Kazu Omukai 　 (NAOJ)

Formation of the first and second generation stars

Aug. 17 @ Tartu Workshop

Outline

Formation of the First Stars Why they are supposed to be very massive (100-1000Msun)?

Formation of the 2nd-generation starsWhen and how did the transition to low-mass stars occur ?

“Second-generation” includes2nd-gen. zero-metal starsExtremely metal-poor stars

Formation of the First Stars

First Stars (definition) made of primordial pristine gas (H, He, small Li) formed from the cosmological initial condition (no astrophysical feedback)

2. Fragmentationof the First Objects

Scenario of the First Star Formation

1. Formationof the First Object

3. Collapse of Dense Cores: Formation of Protostar

4. Accretion of ambient gas andRelaxation to Main Sequence Star

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

Bromm et al.. 2001

Typical mass scale of fragmentation;

Dense cores

~a few x 102-103Msun

Fragmentation of First Objects: Formation of Dense Cores

These massive cores will collapse and form protostars.

First Star Accretion(1) High accretion rate

Mdot = cs3/G ~ T3/2

=0.001-0.01Msun/yr

for metal-free clouds (T ~300 K)

short formation time

(c.f.10-6-10-5Msun/yr

for the present-day case)

(2) low opacity in accreting matter

because of no dust

lower radiation pressure

(smaller stellar feedback)

Protostellar Evolution with Accretion

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

Owing to fast accretion, the star becomes massive before H burning. (H burning via CN cycle starts at 40-100Msun)

Accretion continues if accretion rate <Mdotcrit=4x10-3Msun/yr

(if >Mdotcrit , no stationary solution for >~100Msun)

(K.O. & Palla 2003)

Protostellar Evolution with ABN(2002) Accretion Rate

Accretion continues….the final stellar mass will be 600Msun

Or accretion may stop owing to photoevaporation of the disk at 200Msun (Tan & McKee 2004)

Evolution of radius under the ABN accretion rate

Mass of First Stars

Mstar=min( Mfrag, Mdot tOB, Mfeedback)

Mfrag:fragmentation mass ~1000Msun

Mdot:accretion rate ~10-3Msun

tOB: massive star lifetime ~106yr Mdot tOB ~1000Msun

Mfeedback: mass of star when the accretion is halted by stellar feedback > 100Msun

Mstar=100-1000Msun

2nd Generation Star Formation

Different Initial Condition • Ionization by the first stars• Density fluctuation by SN blast wave, or

HII region

Different Environment • External Radiation (UV, Cosmic Ray)

Different Composition Metal Enrichment Dust formation

ionized

Star formation

in fossil HII regions

After the death of the exciting star, star formation restarts inside the fossil HII region.

(Oh & Haiman 2004;Nagakura & K.O. 2005)

ionizedneutral

Star formation in fossil HII regions

High ionization degree facilitates the formation of H2 and HD.

HD cooling T~30K

low-mass star formation (<Msun; e.g. Uehara & Inutsuka 1999).

(Nagakura & K.O. 2005)

Temperature evolution

Chemical evolution

Metallicity Effects

Omukai, Tsuribe, Schneider & Ferrara (2005)

1

1) Dust cooling: [Z/H]~-5

2

2) H2 formation on dust: [Z/H]~-4

3

3) metal-line cooling: [Z/H]~-3

Metals and Fragmentation scales

Formation of massive fragments by H2 cooling continues until some metallicity, say Z~10-5Zsun

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

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

ConclusionMetal-free stars

consist of first-generation stars

(cosmological initial condition, H2cooling)

typically very massive ~102-103Msun

second-generation stars (e.g., HD cooling)

can be less massive

Metal enrichment Slight amount of metals (~10-5Zsun)can induce the tr

ansition from massive to low-mass star formation mode.

END

Metallicity Effect: Radiation Pressure on to Dust Grains

ifd>es, radiation pressure onto dust shell is more important.

=> massive SF This occurs ~0.01Zsun

For Z<0.01Zsun

Accretion process is not changed from Z=0

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.

Effects of external FUV radiation

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

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

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.

3D simulations for prestellar collapse

The 3D calculations have reached n>1012

cm-3 (radiative transfer needed for

higher density; cf. n~1022cm-3 for protostars)

Overall evolution is similar to the 1D calculation.

Abel, Bryan & Norman 2002

Bromm & Loeb 2004

Pop I vs Pop III Star Formation

Pop I coreMstar : 10-3Msun

Mfrag: >0.1Msun

Mdot: 10-5Msun

With dust grains

Pop III coreMstar : 10-3Msun

Mfrag : >103Msun

Mdot : 10-2Msun

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

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

Metals from the First SNe

Type II SN 8-25Msun

Pair-instability SN 150-250Msun

Heger, Baraffe, Woosley 2001

PISNSN II

Two windows

Metals and Dusts from the First Stars

Progenitor: SN II (22Msun) Progenitor: PISN (195Msun)

Dust from SNe (c.f. present-day dust from AGB stars) larger area per unit dust mass (smaller radius) more refractory composition (silicates, amorphous carbon)Becomes important even with smaller amount of dust

Schneider, Inoue, K.O., Ferrara in prep.

Scenario of Present-day Star Formation My talk covers these phases.

How much is the accretion rate onto the first protostars?

Several groups found

similar accretion rates.

The rate is very high ~0.01Ms

un/yr

because of high prestellar temperature ~300 K

(c.f.10-6-10-5Msun/yr

for the present-day case)

The rate decreases with time.

Glover (2004)

Key Observations

Early reionization of IGM

ezreion=17 (WMAP)

caused by first stars?

Number and abundance pattern of metal poor stars ( [Fe/H] = -5 – -2 )

So far still very limited !!!

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 StarsFeedback (SN, stellar wind) turbulent ISMmetal /dust enriched gas radiation field (except CMB), CR

complicated microphysics magnetic field MHD

SIMPLE

COMPLICATED

（ K.O. & Nishi 1998) self-similar collapseup to n~1020cm-3

protostar formation

state 6; n~1022cm-3, T~30000K, Mstar~10-3Msun

（ very similar to Pop I protostars ）

Pop III Dense Cores to Protostars: Dynamical Evolution