Star Formation Triggered By First Supernovae

1. Star Formation Triggered By First Supernovae Fumitaka Nakamura (Niigata Univ.)

2. Questions • What is the typical mass of the first stars? • Can primordial cloud cores break up into multiple fragments? Binary formation? • Can first supernovae trigger subsequent star formation? • What is the typical mass of the stars formed by shock compression? low mass star formation? (e.g., HE0107-5240)

3. HII region What is the typical mass of first stars? • Typical mass of fragments ~ 100M8 • No fragmentation for the polytrope gas with g = 1.1. (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 should affect subsequent evolution of other prestellar fragments.

4. (Nakamura & Umemura 2002) Formation of HD molecules Threshold H2 abundance xH2 > 3 x 10-3 (Nakamura & Umemura 2002) Positive feedback of UV radiation • Enhanced H2 formation HD cooling is more dominant for T < 100 ~ 200 K

5. g ~ 5/3 g ~ 1 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 ~ 10-40 M8. Thermal Property of Primordial Gas for HD Controlled Case • H2 controlled collapse • HD controlled collapse sphere g ~ 1.1 Temperature cylinder density Machida et al. (in prep.) Omukai 2000

6. Summary part 1: typical mass of first generation stars • Truly first stars may be very massive as ~100 M8. • But, many first generation stars may have masses of 10~40 M8. • Massive binary stars may be common product. Effect of HD cooling ! Fragmentation ! HD cooling

7. SNR Shock-cloud interaction (e.g., Shigeyama & Tsujimoto 1998) Fragmentation of cooling shells Compression of cloud cores Complete mixing No mixing Cloud destruction? Induced SF? Induced star formation? Can First Supernovae Trigger Subsequent Star Formation? Supernovae of first stars

8. Step 2: 2D hydrodynamic simulation Then, we follow fragmentation of the cooling shell with the thin-disk approximation. Evolution of SNR cooling adiabatic 1. Free expansion 2. Sedov-Taylor 3. Pressure-driven expansion Step 1: 1D calculation We follow the evolution of the SNR shell with the thin-shell approximation. ・Dynamical evolution : analytic model ・Thermal evolution : radiative cooling + time-dependent chemical evolution

9. Evolution of SNR: Step 1 Machida et al. (in prep.) Radius and expansion velocity Evolution of density Evolution of temperature

10. Formation of Self-Gravitating Shells • The cooling shell is expected to become self-gravitating by the time 106 - 107 yr. Formation of self-gravitating Shell ↓ Tff = Tdyn Ｔｆｆ Ｔcool Ｔdyn Ｔexp Texp is sufficiently longer than Tff and Tdyn at the final stage.

11. Fragmentation of Cooling Shells: Step 2 • Fragmentation of a self-gravitating sheet • Thin-disk approximation • isothermal EOS • Power law velocity fluctuations • 2D hydro simulation Nakamura & Li (in prep.)

12. Fragmentation of Cooling Shells • Mass fraction of dense regions reaches ~0.7. → star formation efficiency may be high. M: Mach number of the velocity perturbations • Dense cores are rotating very rapidly.

13. Fragmentation Condition of SNR • The shell should be self-gravitating before blow out. • Expansion velocity should be larger than the sound speed.

14. Summary part2: Star Formation Triggered by First Supernovae Supernovae of first stars SNR Shock-cloud interaction Fragmentation of cooling shells Compression of cloud cores Complete mixing No mixing ~1M8. Induced SF Z ~ 10-3Z8 HD cooling Formation of low-massmetal-free stars Metal cooling Formation of massivemetal-free stars ~10-40M8. Similar to present-day SF ~1M8.

15. Effect of Mixing The temperature goes down to 20-40 K. • Dense cores are rotating very rapidly. → binary formation • Dense cores may fragment into small cores with masses of ~ 1 M8. • The efficiency of star formation may be high.

16. Shock-Cloud Interaction The density can become greater than 104 cm-3 for nearly isothermal case. Shock can trigger gravitational collapse before KH instability grows significantly. Polytrope gas, 2D axisymmetric, no self-gravity Nakamura, McKee, & Klein (in prep.) Fragmentation into 1M8 cores is expected due to efficient H2 cooling by three-body reaction.