5 has been identified
Follow-up by high-resolution spectroscopy: >4 00
Database: SAGA (Stellar Abundance for Galactic Archaeology)
Globular clustersdSph galaxies (Ishigaki-san)
[Fe/H]=log((nFe/nH)/(nFe/nH)☉)
EMP stars
HMP stars
http://saga.sci.hokudai.ac.jp3
Contents 1. Introduction
Observations : r-process elements on EMP stars Previous chemical evolution studies
2. Hierarchical chemical evolution model Merging history of building blocks of MW
Pre-enrichment of intergalactic matter Surface pollution of EMP stars
3. Results & Discussion R-process source
SN v.s. Neutron star merger Surface pollution : origin of r-deficient EMP stars Light r-process elements (Sr)
light-r/heavy-r
4
Observations: Ba, Eu Large scatter (2-3 dex)
(Non-LTE: slightly shifted toward the solar ratio (Andrievski et al. 2009))
R-II stars: [Eu/Fe]>1. at [Fe/H]~ -2.8
Decreasing trend as metallicity decrease at [Fe/H]< -2.3
Abundance pattern of r-process elements heavier than Ba (Z ≥ 56) matches the scaled solar system r-process abundances
Cowan & Sneden(2006)
(Ba is also “r-process element”)
5
Previous chemical evolution studies
Ishimaru & Wanajo (1999, 2003) Homogeneous ISM + SN shell R-process souce: 8-10 M☉
Argast et al.(2004) SNe pollute spherical shell with 5*10^4M☉
R-process source: type II SN Neutron star merger is ruled out
Cescutti (2008) 100 independent regions R-process source:
10-30 M☉ (stars with 10-15 M☉ are dominant)
6
Observations: Ba, Eu
Large scatter (2-3 dex) (Non-LTE: slightly shifted toward the solar
ratio (Andrievski et al. 2009))
Decreasing trend as metallicity decrease at [Fe/H]< -2.3
But, Ba is detected for almost all EMP stars. (Roederer 2013)
But, at [Fe/H]<-3, plateau is reached(Andrievski+ 09)
(Ba is also “r-process element”)
?
?
Observations: Sr/Ba Light-element primary process (LEPP)
(weak r-process) Enhancements of lighter r-process elements
(Z<56; Sr, Y, Zr…) relative to heavier elements (Ba, Eu, Pb…)
Anti correlation between [Sr/Ba] – [Ba/Fe]
[Sr/B
a]
[Ba/Fe]
?
But, at [Fe/H]<-3.5, no light element enhanced stars. (Aoki+ 2013)
But, stars with very low [Ba/Fe] and [Sr/Ba]~0.(Qian+ 2008, 3rd source origin stars)
?
8(Aoki+ 2013)
2. THE HIERARCHICAL CHEMICAL EVOLUTION MODEL
9
Hierarchical model for chemical evolution
Mini-halo~106M☉
Milky Way
Chemical evolution along a merger tree All the individual EMP stars are registered
Yield from each individual SN Metal pre-enrichment of intergalactic medium (IGM) Surface pollution of EMP stars by accretion of interstellar medium (ISM)
Proto-galaxy
First star
First supernova
10
Model: Galaxy formation Merger tree:
Extended Press-SchechterMethod Somerville & Kollat (1999)
MMW=1012 M☉, Mmin=M(Tvir=103K)
Proto-galaxy Gass infall: M⊿ h×Ωb/ΩM
At the beginning, stars are formed in mini-halo with Tvir > 103K (Tegmark+ 1997, Yoshida+ 2003)
At z < 20, by Lyman-Werner photon, stars are not formed in newly formed mini-halos with Tvir < 104K
Reionization: At z < 10 gas do not accrete to mini-halos with Tvir < 104K
Proto galaxies are chemically homogeneous
11
Model: Chemical evolution Star formation
All the individual EMP stars are registered in computations Star Formation Rate: ψ = Mgas×10-10/yr Lognormal IMF:
Pop.III stars (Z<10-6Z☉) Pop. III.1: Mmd = 200 M☉, Pop.III.2 : Mmd = 40 M☉
EMP (Pop.II) stars: Mmd = 10 M☉ (Komiya et al. 2007) Binary fraction: 50%
Mass ratio distribution: n(q) = 1 Massive Pop.III.1 stars suppress star formation in their host halos.
