1. Formation of the First Stars
Seminario Italia-Giappone
Formation of the First Stars
Kazuyuki Omukai
(NAO Japan)

2. Let’s study their formation process !
First Stars:
proposed as an origin of heavy elements
Sun 2%, metal poor stars %
Cause of early reionization of IGM
te= zreion=17 (WMAP)
Depend on mass /formation rate of first stars
Let’s study their formation process !

3. Before the First Stars After the First Stars
SIMPLE
Cosmological initial condition (well-defined)
Pristine H, He gas, no dusts, no radiation field (except CMB), CR
simple chemistry and thermal process
No magnetic field (simple dynamics)
After the First Stars
COMPLICATED
Feedback (SN, stellar wind) turbulent ISM
metal /dust enriched gas
radiation field (except CMB), CR
complicated microphysics
magnetic field MHD

4. Formation of First Objects: condition for star formation
Hierarchical clustering
　 small objects form earlier
Condition for star formation
　　radiative cooling is necessary for further contraction and star formation
First Objects （３s）
　ｚ～30, M～106Msun
Tvir～3000K
cool by H2
Tegmark et al. 1997

5. Easy
Microphysics of Primordial Gas
Radiative cooling rate
In primordial gas
Atomic cooling only effective for T>104K
Below 104K, H2 cooling is important
H2 formation
(H- channel: e catalyst)
H + e -> H- + g
H- + H -> H2 + e
Efficient cooling for T>1000K

6. Simulating the formation of first objects
600h-1kpc
Yoshida, Abel, Hernquist & Sugiyama (2003)
ab initio calculation is already possible !

7. Road to the First Star Formation 1
of the First Object
95%known

8. Road to the First Star Formation 2
2. Fragmentation
of the First Objects
50%known

9. Typical mass scale of fragmentation; Dense cores a few x 102-103Msun
Fragmentation of First Objects
3D numerical simulation is getting possible
3D similation
(Abel et al. 2002,Bromm et al. 2001)
filamentary clouds
(Nakamura & Umemura 2001)
Typical mass scale of fragmentation;
Dense cores
a few x Msun
no further fragmention
Bromm et al
These cores will collapse and form protostars eventually.

10. Road to the First Star Formation 3
3. Collapse of Dense Cores: Formation of Protostar
60% known

11. Pop III Dense Cores to Protostars: Thermal Evolution
cooling agents:
　H2 lines　
(log n＜14)
　H2 continuum
(14-16)
becomes opaque
at log n=16
　H2 dissociation
(16-20)
g=1.1
（K.O. & Nishi 1998)
Temperature evolution
approximately, g =d log p/d log n= 1.1

12. Pop III Dense Cores to Protostars: Dynamical Evolution
（K.O. & Nishi 1998)
protostar formation
state 6; n~1022cm-3, Mstar~10-3Msun
（~Pop I protostar ）
self-similar collapse
　up to n~1020cm-3
Tiny Protostar

13. 3D simulation for prestellar collapse
The 3D calculation has reached n~1012cm-3
(radiative transfer needed for higher density; cf. n~1022cm-3 for protostars)
Overall evolution is similar to the 1D calculation.
The collapse velocity is slower.
(why? the effect of rotation, initial condition, turbulence)
Abel, Bryan & Norman 2002

14. Road to the First Star Formation 4
4. Accretion of ambient gas and
Relaxation to Main Sequence Star
25% known

15. Density Distribution at protostar formation
Density around the primordial protostar is higher
Than that around prensent-day counterpart.
(For hot clouds, the density must be higher to overcome the stronger pressure and form stars.)
This difference affects the evolution after the protostar formaition
via accretion rate.

