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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).

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Formation of the first stars

Seminario Italia-Giappone

Formation of the First Stars

Kazuyuki Omukai

(NAO Japan)

First stars
First Stars:

  • proposed as an origin of heavy elements

    Sun 2%, metal poor stars 0.001-0.00001%

  • Cause of early reionization of IGM

    te=0.17 zreion=17 (WMAP)

Depend on mass /formation rate of first stars

Let’s study their formation process !

Before the first stars


Before the First Stars

  • 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


  • Feedback (SN, stellar wind) turbulent ISM

  • metal /dust enriched gas

  • radiation field (except CMB), CR

  • complicated microphysics

  • magnetic field MHD

Formation of first objects condition for star formation

First Objects(3s)

z~30, M~106Msun


cool by H2

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

Tegmark et al. 1997


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

Simulating the formation of first objects
Simulating the formation of first objects


Yoshida, Abel, Hernquist & Sugiyama (2003)

ab initio calculation is already possible !

Road to the first star formation 1
Road to the First Star Formation 1

1. Formation

of the First Object


Road to the first star formation 2
Road to the First Star Formation 2

2. Fragmentation

of the First Objects


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 102-103Msun

no further fragmention

Bromm et al.. 2001

These cores will collapse and form protostars eventually.

Road to the first star formation 3
Road to the First Star Formation 3

3. Collapse of Dense Cores: Formation of Protostar

60% known

Pop iii dense cores to protostars thermal evolution


Pop III Dense Cores to Protostars: Thermal Evolution

cooling agents:

H2 lines

(log n<14)

H2 continuum


becomes opaque

at log n=16

H2 dissociation


(K.O. & Nishi 1998)

Temperature evolution

approximately, g =d log p/d log n= 1.1

Pop iii dense cores to protostars dynamical evolution
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

3d simulation for prestellar collapse
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

Road to the first star formation 4
Road to the First Star Formation 4

4. Accretion of ambient gas and

Relaxation to Main Sequence Star

25% known

Density distribution at protostar formation
Density Distribution at protostar formation

(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.

Mass accretion rate
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 ~ 10-6 - 10-5Msun/yr

The accretion rate is very high

for Pop III protostars

Protostellar evolution in accretion phase
Protostellar Evolution in Accretion Phase

Protostellar Radius


1、adiabatic phase

tKH >tacc

2, KH contr.

3a, ZAMS

(K.O. & Palla 2003)

  • 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.

Critical accretion rate
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.

How much is the Actual Accretion Rate ?

From the density distribution

around the protostar…

Abel, Bryan, & Norman (2002)

Protostellar evolution for abn accretion rate
Protostellar Evolution for ABN Accretion Rate

Evolution of radius

under the ABN accretion rate

  • The protostar reaches ZAMS after Mdotdecreases < Mdot_crit.

  • Accretion continues….

  • The final stellar mass will be 600Msun.

Pop i vs pop iii star formation
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


Massive stars (>10Msun)

are difficult to form.

A 2 nd generation star found
a 2nd generation star found !

Most iron-deficient star

HE0107-5240 [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

Key ingredients in 2 nd generation star formation
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

Metals from the first sne
Metals from the First SNe

Heger, Baraffe, Woosley 2001

  • Type II SN 8-25Msun

  • Pair-instability SN 150-250Msun



Metals and fragmentation scales
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.

Radiation pressure onto dusts
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

Metals and mass of stars
Metals and Mass of Stars





Massive frag.

Low-mass frag. possible

Accretion halted by

dust rad force

Accretion not halted

Massive stars


& massive




Effects of uv radiation field


Effects of UV Radiation Field

Star Formation in Small Objects (Tvir < 104K)

(K.O. & Nishi 1999)

  • 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.

Fuv radiation effect on fragmentation scale

Fragmentaion scale vs UV intensity

FUV radiation effect on fragmentation scale

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

  • log(W)=-15 ;

    critical value

    • W<WcritH2 formation, and cooling

    • W>Wcrit no H2

      (Lyα –– H- f-b cooling)

  • Fragmentation scale

  • H2 cooling clumps

    (logW < -15)


  • Atomic cooling clumps(logW > -15)


In starburst of large objects, subsolar mass Pop III

Stars can be formed.

Fragmentaion scale decreases for stronger radiation

Effects of sn blast wave
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


  • 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.