ROTATING MASSIVE STARS

1. ROTATING MASSIVE STARS as Long Gamma-Ray Burst progenitors Matteo Cantiello - Sterrekundig Instituut Utrecht

2. What’s this talk about? • Rotation and Massive Stars • Chemically Homogeneous Evolution • Long GRB progenitors What !? Matteo Cantiello Rotating Massive Stars

3. Rotating Stars • A couple of good reasons: • Observations just says stars are rotating, some of them pretty fast (Fukuda, 1982 - Mokiem et al., 2005) • At low Z stars are expected to be rotating faster because of weaker stellar winds (See talks from I.Brott and L. Muijres ) • And what we expect from rotation ? MIXING Rotation Rotational Instabilities Matteo Cantiello Rotating Massive Stars

4. Meridional circulation Convective Core Temperature Meridional Circulation (Vega, a Fast rotating star - J.Aufdenberg) • For Massive stars the most important contribution to rotational mixing is due to the Meridional (Eddington-Sweet) circulation • It’s due to the fact that the pole of a rotating star is hotter than the equator (Von Zeipel Theorem) • Mixing Act on the thermal timescale (Kelvin Helmoltz) Matteo Cantiello Rotating Massive Stars

5. Chemically Homogeneous Evolution • If rotationally induced chemical mixing during the main sequence occurs faster than the built-up of chemical gradients due to nuclear fusion the star evolves chemically homogeneous(Maeder, 1987) • The star evolves blueward and becomes directly a Wolf Rayet (no RSG phase). This is because the envelope and the core are mixed by the meridional circulation -> no Hydrogen envelope • Because the star is not experiencing the RSG phase it retains an higher angular momentum in the core(Yoon & Langer, 2005) R~1 Rsun R~1000 Rsun Matteo Cantiello Rotating Massive Stars

6. The only evolutionary sequences of collapsing massive stars that satisfy the Collapsar scenario are the ones that evolve Chemically Homogeneous (fast rotating massive stars) Gamma Ray Bursts • Short Gamma Ray Bursts (<2s): Coalescence of compact objects • Long Gamma Ray Bursts (>2s): Death of Massive stars Collaspar Scenario for Long GRB (3 ingredients) • Massive core (enough to produce a BH) • Removal of Hydrogen envelope • Rapidly rotating core (enough to produce an accretion disk) (Woosley ,1993) Matteo Cantiello Rotating Massive Stars

7. Single Stars Progenitors of GRB • We used a 1D evolutionary code that account for rotation and magnetic fields (STERN Langer, Heger, Yoon et al.) • The evolution of a star here depends not only on its initial M and Z, but also on the initial rotational velocity (W/Wk). • We found that models that undergo chemically homogeneous evolution can retain enough angular momentum and fullfill the collapsar scenario. These models can be GRB progenitors. • We computed grids of evolutionary models (Z,M,We found that GRB are more likely to happen in low metallicity regions because of the weaker spin down of the winds (Yoon, Langer and Norman 2006) • This prediction agrees with observations Matteo Cantiello Rotating Massive Stars

8. Conclusions • Stellar Evolution = F ( M, Z, ) • Fast rotating massive stars can evolve chemically homogeneous (due to rotational mixing) • Fast rotating single massive stars could be long Gamma Ray Burst progenitors • This model predicts Long GRB to be more likely at low Z Matteo Cantiello Rotating Massive Stars

9. Thank you! Matteo Cantiello Rotating Massive Stars

10. 1D Approximation • Anisotropic turbulence acts much stronger on isobars, which coincide with equipotential surfaces, than in the perpendicular direction. This enforces “Shellular” rotation rather than cylindrical and sweeps out compositional differences on equipotential surfaces. Therefore it can be assumed that the matter on equipotential surfaces is chemically homogeneous. This assumption it’s actually the assumption that baroclinic instabilities (which act on a dynamical timescale) are very efficient on mixing horizontally the star (A.Heger, PhD Thesis) Matteo Cantiello Rotating Massive Stars

11. Chemically Homogeneous Evolution Matteo Cantiello Rotating Massive Stars

12. Final angular momentum Matteo Cantiello Rotating Massive Stars

13. Normal Evolution vs CHES Matteo Cantiello Rotating Massive Stars

14. A Bifurcation in the HR diagram Matteo Cantiello Rotating Massive Stars

15. The Collapsar Model Collaspar Scenario for Long GRB (3 ingredients) • Massive core (enough to produce a BH) • Removal of Hydrogen envelope • Rapid rotating core (enough to produce an accretion disk) (Woosley ,1993) Matteo Cantiello Rotating Massive Stars