Seminario Italia-Giappone. Formation of the First Stars. Kazuyuki Omukai (NAO Japan). First Stars:. proposed as an origin of heavy elements Sun 2%, metal poor stars 0.001-0.00001% Cause of early reionization of IGM t e =0.17 z reion =17 (WMAP).
Sun 2%, metal poor stars 0.001-0.00001%
te=0.17 zreion=17 (WMAP)
Depend on mass /formation rate of first stars
Let’s study their formation process !
simple chemistry and thermal process
After the First Stars
cool by H2Formation of First Objects: condition for star formation
small objects form earlier
radiative cooling is necessary for further contraction and star formation
Tegmark et al. 1997
Microphysics of Primordial Gas
Radiative cooling rate
In primordial gas
(H- channel: e catalyst)
H + e -> H- +g
H- + H -> H2 + e
Efficient cooling for T>1000K
Yoshida, Abel, Hernquist & Sugiyama (2003)
ab initio calculation is already possible !
of the First Object
of the First Objects
3D numerical simulation is getting possible
(Abel et al. 2002,Bromm et al. 2001)
(Nakamura & Umemura 2001)
Typical mass scale of fragmentation;
a few x 102-103Msun
no further fragmention
Bromm et al.. 2001
These cores will collapse and form protostars eventually.
3. Collapse of Dense Cores: Formation of Protostar
at log n=16
（K.O. & Nishi 1998)
approximately, g =d log p/d log n= 1.1
（K.O. & Nishi 1998)
state 6; n~1022cm-3, Mstar~10-3Msun
（~Pop I protostar）
up to n~1020cm-3
(radiative transfer needed for higher density; cf. n~1022cm-3 for protostars)
(why? the effect of rotation, initial condition, turbulence)
Abel, Bryan & Norman 2002
4. Accretion of ambient gas and
Relaxation to Main Sequence Star
(For hot clouds, the density must be higher to overcome the stronger pressure and form stars.)
Density around the primordial protostar is higher
Than that around prensent-day counterpart.
This difference affects the evolution after the protostar formaition
via accretion rate.
After formation, the protostars grow in
mass by accretion.
The accretion rate is related to density distribution
(the temperature in prestellar clumps):
Pop III T~300K Mdot ~ 10-3 – 10-2Msun/yr
Pop I T~10K Mdot ~ 10-6 - 10-5Msun/yr
The accretion rate is very high
for Pop III protostars
2, KH contr.
(K.O. & Palla 2003)
Total Luminosity (if ZAMS)
Exceeds Eddington limit
if the accretion rate is larger than
In the case that Mdot > Mdot_crit, the stars cannot reach the ZAMS structure with continuing accretion.
From the density distribution
around the protostar…
Abel, Bryan, & Norman (2002)
Evolution of radius
under the ABN accretion rate
Pop I core
Mstar : 10-3Msun
With dust grains
Pop III core
Mstar : 10-3Msun
Mclump : >103Msun
Mdot : 10-3Msun
No dust grain
Very massive star formation
Massive stars (>10Msun)
are difficult to form.
Most iron-deficient star
10-5 of solar;
What mechanism causes the transition to low-mass star formation mode?
Christlieb et al. 2002
Heger, Baraffe, Woosley 2001
K.O.(2000), Schneider, Ferrara, Natarajan, & K.O. (2002)
if kd>kes, radiation pressure onto dust shell is more important.
=> massive SF
Accretion is not halted
Low-mass frag. possible
Accretion halted by
dust rad force
Accretion not halted
Star Formation in Small Objects (Tvir < 104K)
(K.O. & Nishi 1999)
Only One star is formed at a time.
Star formation in large objects (Tvir>104K)
K.O. & Yoshii 2003
Evolution of T in the prestellar collapse
radiation： Jn=W Bn(105K) from massive PopIII stars
（Lyα –– H- f-b cooling)
（logW < -15）
In starburst of large objects, subsolar mass Pop III
Stars can be formed.
Fragmentaion scale decreases for stronger radiation
(Wada & Venkatesan 2002; Salvaterra et al. 2003)
(Umeda & Nomoto 2002)
SN II (10Msun-30Msun; 1051 erg)
pair instability SN
fragmentation of the shell
low-mass star formation?
Bromm, Yoshida, & Hernquist 2003
However, the conclusion is still rather qualitative.
Metallicity/ radiation can induce the transition from massive to low-mass star formation mode.