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EVOLUTION MODELS OF LOW METALLICITY STARS

EVOLUTION MODELS OF LOW METALLICITY STARS. Georges Meynet, André Maeder, Sylvia Ekström. Geneva Observatory. and Raphael Hirschi Basel University. Spite et al. 2005 Cayrel et al. 2004. Early chemical evolution of galaxies. Reionization at high redshift. Pelló et al. 2004.

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EVOLUTION MODELS OF LOW METALLICITY STARS

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  1. EVOLUTION MODELS OF LOW METALLICITY STARS Georges Meynet, André Maeder, Sylvia Ekström Geneva Observatory and Raphael Hirschi Basel University Spite et al. 2005 Cayrel et al. 2004 Early chemical evolution of galaxies Reionization at high redshift Pelló et al. 2004 Stellar population in metal poor and/or high redshifted galaxies

  2. What is different at very low Z ? • The initial masses of the stars (?) • The ignition of H-burning in massive stars (no CNO element catalysts at the beginning) • The opacities are lower  Stars more compact: R(popIII) = R(Zsol)/4  Stellar winds are weaker El Eid et al 1983; Ober et al 1983; Bond et al 1984; Klapp 1984; Arnett 1996; Limongi et al. 2000; Chieffi et al. 2000; Chieffi and Limongi 2002; Siess et al. 2002; Heger and Woosley 2002; Umeda and Nomoto 2003; Nomoto et al. 2003; Picardi et al. 2004; Gil-Pons et al. 2005

  3. Very important process for the transport of the angular momentum Cells of meridional circulation 20 Msol on the ZAMS Inner cell  inwards transport of angular momentum Outer cell  outwards transport of angular momentum GRATTON- ÖPIK CELL Timescale a few times the Kelvin-Helmholtz timescale

  4. THE SHEAR INSTABILITY Where does the energy come from ? From the excess energy in the shear When does it occur ? When the excess energy in the shear can overcome the stable pressure gradients The timescale Secular shear  much longer than MS lifetime Dynamical shear  dynamical timescale Brueggen & Hillebrandt 2001

  5. ROTATION AND MASS LOSS WHEN Z SURFACE VELOCITY DURING THE MAIN-SEQUENCE PHASE

  6. WHAT CHANGES AT VERY LOW Z FOR ROTATING MODELS ? Meridional velocities smaller MORE ANGULAR MOMENTUM IN THE CORE Steeper gradients of the angular velocity in the interiors MORE EFFICIENT MIXING Less angular momentum removed by stellar winds BREAK-UP LIMIT

  7. New grids of stellar models Also Z=0.040; 0.008, 0.004, 0.00001,10-8 +Pop III See talk by S. Ekström Meynet and Maeder 2003

  8. Rotating model • Enter WR phase during the MS phase WR:post H-b. Mass loss WR:in H-b. Rot. mixing  With a much higher actual mass: 45 Msol instead of 27 Msol 60 Msol, vini= 0 km s-1 vini= 300 km s-1 <V>O=189km/s Mtot Mcc WR phase longer Mass loss and Mixing Both important H-b. He-b. cf also Maeder 1987 Fliegner and Langer 1995

  9. For a given metallicity, the minimum initial mass of single stars which become Wolf-Rayet star is decreased for higher rotation velocities WR lifetimes also increased for a given initial mass 22Msol 37Msol

  10. Meynet and Maeder 2005

  11. Meynet and Maeder 2005 Observed points from Prantzos and Boissier (2003)

  12. IMPORTANT AMOUNTS OF PRIMARY NITROGEN NEED TO BE PRODUCED AT LOW Z Israelian et al. 2004 Centurion et al 2003 (DLA) Spite et al 2005

  13. 60 Msol, Z=10-5, Wini/W = 0.85

  14. ROTATION INDUCES NUMEROUS PROCESSES WHICH ENHANCES MASS LOSS 1) Reaching of the break-up limit during the Main Sequence phase 60 Msol, Z = 0.020 60 Msol, Z = 0.00001 Vcrit Vcrit Vini=800 km/s Vini=500 km/s Vini=300 km/s Vini=300 km/s Cf also Sackman & Anand 1979; Langer 1998

