Structure and Evolution of Cosmological HII Regions

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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) - PowerPoint PPT Presentation

Transcript of Structure and Evolution of Cosmological HII Regions

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∝ 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∝ 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.