Metal enrichment Stellar lifetime : Schaerer et al. (2002) Instantaneous mixing inside mini-halos. Yield : (He-Zn)
Kobayashi et al.(2006, Type II SN) Nomoto et al. (1984, Type Ia SN)
Umeda & Nomoto (2002, Pair-instability SN; PISN) 12
Model: Accretion of interstellar matter (ISM)
222
22
2s
s
cvcv
Gmm
EMP star
13
Surface abundances of EMP stars can be changed by accretion of ISM. Trace the changes of the surface abundance of EMP stars
along the evolution of galaxies Accretion rate in mini-halos are much higher than in the MW halo
due to small relative velocity between stars and ISM.
Accretion rate Bondi accretion
In the mini-halos in which stars are formed, v = cs(T), ρ=ρav×(Tvir/T)T = 200K for primordial clouds, T = max(10K, TCMB) for Z > 10-6Z☉
After their host halos merge with larger halo, stars moves with circular velocity v = vcir, ρav (average density of virialized halo)
Accreted matter is mixed in surface convective zone of EMP stars. Mscz= 0.2M☉ for giant = 0.0035M☉ for main sequence
Result: Chemical evolutionMetallicity distribution function (MDF)
formation redshift
Histograms: observvationsRed: Predicted MDF (after ISM accretion)
HES survsy (Sch¨orck et al. 2009)
SAGA database
14
Metallicity ditribution of EMP stars is well reproduced
3. RESULTS & DISCUSSION
15
R-process sources Main-r: SN? Neutron star binary?
Abundance distribution @ [Fe/H]<-3 Why Ba is detected in almost all EMP
stars
LEPP (light element primary process) source
Origin of vey Ba-poor and Sr-poor stars
16
3.1 R-process site Core-collapse SN (e.g. Burbidge+ 1957)
Neutrino driven wind >20M ☉ (Woosley & Hoffman 1992) Very high entropy is required to synthesize heavy r-process elements
Electron-capture (O-Ne-Mg) SN (8-12M☉) (e.g. Wheeler et al. 1998) Artificial enhancement of the explosion energy (Wanajo et al. 2003) n + νe → p+ + e- :not so n rich
(Light r-process elements can be synthesized)
Neutron star merger (e.g. Lattimer+ 1974, Rosswog+ 1999) Abundance pattern : ○ (e.g. Wanajo & Janka 2011) Chemical evolution: ×(Argast et al. 2004)
Event rate: ~1/1000 of SN Long delay time ( ~ Gyr )
17
Core-collase SN Mass range of r-process source
30-40 M☉ 9-10 M☉9-40 M☉
18
Color : predicted distributionBlue lines: 95, 75, 50, 25, 5 percentile curves of predicted distributionsBlack symbols: SAGA sample
Main R-process site is ~10% of SNe with progenitor stars at low-mass end of the SN mass range.
Results: [9 M☉,10M☉]
Color : predicted distributionBlue lines: 95, 75, 50, 25, 5 percentile curves of predicted distributionsBlack symbols: SAGA sample
9-10 M ☉ model well reproduce the abundance distributions of heavy r-process elements.
