Star Formation Triggered Star Formation Triggered By First SupernovaeBy First Supernovae

Fumitaka Nakamura (Niigata Univ.)Fumitaka Nakamura (Niigata Univ.)

QuestionsQuestions What is the What is the typical masstypical mass of the first stars? of the first stars?

Can first supernovae Can first supernovae triggertrigger subsequent star formation?subsequent star formation?

Can primordial cloud cores Can primordial cloud cores break up break up into into multiple fragments?multiple fragments?

Binary formation?Binary formation?

What is the typical mass of the stars formed What is the typical mass of the stars formed by shock compression?by shock compression?low mass star formationlow mass star formation? (e.g., HE0107-5240)? (e.g., HE0107-5240)

What is the typical mass of first What is the typical mass of first stars?stars? Typical mass of fragments ~ 100MTypical mass of fragments ~ 100M

No fragmentation for the polytrope gas with No fragmentation for the polytrope gas with = 1.1. = 1.1. (e.g., Tsuribe’s talk)(e.g., Tsuribe’s talk)

Size of HII region ~ 100 pc Free-fall time of fragments ~ 106yr

↓Positive feedback of UV radiation

↓Enhanced H2 formation

30 pc

(Bromm, Coppi, Larson 1999)

If a truly first star is massive, it emits strong UV radiation, which sIf a truly first star is massive, it emits strong UV radiation, which should affect subsequent evolution of other prestellar fragments. hould affect subsequent evolution of other prestellar fragments.

HII region

Positive feedback of UV Positive feedback of UV radiationradiation Enhanced H2 formationEnhanced H2 formation

eHHH 2

hHeH

HD cooling is more dominant for T < 100 ~ 200 K

Threshold H2 abundance

xH2 > 3 x 10-3

(Nakamura & Umemura 2002)

Formation of HD molecules

HHDHD 2

HHDHD 2

H2

HD

LiH

(Nakamura & Umemura 2002)

Thermal Property of Primordial Thermal Property of Primordial Gas for HD Controlled Case Gas for HD Controlled Case

HDHD controlled collapse controlled collapse

Fragmentation !For HD dominant clouds, EOS is almost isothermal. Thus, there is a possibility for the fragments to break up into multiple cores.

Fragment mass ~ 10-Fragment mass ~ 10-40 M40 M..

H2H2 controlled collapse controlled collapse

Omukai 2000Machida et al. (in prep.)

sphere

cylinder

density

Tem

pera

ture

Summary part 1: typical Summary part 1: typical mass of first generation mass of first generation

starsstars Truly first stars may be very massive as ~100 Truly first stars may be very massive as ~100

MM.. But, many first generation stars may have But, many first generation stars may have

masses of 10~40 Mmasses of 10~40 M.. Massive binary stars may be common product.Massive binary stars may be common product.

HD coolingHD cooling

Effect of HD cooling !

Fragmentation !

Can First Supernovae Can First Supernovae Trigger Subsequent Star Trigger Subsequent Star

Formation?Formation?Supernovae of first stars

Complete mixing No mixing

Cloud destruction?Induced SF?

Compression of cloud cores

Shock-cloud interaction

Induced star formation?

Fragmentation of cooling shells

SNR(e.g., Shigeyama & Tsujimoto 1998)

Evolution of SNREvolution of SNR

Step 1: 1D calculationWe follow the evolution of the SNR shell with the thin-shell approximation.

1. Free expansion 2. Sedov-Taylor 3. Pressure-driven expansion

・ Dynamical evolution : analytic model ・ Thermal evolution : radiative cooling + time-dependent chemical evolution

Step 2: 2D hydrodynamic simulation Then, we follow fragmentation of the cooling shell with the thin-disk approximation.

adiabaticcooling

Evolution of SNR: Step 1Evolution of SNR: Step 1Radius and expansion velocity Evolution of density

Evolution of temperature

Machida et al. (in prep.)

Formation of Self-Gravitating Formation of Self-Gravitating ShellsShells

The cooling shell is expected to become self-The cooling shell is expected to become self-gravitating by the time 10gravitating by the time 1066 - 10 - 1077 yr. yr.

Ｔｆｆ

Ｔ dyn

Ｔ exp

Ｔ cool

Formation of self-gravitating Shell↓

Tff = Tdyn

Texp is sufficiently longer than Tff and Tdyn at the final stage.

Fragmentation of Cooling Fragmentation of Cooling Shells: Step 2Shells: Step 2

Fragmentation of a self-gravitating sheetFragmentation of a self-gravitating sheet

Thin-disk approximation isothermal EOS Power law velocity fluctuations 2D hydro simulation

Nakamura & Li (in prep.)

Fragmentation of Cooling Fragmentation of Cooling ShellsShells

Mass fraction of dense regions reaches ~0.7.Mass fraction of dense regions reaches ~0.7. → → star formation efficiency may be high.star formation efficiency may be high.

Dense cores are rotating very rapidly.Dense cores are rotating very rapidly.

M: Mach number of the velocity perturbations

Fragmentation Condition of Fragmentation Condition of SNRSNR

The shell should be self-gravitating before blow out.Expansion velocity should be larger than the sound speed.

Summary part2: Star Summary part2: Star Formation Triggered by First Formation Triggered by First

SupernovaeSupernovaeSupernovae of first stars

Fragmentation of cooling shellsCompression of cloud cores

Complete mixing No mixing

Metal cooling

Z ~ 10-3Z HD cooling

Similar to present-day SF

Formation of massive metal-free stars

Induced SF

SNR Shock-cloud interaction

Formation of low-mass metal-free stars

~10-~10-40M40M..

~1M~1M..

~1M~1M..

Effect of MixingEffect of Mixing

Dense cores are rotating very rapidly. Dense cores are rotating very rapidly. → → binary formationbinary formation Dense cores may fragment into small cores with masses of ~ Dense cores may fragment into small cores with masses of ~

1 M1 M.. The efficiency of star formation may be high.The efficiency of star formation may be high.

The temperature goes down to 20-40 K.

Shock-Cloud InteractionShock-Cloud InteractionShock can trigger gravitational collapse before KH

instability grows significantly.

Nakamura, McKee, & Klein (in prep.)

The density can become greater than 104 cm-3 for nearly isothermal case.

Fragmentation into 1M cores is expected due to efficient H2 cooling by three-body reaction.

Polytrope gas, 2D axisymmetric, no self-gravity