Matteo Cantiello Astronomical Institute Utrecht

In collaboration with

S.C.Yoon, N.Langer & M.Livio

Progenitors of long GRBs

Darjeeling 2008 Matteo Cantiello LGRBs progenitors

Outline

Collapsar scenarioRotating stellar models Single star progenitorsBinary star progenitorsObservational consequencesConclusions

Collapsar Scenario (Paczinski, Woosley)

Massive core (BH)

Rapidly rotating core (accretion disk)

Compact size

€

R* /c ≈ τ engine

Darjeeling 2008 Matteo Cantiello LGRBs progenitors

Recipe to make a long GRB

Darjeeling 2008 Matteo Cantiello LGRBs progenitors

( Zen and The art of )

Evolving stars toward Long GRBs

The “angular momentum” problem

Collapsar needs compact progenitor with massive, fast rotating core

Canonical evolution of single stars including rotation and B fields cant produce such an object

A possible solution: Chemically Homogeneous evolution (Yoon & Langer 2005 - Heger & Woosley 2006)

A possible solution: Chemically Homogeneous evolution (Yoon & Langer 2005 - Heger & Woosley 2006)

Darjeeling 2008 Matteo Cantiello LGRBs progenitors

Rotational Mixing

Darjeeling 2008 Matteo Cantiello LGRBs progenitors

Convective Core

Meridional circulation

Rotational instabilities mix rotating massive stars

Eddington-Sweet circulation most efficient process

This instability acts on tKH

€

τES ∝ τ KHωKω

⎛

⎝ ⎜

⎞

⎠ ⎟2

€

ω

Magnetic fields

Darjeeling 2008 Matteo Cantiello LGRBs progenitors

Spruit-Tyler Dynamo (Spruit 2002)

Core - Envelope coupling

1. Differential rotation winds up toroidal component of B.

2. Magnetic torques tend to restore rigid rotation

Convective Core

If the envelope slows downangular momentum is also removed from the core

Chemically Homogeneous Evolution

Darjeeling 2008 Matteo Cantiello LGRBs progenitors

€

τ Mix

τ MS<1

The star cant build a compositional gradient and evolves quasi chemically homogeneous

Rotational mixing can efficiently mix massive stars.

If

Chemically Homogeneous Evolution II

Darjeeling 2008 Matteo Cantiello LGRBs progenitors

TimeRSG

WR

Slow rotator

Fast rotator

Chemically Homogeneous Evolution III

Darjeeling 2008 Matteo Cantiello LGRBs progenitors

GRB

Fast rotating massive stars can evolve q.chemically homogeneous If massloss is not too efficient (Low Z) -> Long GRB

R~1000 Rsun

RSG

R~1 Rsun

WR

Fast rotator

Slow rotator

CCSN

Chemically homogeneous evolution needs high rotational velocity (and low metallicity)

Stars born with high rotational velocity Single star progenitors (Yoon at al. 2006)

Darjeeling 2008 Matteo Cantiello LGRBs progenitors

Rotational velocity

Stars spun-up in binary systemsBinary star progenitors (Cantiello et al. 2007)

Darjeeling 2008 Matteo Cantiello LGRBs progenitors

Single star progenitors (review)

Yoon, Langer and Norman, 2006

Long GRBs prefer low metallicity (i.e. weaker winds) Z 0.004 (SMC)

But important role of wind massloss in determining the metallicity threshold

€

≤

Darjeeling 2008 Matteo Cantiello LGRBs progenitors

Binary star progenitors We want to spin-up a star and induce chemically

homogeneous evolution

Mass (angluar momentum) accretion

Darjeeling 2008 Matteo Cantiello LGRBs progenitors

Spin up by accretion

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)

SN

Darjeeling 2008 Matteo Cantiello LGRBs progenitors

Spin up by accretion16MSun 15MSun

LGRB

Runaway Wolf-Rayet

Rotational Mixing!!

4MSun 21MSun

M* ~ 13MSun

Mco~ 10MSun

Jco~ 2x1016cm2/s

Case B mass transfer

V ~ 30 km/s

Results 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

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 becomes a Runaway WR star and travels

few hundred pc before producing a Long GRB

Runaway GRBs

Darjeeling 2008 Matteo Cantiello LGRBs progenitors

NGC 346: a cluster of young stars in the SMC

Credit: Mokiem et al. 2007

Rotational Velocity vs Surface Helium Rotational Velocity vs Radial Velocity

Low number statistics... But interesting!

Darjeeling 2008 Matteo Cantiello LGRBs progenitors

Darjeeling 2008 Matteo Cantiello LGRBs progenitors

Observational consequences

Observational Consequences Position of GRB in the sky

Darjeeling 2008 Matteo Cantiello LGRBs progenitors

Hammer et. al 2006

Observational Consequences II

Darjeeling 2008 Matteo Cantiello LGRBs progenitors

Constant Density

Van Marle et al. 2006

Afterglow properties

Conclusions

Fast rotating massive stars can evolve chemically homogeneous and become long GRBs

Two classes of progenitors: single and binary stars In massive binaries it’s possible to spin up a star and

obtain a collapsar This scenario is likely to produce a runaway WR which

travel several hundred pc before collapse Observational consequences for the Runaway GRBs

– Position in the sky– Afterglow (maybe) characterized by a constant

density medium Both single and binary progenitors prefer low Z

Darjeeling 2008 Matteo Cantiello LGRBs progenitors

Thanks!

Darjeeling 2008 Matteo Cantiello LGRBs progenitors