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Chemical E volution of R- p rocess E lements in Hierarchical G alaxy F ormation Models

RIKEN-IPMU-RESCEU Joint Meeting 2014-7-4 理研 , 和光. Chemical E volution of R- p rocess E lements in Hierarchical G alaxy F ormation Models. Yutaka Komiya （ Tokyo Univ., RESCEU ） Collaborators Takuma Suda （ Tokyo Univ., RESCEU ） Shimako Y amada （ Hokkaido Univ. ）

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Chemical E volution of R- p rocess E lements in Hierarchical G alaxy F ormation Models

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  1. RIKEN-IPMU-RESCEU Joint Meeting 2014-7-4 理研, 和光 Chemical Evolution of R-process Elements in Hierarchical Galaxy Formation Models Yutaka Komiya（Tokyo Univ., RESCEU） Collaborators Takuma Suda（Tokyo Univ., RESCEU） ShimakoYamada（Hokkaido Univ.） Masayuki Y. Fujimoto（Hokkaido Univ.） Komiya et al. (2014) ApJ, 783, 132

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

  3. 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] < -2.5has been identified • Follow-up by high-resolution spectroscopy: >４00 • 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.jp

  4. Contents • Introduction • Observations : r-process elements on EMP stars • Previous chemical evolution studies • Hierarchical chemical evolution model • Merging history of building blocks of MW • Pre-enrichment of intergalactic matter • Surface pollution of EMP stars • 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

  5. Observations: Ba, Eu • (Ba is also “r-process element”) • 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)

  6. 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)

  7. Observations: Ba, Eu • (Ba is also “r-process element”) • 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) ? ?

  8. Observations: Sr/Ba But, at [Fe/H]<-3.5, no light element enhanced stars. (Aoki+ 2013) • 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] But, stars withvery low [Ba/Fe] and [Sr/Ba]~0.(Qian+ 2008, 3rd source origin stars) ? ? [Sr/Ba] [Ba/Fe] (Aoki+ 2013)

  9. 2. The Hierarchical chemical evolution model

  10. Hierarchical model for chemical evolution First star First supernova Mini-halo~106M☉ Milky Way Proto-galaxy • 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 starsby accretion of interstellar medium (ISM)

  11. Model: Galaxy formation • Merger tree: • Extended Press-SchechterMethod Somerville & Kollat (1999) • MMW=1012 M☉, Mmin=M(Tvir=103K) • Proto-galaxy • Gassinfall: ⊿Mh×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

  12. 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)

  13. Model: Accretion of interstellar matter (ISM) • 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 halodue 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 EMP star

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

  15. 3. Results & discussion

  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

  17. 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 )

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

  19. Results: [9 M☉,10M☉] Color : predicted distribution Blue lines: 95, 75, 50, 25, 5 percentile curves of predicted distributions Black symbols: SAGA sample (S-process) R-process yield: Ba: 6.55×10－6Mʘ/event Eu: 8.62 ×10－7 Mʘ/event 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 • 7.5% of EMP stars shows [Ba/H] < -5.5

  20. Neutron star (NS) merger scenario • tc: Timescale to coalesce NS binary • ~109yr (e.g. Fryer+ 1999) • ~106yr (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

  21. NS merger scenario tc=1Myr tc=10Myr tc=100Myr ψ = 10-10Mgas/yr pNSM = 1.0 ENSM = 1050erg tc=10Myr ψ = 10-11Mgas tc=10MyrpNSM = 0.1 tc=10MyrENSM = 1051erg SN with 9-10M☉progenitor lifetime: 2×10^7yr event rate: 0.1×ψSN • delay time: tc~ 107yr • coalescence probability : pNSM = 100% • Ba: 5.7×10－6Mʘ/event But, consider the dynamics of very high velocity (~ 0.2c ) ejecta (Shigeyama-san)

  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 SN with progenitor mass with 9-10M☉

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

  24. ISM accretion Before surface pollution no ISM accretion Majority of stars with [Fe/H]<-3 was [Ba/H] = -∞. 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 with ISM accretion Accretion dominant • 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 SN with progenitor mass with 9-10M☉ Surface pollution by ISM accretion → [Ba/H] ~ - 4

  26. 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 Aoki+ (2013) R-II stars [Sr/Ba] [Ba/Fe]

  27. Result MLEPP = 10-12 M☉ [Sr/Fe] Main-r source: 9 -10 M☉ [Sr/Ba] = - 0.5 LEPP source: 10 – 12 M☉ YBa = 0YSr is set to be <Sr/Ba> = solar r-process [Ba/Fe] -5 -4 -3 -2 -1 [Fe/H]

  28. Result: [Sr/Ba] LEPP (10-12Mʘ) Main-r(9-10Mʘ) 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 SN with progenitor mass with 9-10M☉ Surface pollution by ISM accretion → [Ba/H] ~ - 4 SN with progenitor mass with 10-12M☉ Surface pollution by ISM accretion → [Sr/Ba] ~ 0 29

  30. 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) • 3rdsource origin stars: Surface pollution

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