1. Probing the First Star Formation by 21cm line Kazuyuki Omukai (Kyoto U.)

2. Contents Formation of first & second generation stars Their observational signatures in 21-cm line

3. 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.

4. Birth of First Cosmological Objects Yoshida, Abel, Hernquist & Sugiyama (2003) 600h -1 kpc ΛCDM model Simulates the evolution from over-density to formation of first objects First Objects

5. First Protostar Formation Now we have reached the protostar even in 3D simulation. Yoshida, KO, Hernquist 2007 ~1000M sun ~1/100M sun

6. 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 M  protostar 10 3 M  dense gas

7. Snapshot at M * =64.5 M   HII region expansion  Photoevaporation 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).

8. Pop III-II transition First stars (Pop III stars ) theoretically predicted to be very massive (~100M sun ) Stars in the solar neighborhood (Pop I) typically low-mass (0.1-1M sun ) 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 )

9. Two characteristic fragmentation epochs 1) T minimum by line cooling line-induced 2) T minimum by dust cooling dust-induced Low-mass fragments are formed only in the dust-induced mode.

10. For [M/H]=-5, Rapid cooling by dust at high density (n~10 14 cm -3 ) leads to fragmentation. Fragment mass ~ 0.1 Msun 5AU Dust-induced fragmentation Z cr ~10 -6 -10 -5 Z sun 2 nd gen. stars have low-mass components Critical metallicity Yoshida, KO + 2011

11. Were the population III stars indeed massive ? Which population of stars reionized the universe ? SKA will probe them by 21cm line !

12. Basics of 21cm transition Collisinal de-ex. coeff. Ly  coupling: Wouthuysen-Field effect T S  T K In the following environments: dense /hot/moderately ionized gas Abundant Ly  photons Furlanetto et al. (2006) x , x c: Ly  /collisional coupling coefficients Ly  color temperature T C (=~T K ) : Ly  color temperature For 21cm line to be observable, T S must deviate from T 

13. Global IGM evolution and its signal TKTK TT TSTS Absorption: cosmological Abs. & emi.: astrophysical z reion This trough shows the strength of Ly  flux Pritchard & Loeb (2008)

14. Reionization by Pop III vs Pop II Pop II Pop III Pop III stars: hot & top-heavy emit fewer Ly  photons than Pop II stars do. Pop II stars make deeper absorption trough (i.e., more Ly  coupling) than Pop III. Furlanetto (2006)

15. T b fluctuation signal Pritchard & Loeb (2008) 3. 2. 1. 21cm power spectrum 1. High-z regime collisional coupling, tracks density field 2. Int.med.-z regime star formation  enhances Lya coupling reionization  reduces neutral gas rich in astrophysics 3. Post-z reion regime reflects distribution of residual neutral matter reionization First star formation

16. Relic HII regions of the first stars Tokutani, Yoshida, Oh, Sugiyama 2009 Greif, Johnson, Klessen, Bromm 2009 Cumulative effect of relic HII regions

17. 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 stars around ~100MHz.