# Presentation: 349373

## Evolution of massive stars towards GRBs

- Sung-Chul Yoon (Amsterdam)
- In collaboration with
- Norbert Langer (Utrecht)

- Santorini, Aug. 29, 2005

## 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)

## 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 > 10****16**** cm****2****/s**
- Removal of H envelope
- Difficult at very low Z
- He core massive enough to form BH

## 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

## Role of magnetic fields in J-transport

|  | Observations | Models without B-fields | Models with 
B-fields (Spruit-Talyer dynamo) |
| --- | --- | --- | --- |
| Young NS spin |  15 – 150 ms | < 1ms (Heger et al.00; Hirschi et al. 05) | 4 – 15  ms 
(Heger et al. 05) |
| WD spin |  < 10 km/s | ~ 150 km/s
(Langer et al. 98) |  < 10 km/s
(Suijs et al. 05) |
| Sun | Solid rotation in the core | Differential rotation | Rigidly rotating core (Eggenberger et al. 05) |
| RGRB/
RSNIbc |  0.01 – 0.001
GRBs are rare! | Too high! | Difficult to make GRBs |

- Models with B-fields are more consistent with observations!!

## Binary Evolution

- Mass accretion will spin up the secondary star.

## Mass and J accretion in close binaries

- Petrovic, Langer, Yoon & Heger (2005)

- M1 = 56, Pinit =6 days

- 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.

## 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

## Quasi- Chemically Homogeneous evolution of single stars

- Found by Maeder (1987)

- The core is spun down by the slowly rotating envelope

- The envelope cannot spin down the core;
- The angular momentum redistribution is mainly determined by stellar winds

## Quasi- Chemically Homogeneous evolution

- Rapidly rotating helium stars can be made even without removing hydrogen envelope => particularly important at low Z

## Quasi- Chemically Homogeneous evolution

- Yoon & Langer (2005); see also Woosley & Heger (2005)

## 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

## Observational evidence for homogeneous evolution

- N enriched blue stars in Magelanic cloulds:
- GRB progenitor?

- NGC346 in SMC; Bouret et al. (2003)

## Z dependence of GRB progenitor evolution

## 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)