1. 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

2. Galaxy formation Star formation Stellar evolution Metal-poor stars Big Bang 2 R-process ? Neutron star merger R-process ? Supernova

3.  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 clusters dSph galaxies (Ishigaki-san) [Fe/H]=log((n Fe /n H )/(n Fe /n H ) ☉ ) EMP stars HMP stars http://saga.sci.hokudai.ac.jp 3

4. 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

5.  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

6.  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

7.   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”) ? ? ?

8.  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, 3 rd source origin stars) ? 8 (Aoki+ 2013)

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10. Mini-halo ~10 6 M ☉ 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

11.  Merger tree:  Extended Press-Schechter Method Somerville & Kollat (1999)  M MW =10 12 M ☉, M min =M(T vir =10 3 K)  Proto-galaxy  Gass infall: ⊿ M h ×Ω b /Ω M  At the beginning, stars are formed in mini-halo with T vir > 10 3 K (Tegmark+ 1997, Yoshida+ 2003)  At z < 20, by Lyman-Werner photon, stars are not formed in newly formed mini-halos with T vir < 10 4 K  Reionization: At z < 10 gas do not accrete to mini-halos with T vir < 10 4 K  Proto galaxies are chemically homogeneous 11

12.  Star formation  All the individual EMP stars are registered in computations  Star Formation Rate: ψ = M gas ×10 -10 /yr  Lognormal IMF:  Pop.III stars (Z<10 -6 Z ☉ ) Pop. III.1: M md = 200 M ☉, Pop.III.2 : M md = 40 M ☉  EMP (Pop.II) stars: M md = 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

13. 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 = c s (T), ρ=ρ av ×(T vir /T) T = 200K for primordial clouds, T = max(10K, T CMB ) for Z > 10 -6 Z ☉  After their host halos merge with larger halo, stars moves with circular velocity v = v cir, ρ av (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

14. Metallicity distribution function (MDF) formation redshift Histograms: observvations Red: Predicted MDF (after ISM accretion) HES survsy (Sch¨orck et al. 2009) SAGA database 14 Metallicity ditribution of EMP stars is well reproduced

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16.  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

17.  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 + ν e → 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

18. 30-40 M ☉ 9-10 M ☉ 9-40 M ☉ 18 Color : predicted distribution Blue lines: 95, 75, 50, 25, 5 percentile curves of predicted distributions Black symbols: SAGA sample Main R-process site is ~10% of SNe with progenitor stars at low-mass end of the SN mass range.

19. Color : predicted distribution Blue lines: 95, 75, 50, 25, 5 percentile curves of predicted distributions Black 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] < -5.5 19 R-process yield: Ba: 6.55×10 － 6 M ʘ /event Eu: 8.62 ×10 － 7 M ʘ /event

20.  t c : Timescale to coalesce NS binary  ~10 9 yr (e.g. Fryer+ 1999)  ~10 6 yr (Belczynski+ 2002)  p NSM : number fraction of r-process source among massive star binary  Event rate  Galactic event rate: 40 – 660 /Myr  p NSM =0.01 for the high-mass IMF and ψ = 10M ☉ /yr 20

21. SN with 9-10M ☉ progenitor lifetime: 2×10^7yr event rate: 0.1×ψ SN t c =1Myr t c =10Myr t c =100Myr t c =10Myr ψ = 10 -11 M gas t c =10Myr p NSM = 0.1 t c =10Myr E NSM = 10 51 erg But, consider the dynamics of very high velocity (~ 0.2c ) ejecta (Shigeyama-san) ψ = 10 -10 M gas /yr p NSM = 1.0 E NSM = 10 50 erg 21 delay time: t c ~ 10 7 yr coalescence probability : p NSM = 100% Ba: 5.7×10 － 6 M ʘ /event

22.  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 ☉

23. One r-process source event ⇒ [Ba/H]=-2.5~-3 First SN eject Fe but no r-process elements First stars (Population III) 23

24. no 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

25.  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

26.  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

27. M LEPP = 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 ☉ Y Ba = 0 Y Sr is set to be = solar r-process 27

28. 28 LEPP (10-12M ʘ ) Main-r (9-10M ʘ ) Polluted stars formed with [Ba/Fe] = − ∞ [Sr/Fe] = − ∞

29.  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

30.  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)  3 rd source origin stars: Surface pollution 30