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 te=0.17 zreion=17 (WMAP) Depend on mass /formation rate of first stars Let’s study their formation process ! Before the First Stars SIMPLE 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 COMPLICATED 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 Hierarchical clustering small objects form earlier Condition for star formation radiative cooling is necessary for further contraction and star formation Tegmark et al. 1997 First Objects （３s） ｚ～30, M～106Msun Tvir～3000K cool by H2 Easy 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 600h-1kpc Yoshida, Abel, Hernquist & Sugiyama (2003) ab initio calculation is already possible ! Road to the First Star Formation 1 1. Formation of the First Object 95%known Road to the First Star Formation 2 2. Fragmentation of the First Objects 50%known 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 Bromm et al.. 2001 a few x 102-103Msun no further fragmention 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 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) （K.O. & Nishi 1998) Temperature evolution approximately, g =d log p/d log n= 1.1 Pop III Dense Cores to Protostars: Dynamical Evolution self-similar collapse up to n~1020cm-3 （K.O. & Nishi 1998) protostar formation state 6; n~1022cm-3, Mstar~10-3Msun Tiny Protostar （~Pop I protostar ） 3D simulation for prestellar collapse Abel, Bryan & Norman 2002 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) 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 around the primordial protostar is higher Than that around prensent-day counterpart. (For hot clouds, the density must be higher to overcome the stronger pressure and form stars.) This difference affects the evolution after the protostar formaition via 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): 3 3/ 2 M M Jeans / t ff cs / G T Pop III T~300K Mdot ~ 10-3 – 10-2Msun/yr Pop I Mdot ~ 10-6 - 10-5Msun/yr T~10K The accretion rate is very high for Pop III protostars Protostellar Evolution in Accretion Phase Protostellar Radius M = 8.8, 4.4, 2.2, 1.110 3 M / yr ３ｂ、expansion Nuclear burning is delayed by accretion. (K.O. & Palla 2003) (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 Total Luminosity (if ZAMS) Ltot LZAMS GM M / RZAMS Exceeds Eddington limit if the accretion rate is larger than M crit = 4c(1 LZAMS / LEdd ) RZAMS / es 4 10 M / yr -3 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 Evolution of radius under the ABN accretion rate The protostar reaches ZAMS after Mdot decreases < Mdot_crit. Accretion continues…. The final stellar mass will be 600Msun. 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 Massive stars (>10Msun) are difficult to form. Accretion continues. Very massive star formation (100-1000Msun) 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 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 Heger, Baraffe, Woosley 2001 SN II Type II SN Pair-instability SN PISN 8-25Msun 150-250Msun 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 if d>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 10-2Zsun 10-5Zsun 0 Massive frag. Low-mass frag. possible Accretion not halted Massive stars Zsun Low-mass & massive stars Accretion halted by dust rad force Low-mass stars Effects of UV Radiation Field Star Formation in Small Objects (Tvir < 104K) (K.O. & Nishi 1999) Photodissociation 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 Star formation in large objects (Tvir>104K) Evolution of T in the prestellar collapse Fragmentaion scale vs 5UV intensity radiation： Jn=W Bn(10 K) from massive PopIII stars K.O. & Yoshii 2003 Fragmentation log(W)=-15scale ; H2 cooling clumps critical value （logW < -15） W＜Wcrit H2 formation, Mfrag ~2000-40M and cooling sun Atomic cooling clumps W>W crit no H2 (logW > -15) （Lyα –– H- f-b Mfragcooling) ~0.3Msun In starburst of large objects, subsolar mass Pop III Fragmentaion scale decreases for stronger radiation Stars can be formed. 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 Conclusion Typical 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.