Structure and Evolution of Cosmological HII Regions

T. Kitayama (Toho 　 University)

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

HII

Static solution: photoionization = recombination ⇒ 　 Stroemgren sphere (1939)

HII regions in a uniform medium (2)

Dynamical evolution

Two phases!

formation ofthe HII region→pressure gap→shock→expansion of the HII region

HII regions in a uniform medium (3)

R-type front

HII

HIIHII

D-type front

rion < Rst

vion >> vshock

rion > Rst

vion ～ vshock

shock formation

HII regions in a uniform medium (4)

Rst

Essential ingredients:

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

MethodCollapsed 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)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

M,w: free

n(r) r∝ -w, w=2M=3×106 Msun

high central density →confined I-front →sweep out of gas by shock →prompt ionization

D-type →R-typeopposite to the uniform medium

Structure of HII regions (1)

Structure of HII regions (2)

n(r) r∝ -w, w=2M=3×107 Msun

higher mass→ confined shock→ no further ionization 　　

D-type only

Structure of HII regions (3)

n(r) r∝ -w, w=1.5M=3×107 Msun

shallower slope・ lower n at the center・ higher n at the envelope

R-type →D-type

Density profile and I-front types

ｎ

ｒ

ｎ

ｒ

ｎ∝ Rst-3/2

ｎ∝ Rst-3/2

n r∝ -w

w<3/2

n r∝ -w

w>3/2

R-type → 　 D-type D-type → 　 R-type

r<Rst → 　 r>Rst r>Rst → 　 r<Rst

Evolution of HII regions (1)

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

I-front

shock

Evolution of HII regions (2)

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

I-front

shock

Final HI and H2 fractions

Critical masses- ionization ～ 107 Msun

- H2 dissociation ～ 108 Msun

H2 fraction positive feedback near Mcrit

Fate of collapsed clouds

HII

HI & H2

HI H2 dissociated

Fate of collapsed clouds

HII

HI & H2

HI H2 dissociated

large: R-typesmall: D-type

Density profile and I-front types

ｎ

ｒ

ｎ

ｒ

ｎ∝ Rst-3/2

ｎ∝ Rst-3/2

n r∝ -w

w<3/2

n r∝ -w

w>3/2

R-type → 　 D-type D-type → 　 R-type

r<Rst → 　 r>Rst r>Rst → 　 r<Rst

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.