probing the first star formation by 21cm line
DESCRIPTION
Probing the First Star Formation by 21cm line. Kazuyuki Omukai (Kyoto U.). Contents. Formation of f irst & s econd generation stars Their observational signatures in 21-cm line. Before the First Stars. Cosmological initial condition (well-defined) - PowerPoint PPT PresentationTRANSCRIPT
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Probing the First Star Formation by 21cm line
Kazuyuki Omukai (Kyoto U.)
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Contents
• Formation of first & second generation stars
• Their observational signatures in 21-cm line
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Before the First Stars
• Cosmological initial condition (well-defined)• Pristine H, He gas, no dusts, no radiation field
(except CMB), no cosmic ray simple chemistry and thermal process • No or only weak magnetic field simple dynamics
Simple physical processes
We can solve all the important processes in computers.
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Birth of First Cosmological Objects
Yoshida, Abel, Hernquist & Sugiyama (2003)
600h-1kpc
ΛCDM modelSimulates the evolution from over-density to formation of first objects
First Objects
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First Protostar Formation
Now we have reached the protostar even in 3D simulation.
Yoshida, KO, Hernquist 2007
~1000Msun
~1/100Msun
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collapse of a dense core ⇒ mass accretion of the protostar
Final mass is set when the accretion terminates.
enlarge
How massive was the first star?
At the end of collapse: 10-2 M8protostar 103 M8 dense gas
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Snapshot at M*=64.5 M8 HII region expansionPhotoevaporation of the disk limit the mass of the star.
Accretion Evolution of the protostar
Hosokawa, KO+ 2010
First stars are typically very massive (50-100Msun).
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Pop III-II transitionFirst stars (Pop III stars ) theoretically predicted to be very massive(~100Msun)
Stars in the solar neighborhood (Pop I) typically low-mass(0.1-1Msun )Low-mass Pop II stars exist in the halo.
transition of characteristic stellar mass in the early universe from
very massive to low-mass (Pop III-II transition)This transition is probably caused by accumulation of a certain
amount of metals and dusts in ISM (critical metallicity )
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Two characteristic fragmentation epochs1) T minimum by line cooling
line-
induc
ed
2) T minimum by dust cooling
dust-induced
Low-mass fragments are formed only in the dust-induced mode.
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For [M/H]=-5,Rapid cooling by dustat high density (n~1014cm-3)leads to fragmentation.Fragment mass ~ 0.1 Msun
5AU
Dust-induced fragmentation
Zcr~10-6-10-5 Zsun
2nd gen. stars have low-mass components
Critical metallicity
Yoshida, KO + 2011
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• Were the population III stars indeed massive ?
• Which population of stars reionized the universe ?
SKA will probe them by 21cm line !
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Basics of 21cm transition
Collisinal de-ex. coeff. Lya coupling: Wouthuysen-Field effect
TS TK
In the following environments: •dense /hot/moderately ionized gas•Abundant Ly a photons Furlanetto et al. (2006)
xa, xc: Lya/collisional coupling coefficientsLya color temperature
TC(=~TK) : Lya color temperature
For 21cm line to be observable, TS must deviate from Tg
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Global IGM evolution and its signal
TK
Tg
TS
Absorption: cosmological
Abs. & emi.: astrophysical
zreion
This trough shows the strength of Lya flux
Pritchard & Loeb (2008)
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Reionization by Pop III vs Pop II
Pop II
Pop III
Pop III stars: hot & top-heavy emit fewer Ly a photons than Pop II stars do.
Pop II stars make deeper absorption trough (i.e., more Lya coupling) than Pop III.
Furlanetto (2006)
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Tb fluctuation signal
Pritchard & Loeb (2008)
3.
2.
1.
21cm
pow
er s
pect
rum
1. High-z regime collisional coupling,
tracks density field2. Int.med.-z regime star formation
enhances Lya coupling reionization reduces neutral gas
rich in astrophysics3. Post-zreion regime reflects distribution of
residual neutral matter
reionization First star formation
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Relic HII regions of the first stars
Tokutani, Yoshida, Oh, Sugiyama 2009
Greif, Johnson, Klessen, Bromm 2009
Cumulative effect of relic HII regions
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Summary
•First stars (Pop III) were (perhaps) very massive ~100Msun.•Pop III-II transition occurred in the early universe with slight amount of dust enrichment. •SKA is able to detect signals by such early starsaround ~100MHz.