The Progenitor Stars of Gamma-Ray Bursts

1. The Progenitor Stars of Gamma-Ray Bursts 沈兆强 17th June, 2014 Long GRBs

2. The First Author Stanford Earl Woosley (1944 - ) • Professor of Astronomy and Astrophysics at UCSC • Director of the Center for Supernova Research at UCSC • Co-investigator on the High Energy Transient Explorer-2 • Bruno Rossi Prize and Hans Bethe Prize Laureate • Education Background: B.S. in Physics, Rice University (1966)Ph.D. in Astrophysics, Rice University (1971) • Research Interest: Theoretical high-energy astrophysics, especially violent explosive events such as supernovae and gamma ray bursts

3. Stanford Earl Woosley

6. Outline • Introduction • What are gamma-ray bursts (GRBs)? • Why must the GRB Progenitors have fast rotation? • If a typical massive star death produces a slow pulsar, what special circumstances produce a GRB? • Models • Package, magnetic torques etc. • Results And Discussion • Helium Cores • Rapidly Rotating Single Stars • Further Discussion • Oe And Be Stars • Presupernova Characteristics • Implications For Gamma-ray Bursts And Supernovae And how do the rapid rotating stars evolve? What can we deduce from the results?

7. What are gamma-ray bursts (GRBs)? Why must the GRB Progenitors have fast rotation? If a typical massive star death produces a slow pulsar, what special circumstances produce a GRB? 1.Introduction

8. What are GRBs? (1) • GRBs are flashes of gamma rays associated with extremely energetic explosions (the most powerful since the Big Bang) that have been observed in distant galaxies. • The GRBs longer than 2s are long GRBs. Because long GRBs occur in galaxies and regions of galaxies where vigorous star formation is going on, they are believe to originate from the death of massive stars.

9. What are GRBs? (2) • If GRB explosions are assumed to be spherical, no known process can explain how to produce this much energy in such a short time. So GRBs are thought to be highly focused explosions, with most of the explosion energy collimated into a narrow jet ().

10. What are GRBs? (3) • GRBs are composed of a four stage phenomenon (Piran 1999): • a compact inner hidden “engine” that produces a relativistic energy flow (in the form of kinetic energy of relativistic particles) • the energy transport stage (in the form of the kinetic energy of a shell of relativistic particles with a width ) • the conversion of this energy to the observed prompt radiation(via internal shocks that arise due to shocks within the flow when fast moving particles catch up with slower ones) • conversion of the remaining energy to radiation in other wavelengths and on a longer time scale - the “afterglow” (via external shocks which are due to interaction with an external medium like the ISM)

11. Why must the GRB Progenitors have fast rotation? Because all currently (2005) favored models for GRBs require so. Collapsar Model (MacFadyen & Woosley 1999) Millisecond Magnetar Model (Usov 1994) “Supranova” Model (Vietri & Stella 1998)

12. Collapsar Model Enough angular momentum must be present to form a disk around a black hole of several solar masses.

13. Millisecond Magnetar Model The rotational energy must be sufficient to produce magnetic fields of order .

14. “Supranova” Model A neutron star supported in part by rotation must exist for an extended period.

15. If a typical massive star death produces a slow pulsar, what special circumstances produce a GRB? Without compelling reasons to the contrary, one must employ the same prescription for magnetic torques in the evolution of GRB progenitors as for pulsar progenitors. But if the magnetic torques are included: NO!! Here is the solution! It seems hard to give a GRB. Really?

16. THE KEY IS TO DECREASE IN WR-MASS LOSS RATE!! • HOW? Rapidly rotating stars to begin with and a decrease of up to a factor of 10 in the standard WR mass-loss rates currently in use by the community may produce GRBs. • WHY? • The scatter in the mass-loss rates of stars having the same mass, composition, and angular momentum on the main sequence. • WR stars of lower metallicity are known to have lower mass-loss rates.

17. Now, it is clear that GRB progenitors must have rapid rotation. It is also clear that in order to get a fast rotating core at collapse, WR-mass loss rate must be decreased. But about how the angular momentum evolves, we need further calculation. 2.Models3.Results And Discussion

18. Models • SIMPLE HELIUM CORES • They are parameterized by their rotational speeds (some fraction of breakup) at helium ignition. • They represent the outcome of stellar mergers or other binary activity. • RAPIDLY ROTATING SINGLE STARS • They experience complete mixing on the main sequence.

