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Structure and Evolution of Early Cosmological HII Regions

Structure and Evolution of Early Cosmological HII Regions. T. Kitayama (Toho University) with N. Yoshida, H. Susa, M. Umemura. Introduction. Feedback from the 1st stars in Pop III objects - Radiation - SN explosions. ⇒ Formation of HII regions (Yorke 1986)

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Structure and Evolution of Early Cosmological HII Regions

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  1. Structure and Evolution of Early Cosmological HII Regions T. Kitayama (TohoUniversity) with N. Yoshida, H. Susa, M. Umemura

  2. Introduction Feedback from the 1st stars in Pop III objects - Radiation - SN explosions ⇒ Formation of HII regions (Yorke 1986) Dissociation of molecules(Omukai & Nishi 1999) Blow-away of gas (Ferrara 1998) Metal enrichment (Gnedin & Ostriker 1997) etc. Great impacts on - reionization history - galaxy formation

  3. Key questions • How large are the HII regions? • How do they evolve? • How much photons can escape from halos? • What happens after the star dies? • blown-away by SN?, next SF? • - Depend on Mhalo, Mstar….? Essential physical processes hydrodynamics radiative transfer, non-equilibrium chemistry heating/cooling…. ⇒1D model, useful ingredients to 3D simulations

  4. HII regions in a uniform medium (1) Static solution # of photons emitted = # of recombination ⇒Stroemgren sphere (1939) HII

  5. HII regions in a uniform medium (2) Rst Dynamical evolution Nγ=3×1050 1/s 1. ionization/heating → pressure gap 2. shock → expansion of the HII region 107yr 105yr Two phases!

  6. HII HII shock formation HII regions in a uniform medium (3) Highly dynamical ! Rst rion < Rst vion >> vshock rion > Rst vion ～ vshock R-type front D-type front

  7. Model 1. Collapsed cloud in a ΛCDM universe zc=10, Mhalo → size Rvir gas: n∝r-2, Ti ～1000K, Xe=10-4, XH2=10-4 DM: NFW profile (fixed) 2. Radiation from a central massive star zero-metallicity (Schaerer 2002) e.g. M=200 Msun Nγ(>13.6eV) = 2.6×1050 1/s Teff = 105 K, τ= 2.2 Myr Mhalo, Mstar free 3. Solve 1D hydro, radiative transfer of UV photons, chemical reactions (e, H, H+, H-, H2, H2+,) & cooling/heating self-consistently

  8. Structure of HII regions (1) Mhalo= 106 Msun Mstar =200 Msun 105yr 1. high central density →confined I-front →sweep-out of gas by shock 2. prompt ionization 106yr D-type →R-type (opposite to the uniform medium)

  9. Structure of HII regions (2) Mhalo= 108 Msun Mstar =200 Msun 105yr 106yr higher mass → confined I-front → no further ionization D-type only

  10. n∝r-w w>3/2 ｎ ｎ n∝Rst-3/2 ｎ∝Rs3/2 ｎ∝ n∝r-w w<3/2 n∝Rst-3/2 ｒ ｒ Density profile and I-front types r>Rst →r<Rst r<Rst →r>Rst D-type →R-type R-type →D-type ※ D-type front can propagate ～10pc within 106 yr →R-type is crucial for ionizing the whole halo !

  11. Final HI and H2 fractions Critical masses ionization ～107 Msun H2 dissociation ～108 Msun HI H2 H2 fraction positive feedback near Mcrit

  12. Escaping fractions of photons 11.2-13.6 eV (x 103) Critical masses >13.6 eV ～107 Msun 11.2-13.6 eV ～108 Msun >13.6 eV fesc sensitive to Mhalo ～1 for Mhalo<Mcrit

  13. Fate of collapsed clouds (1) HI & H2 HI H2 dissociated Estimated threshold for R-type front Mhalo∝ Nγ3/4(1+z)-3/2 HII

  14. n∝r-w w>3/2 ｎ ｎn∝Rst-3/2∝Rs3/2 ｒ Threshold for transition from D- to R-type • Virialzized cloud with n ∝r-w • → rcross∝Nγ-1/(2w-3) • ×(nvir Rvirw)2/(2w-3) • 2. D-type front propagation • rD～ cs tage • ～17 pc for T=104K, t=Myr • rD > rcross • → Mhalo < f(w) Nγ3/2w • ×(1+z)-3(3-w)/w rcross D-type → R-type

  15. Fate of collapsed clouds (2) HI & H2 n ∝r-w HI H2 dissociated Estimated threshold for R-type front HII

  16. Feedback from SN explosions 1. Energy injection at the death of the central star ESN = 1051 erg ～ 1053 erg 2. Prompt heating of surrounding gas with Mgas ～Mstar 3. Propagation of shock & energy dissipation by radiative cooling zc=10 ⇒ How far can it travel?

  17. 104yr SN feedback (1) 107yr Mhalo= 3×107Msun Mstar = 200 Msun ESN =1053 erg blown-away!

  18. 104yr SN feedback (2) 107yr Mhalo= 3×107Msun Mstar = 200 Msun ESN =1051 erg Energy dissipation by cooling

  19. SN feedback (3) Mhalo= 3×107Msun Mstar = 200 Msun ESN=1053 erg Bulk of the energy radiated in ～105 yr ESN=1051 erg

  20. Fate of collapsed clouds (3) ESN 1053 erg large: blown-away 1051 erg

  21. Conclusions 1. Radiative feedback from the 1st massive stars →complete ionization (Mhalo<107 Msun at z～10) & H2 dissociation (Mhalo<108 Msun at z～10) sweep-out of gas down to n ～1 cm-3 ※Highly dynamical ※R-type front is crucial in ionizing the whole halo w<1.5 : R-type → D-type w>1.5 : D-type → R-type (n∝r-w) 2. Subsequent SN feedback → blow-away of clouds with Mhalo<106 Msun (ESN=1051 erg), 107Msun (ESN=1053 erg)

  22. Future work • Escape of photons from r<<1 pc • Instability in SN shocks • - Effects of dust & metal • etc.

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