R-II stars: 7%(4-10%) of EMP stars ([Fe/H]<-2.5) (SAGA sample: 5.6% ) average metallicity: [Fe/H] ~ -2.8
(S-process)
7.5% of EMP stars shows [Ba/H] < -5.519
R-process yield: Ba: 6.55×10 - 6 Mʘ/event Eu: 8.62 ×10 - 7 Mʘ/event
Neutron star (NS) merger scenario
tc: Timescale to coalesce NS binary ~109 yr (e.g. Fryer+ 1999) ~106 yr (Belczynski+ 2002)
pNSM: number fraction of r-process source among massive star binary Event rate
Galactic event rate: 40 – 660 /Myr pNSM =0.01 for the high-mass IMF and ψ = 10M☉/yr
20
NS merger scenario
SN with 9-10M☉progenitor lifetime: 2×10^7yr event rate: 0.1×ψSN
tc=1Myr tc=10Myr tc=100Myr
tc=10Myr ψ = 10-11Mgas
tc=10Myr pNSM = 0.1
tc=10Myr ENSM = 1051erg
But, consider the dynamics of very high velocity (~ 0.2c ) ejecta (Shigeyama-san)
ψ = 10-10Mgas/yr pNSM = 1.0 ENSM = 1050erg
21
delay time: tc~ 107 yr coalescence probability : pNSM = 100%Ba: 5.7×10 - 6 Mʘ/event
R-process sources Main-r: SN? Neutron star binary?
Abundance distribution @ [Fe/H]<-3
Why Ba is detected in almost all EMP stars
LEPP source Origin of Sr-poor & Ba-poor stars
22
SN with progenitor mass with 9-10M☉
Chemical evolution process
One r-process source event ⇒[Ba/H]=-2.5~-3
First SN eject Fe but no r-process elements
First stars (Population III)
23
ISM accretionno ISM accretion
with ISM accretion
Accretion dominant
For stars with [Ba/H] -3.5, ≲accretion of ISM is the dominate source of heavier r-process elements on their surface.
After surface pollution
Before surface pollution
Majority of stars with [Fe/H]<-3 was [Ba/H] = -∞.
24
• All EMP stars have Ba• Plateau at [Fe/H] < -3
R-process sources Main-r: SN? Neutron star binary?
Abundance distribution @ [Fe/H]<-3
Why Ba is detected in almost all EMP stars
LEPP sites Origin of Sr-poor & Ba-poor stars
25
SN with progenitor mass with 9-10M☉
Surface pollution by ISM accretion → [Ba/H] ~ - 4
Observation: Sr
Many stars with large [Sr/Ba] Light Element Primary Process (LEPP)
At [Fe/H] < -3.6, [Sr/Ba] 0≦
Very Ba-poor stars with [Sr/Ba] ~ 0
[Sr/B
a]
[Ba/Fe]
R-II stars
Aoki+ (2013)
26
ResultMLEPP = 10-12 M☉
-5 -4 -3 -2 -1
[Sr/F
e][B
a/Fe
]
[Fe/H]
Main-r source: 9 -10 M☉
[Sr/Ba] = - 0.5 LEPP source: 10 – 12 M☉
YBa = 0 YSr is set to be <Sr/Ba> = solar r-process
27
Result: [Sr/Ba]
28
LEPP (10-12Mʘ)
Main-r (9-10Mʘ)
Polluted stars formed with [Ba/Fe] = −∞ [Sr/Fe] = −∞
R-process sources Main-r: SN? Neutron star binary?
Abundance distribution @ [Fe/H]<-3
Why Ba is detected in almost all EMP stars
LEPP source Origin of Sr-poor & Ba-poor stars
29
SN with progenitor mass with 9-10M☉
Surface pollution by ISM accretion → [Ba/H] ~ - 4
Surface pollution by ISM accretion → [Sr/Ba] ~ 0
SN with progenitor mass with 10-12M☉
29
Summary Hierarchical chemical evolution
Evolution along the merger tree Inhomogeneous model of IGM metal pre-enrichment Surface pollution by ISM accretion
Results Sources of Main r-process elements
9 – 10M ☉ (Electron-capture SN) SNe with progenitors at low-mass end of the SN mass range The abundance distribution is well reproduced
Neutron star merger Not plausible
if most (~100%) massive star binary coalesce in short timescale (~10^7 yr)… R-deficient EMP stars
Surface pollution is dominant for stars with [Ba/H] < -3.5 All EMP stars have Ba on their surfaces
Light elements (Sr) ~10-12 M☉ (core-collapse supernovae)
(9-10M☉ also elect light elements, [Sr/Ba] ~ -0.5) 3rd source origin stars: Surface pollution
30
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