19. KEPLER implicit hydrodynamics package (Weaver et al. 1978) • Angular Momentum Transport And Mixing:the same as in Heger et al. (2000, 2005) • Magnetic Torques:the formalism of Spruit (2002) • Mass Loss Rate: MS & RSG: from Nieuwenhuijzen & de Jager (1990); WR & Stars with Ts>10000 K and X<40%: a mass-dependent mass-loss rate (Langer 1989) using the scaling law established by Braun (1997) and Wellstein & Langer (1999), but lowered by a factor of 3 (Hamann& Koesterke 1998) to account for clumping (Nugiset al. 1998; though see Brown et al. 2004)

20. Wind-driven Mass Loss:a scaling law for hot stars (Kudritzki2000, 2002; Nugis & Lamers 2000; Crowther et al. 2002), with a reduction by up to a factor of 10 from standard values • Angular Momentum Loss:General Cases: angle-averaged angular momentum times the mass loss rateRapid Rotating WR Stars: the angular momentum loss should be reduced because of the anisotropic mass loss • Mass And Composition:Bare Helium Cores: 16 with a composition of 98.5% He, 1.5% , and a solar complement (Lodders 2003) of elements heavier than neon, metallicity varies by scaling 1., 0.3 and 0.1Single Stars: 12, 16, and 35 (zero age MS mass) with metallicity (100%, 10%, and 1% of )

21. Deficiency Of The Package It is one-dimensional. • All rotational quantities are angle averaged, so any deformation due to rotation is not followed. • Rotation is treated as a passive quantity with no back reaction on the stellar structure, i.e., the centrifugal term is not included in the force equation.

22. The more mass lost, the slower was the rotation of the core. BARE HELIUM CORES The magnetic torques enforces rigid rotation. 47% critical speed 26% critical speed

23. BARE HELIUM CORES Summary: • Core:Angular momentum did not change a lot during helium burning. By carbon depletion, It was rotating 3 times slower in its inner core, and by the time the iron core collapsed, this factor had become 8. • Surface:Appreciable Mass Loss: The ratio of centrifugal force to gravity decreases with time. Little Mass Loss And Large Magnetic Torques: The centrifugal forces at the surface increase during the latter stages of helium burning and carbon burning to the point where they are comparable to gravity.

24. RAPIDLY ROTATING SINGLE STARS The table is awful!! Check the appendix of the PPT.

25. Spectral type of the star at core collapse. Equatorial surface rotation velocity halfway through H burning. T: 1% O: 10% S:

26. RAPIDLY ROTATING SINGLE STARS Summary: • Core:Angular momentum in the inner core is essentially preserved throughout the main sequence and only declines by about a factor of 2 at helium depletion. The major angular momentum loss occurs during carbon and oxygen burning, with a large fraction occurring during the final months of the star’s life. • Surface:Some of these rapidly rotating stars develop centrifugally supported surfaces, i.e., centrifugal forces exceed gravity at the equator during helium shell burning. However, none of the solar metallicity calculations or 35 stars developed critical rotation.

27. What can we deduce from the calculation above? 4. Further Discussion

28. Oe and Be stars • The low metallicity, rapidly rotating stars considered here might evolve through a stage having properties similar to Oe and Be stars. • The stars which might develop disks are the same stars most likely to produce GRBs. It may be that the precursor to a GRB is an Oe or Be star.

29. here PresupernovaCharacteristics Stars that have an entry greater than 1 for the Kerr parameter at 3 would have to form a disk to carry the extra angular momentum and are thus good candidates for collapsars. Other models having at 3 may also be candidates because the angular momentum of them increases outwards.

30. Implications For Gamma-ray Bursts And Supernovae The effect of rotation on the explosion mechanism It can be characterized by the rotational energy the resulting pulsar would have if one formed and conserved angular momentum. • Prot>10 ms: won’t have a large effect on the explosion • Prot<5 ms: supernova with kinetic energy 1051 erg • Prot<2 ms: hypernova with kinetic energy 1052 erg • Prot<1 ms: give disks around black holes of several solar masses GRBs maybe GRBs

31. GRBs GRBs Fast rotation Little mass loss Fast rotation Little mass loss Fast rotation Much mass loss Pulsars Pulsars Slow rotation

32. Conclusion

33. Thank you!