16. The accretion rate is very high for Pop III protostars
Mass 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 ~ Msun/yr
The accretion rate is very high
for Pop III protostars

17. Protostellar Evolution in Accretion Phase
Protostellar Radius
３ｂ、expansion
１、adiabatic phase
tKH >tacc
2, KH contr.
３a, ZAMS
Nuclear burning is delayed by accretion.
(H burning via CN cycle at several x10Msun)
Accretion continues in low Mdot cases, while the stellar wind prohibit further accretion in high Mdot cases.
(K.O. & Palla 2003)

18. Critical accretion rate
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.

19. How much is the Actual Accretion Rate ?
From the density distribution
around the protostar…
Abel, Bryan, & Norman (2002)

20. Protostellar Evolution for ABN Accretion Rate
Evolution of radius
under the ABN accretion rate
The protostar reaches ZAMS after Mdot decreases < Mdot_crit.
Accretion continues….
The final stellar mass will be 600Msun.

21. Pop I vs Pop III Star Formation
Pop I core
Mstar : 10-3Msun
Mclump: >0.1Msun
Mdot: 10-5Msun
With dust grains
Pop III core
Mstar : 10-3Msun
Mclump : >103Msun
Mdot : 10-3Msun
No dust grain
Accretion continues.
Very massive star formation
( Msun)
Massive stars (>10Msun)
are difficult to form.

22. a 2nd generation star found !
Most iron-deficient star
HE [Fe/H]=-5.3
Iron less than
10-5 of solar;
Second Generation
Low-mass star ~0.8Msun
What mechanism causes the transition to low-mass star formation mode?
Christlieb et al. 2002

23. Key Ingredients in 2nd Generation Star Formation
Metal Enrichment
UV Radiation Field from pre-existing stars
Density Fluctuation created by SN blast wave, stellar wind, HII regions

24. Metals from the First SNe
Heger, Baraffe, Woosley 2001
SN II
PISN
Type II SN Msun
Pair-instability SN Msun

25. Metals and Fragmentation scales
K.O.(2000), Schneider, Ferrara, Natarajan, & K.O. (2002)
Formation of massive fragments continues until Z~10-4Zsun (If radiation not important)
For higher metallicity, sub-solar mass fragmentation is possible.

26. Radiation pressure onto dusts
if kd>kes, radiation pressure onto dust shell is more important.
=> massive SF
This occurs ~0.01Zsun
For Z<0.01Zsun
Accretion is not halted

27. Metals and Mass of Stars
10-5Zsun
10-2Zsun
Zsun
Massive frag.
Low-mass frag. possible
Accretion halted by
dust rad force
Accretion not halted
Massive stars
Low-mass
& massive
stars
Low-mass
stars

28. Effects of UV Radiation Field
Star Formation in Small Objects (Tvir < 104K)
(K.O. & Nishi 1999)
Photodissociation
Only one or a few massive stars can photodissociate entire parental objects.
Without H2 cooling, following star formation is inhibited.
Only One star is formed at a time.

29. FUV radiation effect on fragmentation scale
Star formation in large objects (Tvir>104K)
K.O. & Yoshii 2003
Fragmentaion scale vs UV intensity
Evolution of T in the prestellar collapse
radiation：　Jn=W Bn(105K) 　from massive PopIII stars
Fragmentation scale
H2 cooling clumps
（logW < -15）
　Mfrag~ Msun
Atomic cooling clumps　(logW > -15)
Mfrag~0.3Msun
log(W)=-15 ;
critical value
W＜Wcrit　H2 formation, and cooling
W>Wcrit no H2
（Lyα –– H- f-b cooling)
In starburst of large objects, subsolar mass Pop III
Stars can be formed.
Fragmentaion scale decreases for stronger radiation

30. Effects of SN blast wave
(Wada & Venkatesan 2002; Salvaterra et al. 2003)
SNe of metal-free stars
(Umeda & Nomoto 2002)
SN II (10Msun-30Msun; 1051 erg)
pair instability SN
(150Msun-250Msun; 1053erg)
Shell formation by blast wave
fragmentation of the shell
low-mass star formation?
Bromm, Yoshida, & Hernquist 2003

31. Conclusion
Typical mass scale of the first stars is very massive ~102-3Msun,
because of
large fragmentation,
continuing accretion at large rate
However, the conclusion is still rather qualitative.
Formation of the second generation of stars is still quite uncertain.
Metallicity/ radiation can induce the transition from massive to low-mass star formation mode.