1 / 30

Formation of the first and second generation stars

Aug. 17 @ Tartu Workshop. Formation of the first and second generation stars. Kazu Omukai (NAOJ). Outline. Formation of the First Stars Why they are supposed to be very massive (100-1000M sun )? Formation of the 2nd-generation stars

galeno
Download Presentation

Formation of the first and second generation stars

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Aug. 17 @ Tartu Workshop Formation of the first and second generation stars Kazu Omukai (NAOJ)

  2. Outline • Formation of the First Stars Why they are supposed to be very massive (100-1000Msun)? • Formation of the 2nd-generation stars When and how did the transition to low-mass stars occur ? • “Second-generation” includes • 2nd-gen. zero-metal stars • Extremely metal-poor stars

  3. 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)

  4. Scenario of the First Star Formation 1. Formation of the First Object 2. Fragmentation of the First Objects 3. Collapse of Dense Cores: Formation of Protostar 4. Accretion of ambient gas and Relaxation to Main Sequence Star

  5. Fragmentation of First Objects: Formation of Dense Cores 3D similation (Bromm et al. 2001; Abel et al. 2002) filamentary clouds (Nakamura & Umemura 2001) Typical mass scale of fragmentation; Dense cores ~a few x 102-103Msun Bromm et al.. 2001 These massive cores will collapse and form protostars.

  6. First Star Accretion • 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)

  7. Protostellar Evolution with Accretion Protostellar Radius ３ｂ、expansion １、adiabatic phase tKH >tacc 2, KH contr. ３a, ZAMS (K.O. & Palla 2003) • 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)

  8. Protostellar Evolution with ABN(2002) Accretion Rate Evolution of radius under the ABN 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)

  9. 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

  10. 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

  11. ionized ionized neutral Star formationin fossil HII regions (Oh & Haiman 2004; Nagakura & K.O. 2005) • After the death of the exciting star, star formation restarts inside the fossil HII region.

  12. Star formation in fossil HII regions (Nagakura & K.O. 2005) Chemical evolution • High ionization degree facilitates the formation of H2 and HD. • HD cooling T~30K low-mass star formation (<Msun; e.g. Uehara & Inutsuka 1999). Temperature evolution

  13. 1) Dust cooling: [Z/H]~-5 2) H2 formation on dust: [Z/H]~-4 3) metal-line cooling: [Z/H]~-3 1 2 3 Metallicity Effects Omukai, Tsuribe, Schneider & Ferrara (2005)

  14. Metals and Fragmentation scales Schneider, Ferrara, Natarajan, & K.O. (2002) • 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.

  15. Conclusion • Metal-free stars consist of • first-generation stars (cosmologicalinitial 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 transition from massive to low-mass star formation mode.

  16. END

  17. Metallicity Effect: Radiation Pressure on to Dust Grains if kd>kes, 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

  18. Photodissociation Effects of UV Radiation Field Star Formation in Small Objects (Tvir < 104K) (K.O. & Nishi 1999) • 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.

  19. Fragmentaion scale vs UV intensity Effects of external FUV radiation Star formation in large objects (Tvir>104K) K.O. & Yoshii 2003 Evolution of T in the prestellar collapse radiation：　Jn=W Bn(105K) from massive PopIII stars • log(W)=-15 ; critical value • W＜WcritH2 formation, and cooling • W>Wcrit no H2 （Lyα –– H- f-b cooling) • Fragmentation scale • H2 cooling clumps （logW < -15） Mfrag~2000-40Msun • Atomic cooling clumps(logW > -15) Mfrag~0.3Msun In starburst of large objects, subsolar mass Pop III Stars can be formed. Fragmentaion scale decreases for stronger radiation

  20. Metals and Mass of Stars 10-2Zsun 0 10-5Zsun Zsun Massive frag. Low-mass frag. possible Accretion halted by dust rad force Accretion not halted Massive stars Low-mass & massive stars Low-mass stars

  21. Critical accretion rate Total Luminosity (if ZAMS) Exceeds Eddington limit if the accretion rate is larger than In the case that Mdot > Mdot_crit, the stars cannot reach the ZAMS structure with continuing accretion.

  22. 3D simulations for prestellar collapse • The 3D calculations have reached n>1012cm-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

  23. Pop I vs Pop III Star Formation Pop I core Mstar : 10-3Msun Mfrag: >0.1Msun Mdot: 10-5Msun With dust grains Pop III core Mstar : 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)

  24. Heger, Baraffe, Woosley 2001 Metals from the First SNe • Type II SN 8-25Msun • Pair-instability SN 150-250Msun SN II PISN Two windows

  25. Metals and Dusts from the First Stars Schneider, Inoue, K.O., Ferrara in prep. 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

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

  27. How much is the accretion rate onto the first protostars? Several groups found similar accretion rates. • The rate is very high ~0.01Msun/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)

  28. Key Observations • Early reionization of IGM te=0.17 zreion=17 (WMAP) caused by first stars? • Number and abundance pattern of metal poor stars ( [Fe/H] = -5 – -2 ) So far still very limited !!!

  29. 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

  30. Pop III Dense Cores to Protostars: Dynamical Evolution （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）

More Related