Structure and Evolution of Early Cosmological HII Regions
description
Transcript of 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
IntroductionFeedback 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
Key questions
Essential physical processes hydrodynamics radiative transfer, non-equilibrium chemistry heating/cooling….
- 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….?
⇒ 1D model, useful ingredients to 3D simulations
HII regions in a uniform medium (1)
HII
# of photons emitted = # of recombination ⇒ Stroemgren sphere (1939)
Static solution
HII regions in a uniform medium (2)
Dynamical evolution
Two phases!
1. ionization/heating → pressure gap2. shock → expansion of the HII region
105yr107yr
Rst
Nγ=3×1050 1/s
HII regions in a uniform medium (3)
R-type front D-type front
rion < Rst
vion >> vshock
rion > Rst
vion ~ vshock
HIIHII
shock formation
Highly dynamical !Rst
Model1. 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
3. Solve 1D hydro, radiative transfer of UV photons, chemical reactions (e, H, H+, H-, H2, H2
+,)
& cooling/heating self-consistently
Mhalo, Mstar free
Mhalo= 106 Msun
Mstar =200 Msun
1. high central density →confined I-front →sweep-out of gas by shock2. prompt ionization
D-type →R-type(opposite to the uniform medium)
Structure of HII regions (1)
106yr
105yr
Mhalo= 108 Msun
Mstar =200 Msun
higher mass→ confined I-front→ no further ionization
D-type only
Structure of HII regions (2)
105yr
106yr
n
r
n
r
n∝ Rs3/2
n∝n r∝ -w
w<3/2
n r∝ -w
w>3/2n R∝ st-3/2
n R∝ st-3/2
Density profile and I-front types
R-type → D-type D-type → R-type
r<Rst → r>Rstr>Rst → r<Rst
※ D-type front can propagate ~ 10pc within 106 yr → R-type is crucial for ionizing the whole halo !
Final HI and H2 fractions
Critical massesionization ~ 107 Msun
H2 dissociation ~ 108 Msun
H2 fraction positive feedback near Mcrit
HI
H2
Escaping fractions of photons
Critical masses >13.6 eV ~ 107 Msun
11.2-13.6 eV ~ 108 Msun
11.2-13.6 eV (x 103)
>13.6 eVfesc sensitive to Mhalo
~ 1 for Mhalo<Mcrit
Fate of collapsed clouds (1)
HII
HI & H2
HI H2 dissociated
Estimated threshold for R-type front Mhalo∝ Nγ
3/4(1+z)-3/2
Threshold for transition from D- to R-type
n
r
n n R∝ st-3/2 R∝ s
3/2
n r∝ -w
w>3/2
rcross
1. Virialzized cloud with n r∝ -w
→ rcross N∝ γ
-1/(2w-3)
×( nvir Rvirw)2/(2w-3)
2. D-type front propagation rD ~ cs tage
~ 17 pc for T=104K, t=Myr
3. rD > rcross
→ Mhalo < f(w) Nγ3/2w
×(1+z)-3(3-w)/w D-type → R-type
Fate of collapsed clouds (2)
HII
HI & H2
HI H2 dissociated
n r∝ -w
Estimated threshold for R-type front
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
⇒ How far can it travel?
zc=10
SN feedback (1)
Mhalo= 3×107Msun
Mstar = 200 Msun
ESN =1053 erg
blown-away!
104yr107yr
SN feedback (2)
Mhalo= 3×107Msun
Mstar = 200 Msun
ESN =1051 erg
Energy dissipationby cooling
104yr107yr
SN feedback (3)
Mhalo= 3×107Msun
Mstar = 200 Msun
Bulk of the energy radiated in ~ 105 yr
ESN=1053 erg
ESN=1051 erg
Fate of collapsed clouds (3)
large:blown-away
1053 erg
1051 erg
ESN
Conclusions1. 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)
Future work
- Escape of photons from r<<1 pc- Instability in SN shocks
- Effects of dust & metal
etc.