  15. 3) WG-limit at the tip of the blue loop 2) Redwards evolution favoured

  16. 4) Surface enrichments • During the MS phase • N increased • C and O decreased but CNO/Z remains constant equal to the initial value • At the end of the core • He-burning phase, apparition • at the surface of both • H and He-burning products • Primary N • Primary C • Primary O CNO/Z increases

  17. 7 Mo stellar model more mixed than the 60 Mo 7 Mo W 60 Mo Vini/Vcrit~0.6 - 0.7 Z=10-5 Gradient of W steeper Radius smaller (~factor 5) Lifetime longer (~factor 9) 7 Mo 

  18. 7 Msol, Z=10-5 E-AGB phase 60 Msol, Z=10-5, C-burning phase

  19. CONCLUSION The effects of rotation are amplified at low metallicity  mixing enhanced  induce mass loss NUMEROUS INTERESTING CONSEQUENCES • Higher surface enrichments at low Z Maeder &Meynet 2001; Venn & Przybilla 2003 • Change with Z of populations of Be stars Maeder et al. 1999 • of blue to red supergiant ratio Langer & Maeder 1995; Maeder &Meynet 2001 • of LBV and WR stars Fliegner & Langer 1995; Meynet & Maeder 2005 • of type Ibc to II SN ratio Prantzos & Boissier 2003; Meynet & Maeder 2005 • of collapsar progenitors MacFadyen & Woosley 1999; Hirschi et al 2005 • Change with Z of the stellar yields Meynet & Maeder 2002; Ekström et al. in prep.  A LOT OF INTERESTING PROBLEMS TO STUDY…

  20. A FEW EXAMPLES Observations • How fast are rotating low metallicity massive stars ? • Are their surface enrichment on average higher than at solar metallicity ? Theory • How stellar winds behave when CNO is increased at the surface during a red supergiant phase ? • How the evolution into the Pair Instability regime is changed by rotation ? • Might the first stellar generations enrich the ISM in new synthesized Helium ? • How the core collapse supernova explosions are affected by rotation ? • How the formation process of massive star is affected by rotation ?

  21. ENERGY OF ROTATION Veq [km s-1] 15 Msol W/Wcrit (ini)=~0.6 Gravitational energy 1050.3 ergs Energy of reference gravitational energy Thermal energy Thermal energy ~50% Energy of radiation ~10% Energy of radiation Rotational energy ~0.3% Energy of rotation Excess of energy in the shear ~0.003% Excess energy of shear Mass fraction of hydrogen at the center

  22. GLOBAL MASS LOSS RATES Maeder and Meynet 2000 Enhancement at break-up velocity Log Teff 4.35 4.30 4.00 3.90 !

  23. Humphreys, 2002 0.90 0.53 0.36 What physics makes the Humphreys- Davidson Limit ? • The G - Limit: classical Eddington limit. Non rotating LBV • The W - Limit: rotational effect dominates. Be – stars. • The WG - Limit: both rotation and radiation. Most LBV.

  24. EVOLUTIONARY STATUS OF LBV • LBV earlier • than WN stars • in general, but • also simultaneous • TRANSITION  Same mass loss for LBV and WN9-11 Crowther 1997 LBV WN

  25. A STRIKING OBSERVATIONAL FACTS • Very Helium-rich stars in  Centuri ?

  26. DY/DZ 60 Msol Z = 10-8 remaining mass in solar masses

  27. Zahn 1992 Meridional circulation Shear turbulence Transport of the angular momentum Transport of the chemical species

  28. ROTATION AND MASS LOSS • Different evolution of the surface velocity • When stellar winds are weak, stars can reach more easily the critical velocity • The account for anisotropic mass • loss favours break-up 40 Msol, Vini=400 km/s Maeder, 2002

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