Probing the First Star Formation by 21cm line

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

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

6. How massive was the first star? collapse of a dense core ⇒　mass accretion of the protostar At the end of collapse：10-2 M8protostar 103 M8 dense gas enlarge Final mass is set when the accretion terminates.

7. Accretion Evolution of the protostar Hosokawa, KO+ 2010 Snapshot at M*=64.5 M8 • HII region expansion • Photoevaporation of the disk • limit the mass of the star. First stars are typically very massive (50-100Msun).

8. Pop III-IItransition • First 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-IItransition) • 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 2) T minimum by dust cooling dust-induced line-induced Low-mass fragments are formed only in the dust-induced mode.

10. Dust-induced fragmentation Yoshida, KO + 2011 For [M/H]=-5, Rapid cooling by dust at high density (n~1014cm-3) leads to fragmentation. Fragment mass ~ 0.1 Msun Critical metallicity Zcr~10-6-10-5 Zsun 2nd gen. stars have low-mass components 5AU

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 For 21cm line to be observable, TS must deviate from Tg Lya coupling: Wouthuysen-Field effect Collisinal de-ex. coeff. xa, xc: Lya/collisional coupling coefficients Lya color temperatureTC(=~TK): Lya color temperature • TS TK • In the following environments: • dense /hot/moderately ionized gas • Abundant Lyaphotons Furlanetto et al. (2006)

13. Global IGM evolution and its signal TS Tg TK zreion Abs. & emi.: astrophysical Absorption: cosmological Pritchard & Loeb (2008) This trough shows the strength of Lya flux

14. Reionization by Pop III vs Pop II Pop II 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. Pop III Furlanetto (2006)

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

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

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 starsaround ~100MHz.