probing the first star formation by 21cm line

Probing the First Star Formation by 21cm line

Kazuyuki Omukai (Kyoto U.)

Contents

• Formation of first & second generation stars

• Their observational signatures in 21-cm line

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.

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

First Protostar Formation

Now we have reached the protostar even in 3D simulation.

Yoshida, KO, Hernquist 2007

~1000Msun

~1/100Msun

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

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).

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 )

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.

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

• Were the population III stars indeed massive ?

• Which population of stars reionized the universe ?

SKA will probe them by 21cm line !

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

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)

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)

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

Relic HII regions of the first stars

Tokutani, Yoshida, Oh, Sugiyama 2009

Greif, Johnson, Klessen, Bromm 2009

Cumulative effect of relic HII regions

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.