Evolution of massive stars towards GRBs

1. Evolution of massive stars towards GRBs Sung-Chul Yoon (Amsterdam) In collaboration with Norbert Langer (Utrecht) Santorini, Aug. 29, 2005

2. GRB progenitors • Some clues • Association with star-forming regions in galaxies • GRB 980425  SN 1998bw (Galama et al. 1998) • GRB 030329  SN 2003dh (eg. Hjorth et al. 2003) • At least, some GRBs are deaths of massive stars! -- especially from massive He stars (or WR stars) • Association with Type Ibc supernovae • Crossing time of jet ~ 10 secs: compact progenitors • GRBs are a subset of SN Ibc? (RGRB/RSNIbc ~ 0.01—0.001)

3. Necessary Conditions for GRB formation • Formation of relativistic Jets => central engines are rapid rotators • Collapsar scenario (Woosley) – formation of a Keplerian disk around a black hole: j > 1016 cm2/s • Removal of H envelope • Difficult at very low Z • He core massive enough to form BH

4. Angular momentum redistribution inside stars • Stellar wind mass loss  Removal of hydrogen envelope • Core contraction/chemical gradient between the core and the envelope • Angular momentum transport by Eddington Sweet circulations, shear instability, magnetic torques, etc.  Angular momentum condition of j > 1016 cm2/s

5. Role of magnetic fields in J-transport Models with B-fields are more consistent with observations!!

6. Binary Evolution Mass accretion will spin up the secondary star.

7. Mass and J accretion in close binaries Petrovic, Langer, Yoon & Heger (2005) • Spin-up by accretion leads to strong stellar winds. • J-transport during He core contraction is very rapid. • the core becomes slow again – as slow as in single stars. • Spin-up during MS does not help. M1 = 56, Pinit =6 days

8. Spin-up in WR stages? • Tidal locking of a massive helium star in a very close binary (Izzard et al.03; Podsiadlowski et al.04)? • Orbit should be very short: P < 5 hr • Massive He stars in X-ray binaries? – eg. Cyg X-3 (P ~ 4.8 hr; van Kerkwijk et al. 1992) • He star merger (Fryer & Heger 05) • BUT: Any evolutionary scenario related to common envelope phase may not work at very low Z S Fryer & Heger 05

9. Quasi- Chemically Homogeneous evolution of single stars • Found by Maeder (1987) The envelope cannot spin down the core; The angular momentum redistribution is mainly determined by stellar winds The core is spun down by the slowly rotating envelope

10. Quasi- Chemically Homogeneous evolution Rapidly rotating helium stars can be made even without removing hydrogen envelope => particularly important at low Z

11. Quasi- Chemically Homogeneous evolution Yoon & Langer (2005); see also Woosley & Heger (2005)

12. Uncertainties in WR winds • Maximun Z for such evolution to GRBs sensitively depends on the WR wind mass loss rate. • With current estimate of WR winds by Vink & de Koter (05): • Zmax ~Zsun/10

13. Observational evidence for homogeneous evolution N enriched blue stars in Magelanic cloulds: GRB progenitor? NGC346 in SMC; Bouret et al. (2003)

14. Z dependence of GRB progenitor evolution

15. Conclusion • Quasi-Chemically homogeneous evolution of single stars at low Z • Very rapidly rotating stars (Vinit/VKepler ~ 0.5) with Z < Zsun/10 and 20 < M/Msun < 60 will be able to make GRBs • Consistent with observational indications of low Z of GRB hosts: e.g. GRB 050730, Z ~ Zsun/100 (e.g.Starling et al. 05) • No-need of removal of H envelope by winds: even first stars will be able to produce GRBs • Binary evolution channel: detailed study needed (=> Lecture by Podsiadlowski)