Matteo Cantiello Astronomical Institute Utrecht

1. Binary star progenitors of long GRBsM. Cantiello, S.-C. Yoon, N. Langer, and M. Livio A&A 465, L29-L33 (2007) Matteo Cantiello Astronomical Institute Utrecht

2. Outline of the talk: • Ingredients for long GRBs: Collapsar Scenario • Chemically Homogeneous Evolution • The binary channel for long GRBs • Runaway GRBs and observations • Conclusions What? Matteo Cantiello Binary star progenitors of GRBs

3. Recipe to make a long GRB Collapsar Scenario (Woosley, 1993) • Massive core (enough to produce a BH) • Compact size • Rapidly rotating core (enough to produce an accretion disk around the BH) Matteo Cantiello Binary star progenitors of GRBs

4. The “angular momentum” issue • Stellar evolution models including rotation and magnetic fields: It’s hard to fulfill all the requirements of the collapsar scenario • It is possible to remove the envelope (WR winds) but too much angular momentum is lost during the RSG and WR phases (magnetic torques) A possible solution: Chemically Homogeneous evolution (Yoon & Langer 2005 - Heger & Woosley 2006) Matteo Cantiello Binary star progenitors of GRBs

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 Red Super Giant 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) The only evolutionary sequences of collapsing, single, massive stars that satisfy the Collapsar scenario are the ones that evolve Chemically Homogeneous (fast rotating massive stars) WR RSG R~1 Rsun R~1000 Rsun Matteo Cantiello Binary star progenitors of GRBs

6. The binary model We used a 1D hydrodynamic binary evolution code to evolve massive binary systems (rotation and magnetic fields included) • 16+15 Msun • P= 5 days • SMC metallicity (Z=0.004) Matteo Cantiello Binary star progenitors of GRBs

7. Results Zero Age Main Sequence Begin of Mass transfer End of Mass transfer Primary dies as a SN Thereafter the accreted companion is a fast rotating, runaway WR star. It evolves chemically homogeneous and at the end of the evolution fulfills the requirements of the collpsar model. Matteo Cantiello Binary star progenitors of GRBs

8. Rotational Velocities • This model explains how a massive star can obtain the high rotational velocity needed to evolve quasi-chemically homogeneous and fulfills the Collapsar scenario for Long GRBs • Unlike the single star model, the star doesn’t need to be born with an high rotational velocity Runaway GRBs • The donor star dies as a SN type Ib/c 7Myrs before the collapse of the accreting companion • The system is likely to be broke up by the SN kick (80%) • The accreting companion (GRB progenitor) becomes a Runaway WR star Matteo Cantiello Binary star progenitors of GRBs

9. NGC 346: a cluster of young stars in the SMC Rotational Velocity vs Surface Helium Rotational Velocity vs Radial Velocity Credit: Mokiem et al. 2007 Low number statistics... But interesting! Matteo Cantiello Binary star progenitors of GRBs

10. Observational Consequences • Position of GRB in the sky: Hammer et. al 2006 • Afterglow properties: Van Marle et al. 2006 Constant Density Matteo Cantiello Binary star progenitors of GRBs

11. Conclusions • Fast rotating massive stars can evolve chemically homogeneous (due to rotational mixing) and becomes long GRB • It is possible to spin up a star in a massive binary system • The accreted star fulfills the collapsar model for long GRB • The progenitor is likely to be a runaway WR and travel several hundred pc before collapse • Observational consequences for the Runaway GRBs • Position in the sky • Afterglow characterized by a constant density medium Matteo Cantiello Binary star progenitors of GRBs

12. Thank you

13. 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 acts on the thermal timescale (Kelvin Helmoltz) Matteo Cantiello Binary star progenitors of GRBs

14. Binary vs Single star Comparison between the internal structure of a long GRB progenitor produced by mass accretion in a binary system (top) and a long GRB progenitor produced by a single, rapidly rotating star (bottom). After mass accretion the two models are almost identical Matteo Cantiello Binary star progenitors of GRBs

15. 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 Binary star progenitors of GRBs

16. Chemically Homogeneous Evolution Matteo Cantiello Binary star progenitors of GRBs

17. Final angular momentum Matteo Cantiello Binary star progenitors of GRBs