Kazuyuki Omukai (NAO Japan)

Formation of the First Stars

Seminario Italia-Giappone

First Stars:

proposed as an origin of heavy elements

Sun 2%, metal poor stars 0.001-0.00001%

Cause of early reionization of IGM

ezreion=17 (WMAP)

Let’s study their formation process !

Depend on mass /formation rate of first stars

Before the First StarsCosmological 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 ISMmetal /dust enriched gas radiation field (except CMB), CR

complicated microphysics magnetic field MHD

SIMPLE

COMPLICATED

Hierarchical clustering

　 small objects form earlier

Condition for star formation

　　 radiative cooling is necessary for further contraction and star formation

First Objects （３） ｚ～ 30, M ～ 106Msun

Tvir ～ 3000K cool by H2

Formation of First Objects: condition for star formation

Tegmark et al. 1997

Atomic cooling only effective for T>104K

Below 104K, H2 cooling is important

H2 formation

(H- channel: e catalyst)

Radiative cooling rate In primordial gas

H + e -> H- + H- + H -> H2 + e

Efficient cooling for T>1000K

Easy Microphysics of Primordial Gas

Simulating the formation of first objects

ab initio calculation is already possible !

Yoshida, Abel, Hernquist & Sugiyama (2003)

600h-1kpc

1. Formationof the First Object

Road to the First Star Formation 1

95%known

2. Fragmentationof the First Objects

Road to the First Star Formation 2

50%known

3D similation (Abel et al. 2002,Bromm et al. 2001)filamentary clouds (Nakamura & Umemura 2001)

Bromm et al.. 2001

Typical mass scale of fragmentation;

Dense cores

a few x 102-103Msun

no further fragmention

Fragmentation of First Objects

3D numerical simulation is getting possible

These cores will collapse and form protostars eventually.

Road to the First Star Formation 3

3. Collapse of Dense Cores: Formation of Protostar 60% known

（ K.O. & Nishi 1998)

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)

Temperature evolution

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

（ K.O. & Nishi 1998)

self-similar collapse

　 up to n~1020cm-3

protostar formation

state 6; n~1022cm-3, Mstar~10-3Msun

（ ~Pop I protostar ）

Pop III Dense Cores to Protostars: Dynamical Evolution

Tiny Protostar

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

4. Accretion of ambient gas andRelaxation to Main Sequence Star

25% known

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 higherThan that around prensent-day counterpart.

This difference affects the evolution after the protostar formaitionvia accretion rate.

Mass Accretion Rate

After formation, the protostars grow in mass by accretion.

2/33 // TGctMM sffJeans

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 Radius yrMM /101.1 ,2.2 ,4.4 ,8.8 3

３ a, ZAMS

３ｂ、 expansion

2, KH

contr.

１、 adiabatic phase

tKH >tacc

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)

Critical accretion rate

Total Luminosity (if ZAMS)

ZAMSZAMStot RMGMLL /

Exceeds Eddington limit if the accretion rate is larger than

yrM

RLLcM esZAMSEddZAMScrit

/104

/)/1(43-

In the case that Mdot > Mdot_crit, the stars cannot reach the ZAMS structure with continuing accretion.

Abel, Bryan, & Norman (2002)

How much is the Actual Accretion Rate ?

From the density distribution around the protostar…

Protostellar Evolution for ABN Accretion Rate

The protostar reaches ZAMS after Mdot decreases < Mdot_crit.Accretion continues….The final stellar mass will be 600Msun.

Evolution of radius under the ABN accretion rate

Pop I vs Pop III Star Formation

Pop I coreMstar : 10-3Msun

Mclump: >0.1Msun

Mdot: 10-5Msun

With dust grains

Pop III coreMstar : 10-3Msun

Mclump : >103Msun

Mdot : 10-3Msun

No dust grainMassive stars (>10Msun)are difficult to form.

Accretion continues.Very massive star formation (100-1000Msun)

a 2nd generation star found !

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?

Most iron-deficient star HE0107-5240 [Fe/H]=-5.3

Christlieb et al. 2002

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

Type II SN 8-25Msun

Pair-instability SN 150-250Msun

Heger, Baraffe, Woosley 2001

PISNSN II

Metals and Fragmentation scales

Formation of massive fragments continues until Z~10-4Zsun (If radiation not important)

For higher metallicity, sub-solar mass fragmentation is possible.

K.O.(2000), Schneider, Ferrara, Natarajan, & K.O. (2002)

Radiation pressure onto

dusts

ifd>es, 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

0 Zsun10-5Zsun 10-2Zsun

Massive frag. Low-mass frag. possible

Accretion not haltedAccretion halted by dust rad force

Massive stars Low-mass & massivestars

Low-massstars

Effects of UV Radiation Field

Only one or a few massive stars can photodissociate entire parental objects.

Without H2 cooling, following star formation is inhibited.

(K.O. & Nishi 1999)

Star Formation in Small Objects (Tvir < 104K)

Photodissociation

Only One star is formed at a time.

FUV radiation effect on fragmentation scale

log(W)=-15 ; critical value

W ＜ Wcrit　 H2 formation, and cooling

W>Wcrit no H2

（ Lyα –– H- f-b cooling)

Evolution of T in the prestellar collapse

radiation ：　 J=W B(105K) 　 from massive PopIII stars

Star formation in large objects (Tvir>104K)

Fragmentation scale H2 cooling clumps （ logW < -15 ）

　 Mfrag~2000-40Msun

Atomic cooling clumps 　 (logW > -15)

Mfrag~0.3Msun

Fragmentaion scale decreases for stronger radiation

Fragmentaion scale vs UV intensity

In starburst of large objects, subsolar mass Pop III Stars can be formed.

K.O. & Yoshii 2003

Effects of SN blast wave

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?

(Wada & Venkatesan 2002; Salvaterra et al. 2003)

Bromm, Yoshida, & Hernquist 2003

ConclusionTypical mass scale of the first stars is very massive ~102-3Msun,

because of

(i) large fragmentation,

(ii) 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.