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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
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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) 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.