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Structure and Evolution of Cosmological HII Regions

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Structure and Evolution of Cosmological HII Regions

T. Kitayama (TohoUniversity)

with

N. Yoshida, H. Susa, M. Umemura

Introduction

Feedback from the 1st stars in a Pop III objects

- radiation

- SN explosion

⇒ 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

Difficulties

- Many relevant physical processes

radiative transfer, non-equilibrium chemistry,

explosive motions….

- Uncertain initial conditions

density, temperature, velocity, composition…..

This work

1D hydro + radiative transfer + H2 chemistry

⇒ Evolution of HII regions around 1st stars

for various Mhalo & ρ(r)

Initial conditions for SN feedback studies

HII regions in a uniform medium (1)

HII

Static solution:

photoionization

= recombination

⇒Stroemgren sphere

(1939)

HII

HII regions in a uniform medium (2)

Dynamical

evolution

formation of

the HII region

→pressure gap

→shock

→expansion of

the HII region

Two phases!

HII

HII

HII regions in a uniform medium (3)shock formation

rion < Rst

vion >> vshock

rion > Rst

vion ～ vshock

R-type front

D-type front

ingredients:

- hydrodynamics
- radiative transfer
- time-dependent
- reactions
- density profile
- of the medium
- etc.

Rst

Method

Collapsed cloud at z=10 in a ΛCDM universe

total M → radius Rvir

gas: power-law density profile n∝r-w

Ti =1000K, Xe=10-4, XH2=10-4

DM: NFW profile (fixed)

M,w: free

Radiation from a central massive star

200 Msun, zero metallicity

→Nγ(>13.6eV) = 2.3×1050 1/s

Teff = 105 K

τ= 2.2 Myr (Schaerer 2002)

Solve 1D hydro, radiative transfer of UV photons,

chemical reactions (e, H, H+, H-, H2, H2+,)

& cooling/heating self-consistently

n(r) ∝r-w, w=2

M=3×106 Msun

high central density

→confined I-front

→sweep out of gas

by shock

→prompt ionization

D-type →R-type

opposite to the

uniform medium

n(r) ∝r-w, w=2

M=3×107 Msun

higher mass

→ confined shock

→ no further

ionization

D-type only

n(r) ∝r-w,w=1.5

M=3×107 Msun

shallower slope

・lower n

at the center

・higher n

at the envelope

R-type →D-type

n∝r-w

w>3/2

ｎ

ｎ

ｎ∝Rst-3/2

ｎ∝Rst-3/2

n∝r-w

w<3/2

ｒ

ｒ

Density profile and I-front typesr<Rst →r>Rst

r>Rst →r<Rst

D-type →R-type

R-type →D-type

Evolution of HII regions (1)

I-front

n(r) ∝r-w, w=2.0

M<107 Msun

fully ionized

H2 fully dissociated

n0 < 1 cm-3

M>107 Msun

almost unionized

H2 partially dissoc.

n0 > 30 cm-3

shock

Evolution of HII regions (2)

I-front

M=107 Msun

w<1.5

fully ionized

H2 fully dissociated

n0 <1 cm-3

w>2.0

almost unionized

H2 partially dissoc.

n0 >10 cm-3

shock

Final HI and H2 fractions

- Critical masses
- - ionization
- ～107 Msun
- H2 dissociation
- ～108 Msun

H2 fraction

positive feedback

near Mcrit

n∝r-w

w>3/2

ｎ

ｎ

ｎ∝Rst-3/2

ｎ∝Rst-3/2

n∝r-w

w<3/2

ｒ

ｒ

Density profile and I-front typesr<Rst →r>Rst

r>Rst →r<Rst

D-type →R-type

R-type →D-type

Conclusions

Radiative feedback from a massive star in Pop III objects

→photoionized & photodissociated HII regions

(M<107 Msun) (M<108 Msun)

sweep-out of gas by shock down to n < 1 cm-3

Evolution & structure of HII regions

sensitive to M & gas density profile (index w)

w<1.5 : R-type → D-type

w>1.5 : D-type → R-type

maintenance/achievement of R-type front is essential!

Future work

- Subsequent SN explosion
- ← initial conditions from the present work
- different z, Mstar, Zstar,…..
- dust in HII regions
- etc.

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