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/

Ba]

[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 homogeneous11

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)

22

2

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/

Ba]

[Ba/Fe]

R-II stars

Aoki+ (2013)

26

ResultMLEPP = 10-12 M☉

-5 -4 -3 -2 -1

[Sr/

Fe]

[Ba/

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