ROTATING MASSIVE STARS

ROTATING MASSIVE STARSROTATING MASSIVE STARS

as Long Gamma-Ray Burst progenitors

Matteo Cantiello - Sterrekundig Instituut Utrecht

as Long Gamma-Ray Burst progenitors

Matteo Cantiello - Sterrekundig Instituut Utrecht

NOVA School 2006NOVA School 2006 Matteo Cantiello Rotating Massive Stars Matteo Cantiello Rotating Massive Stars

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What’s this talk about?What’s this talk about?

Rotation and Massive Stars

Chemically Homogeneous Evolution

Long GRB progenitors

Rotation and Massive Stars


Long GRB progenitors


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Rotating Stars Rotating Stars

A couple of good reasons:1. Observations just says stars are rotating,

some of them pretty fast (Fukuda, 1982 - Mokiem et al., 2005)

2. At low Z stars are expected to be rotating faster because of weaker stellar winds

(See talks from I.Brott and L. Muijres )

A couple of good reasons:1. Observations just says stars are rotating,

some of them pretty fast (Fukuda, 1982 - Mokiem et al., 2005)

2. At low Z stars are expected to be rotating faster because of weaker stellar winds

(See talks from I.Brott and L. Muijres )

RotationalInstabilitiesRotationalInstabilities MIXINGMIXINGRotationRotation

And what we expect from rotation ? And what we expect from rotation ?


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Meridional CirculationMeridional Circulation

(Vega, a Fast rotating star - J.Aufdenberg)

Temperature

For Massive stars the most important contribution to rotational mixing is due to the Meridional (Eddington-Sweet) circulation

For Massive stars the most important contribution to rotational mixing is due to the Meridional (Eddington-Sweet) circulation

€

τES ∝ τ KHωKω

⎛

⎝ ⎜

⎞

⎠ ⎟2

Convective Core

Meridional circulation

It’s due to the fact that the pole of a rotating star is hotter than the equator (Von Zeipel Theorem)

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) Mixing Act on the thermal timescale (Kelvin Helmoltz)


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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)

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

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τESτ MS

<1


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Gamma Ray BurstsGamma Ray Bursts Short Gamma Ray Bursts (<2s): Coalescence of compact objects

Long Gamma Ray Bursts (>2s): Death of Massive stars

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)


a BH) Removal of Hydrogen envelope Rapidly rotating core (enough to


The only evolutionary sequences of collapsing massive stars that satisfy the Collapsar scenario are the ones that evolve Chemically Homogeneous (fast rotating massive stars)

The only evolutionary sequences of collapsing massive stars that satisfy the Collapsar scenario are the ones that evolve Chemically Homogeneous (fast rotating massive stars)


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Single Stars Progenitors of GRB

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

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


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ConclusionsConclusions 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

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


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Thank you!Thank you!


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1D Approximation1D 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)

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)

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Mr


1111

€

τES ∝ τ KHΩ−2 where Ω =ω

ωk and τ KH ∝

GM 2

RL

Chemically homogeneous threshold for τ ES

τ KH~ 1

⇒ M ∝Ω−

2

5




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Final angular momentumFinal angular momentum


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Normal Evolution vs CHESNormal Evolution vs CHES


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A Bifurcation in the HR diagram

A Bifurcation in the HR diagram


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The Collapsar ModelThe Collapsar Model


a BH) Removal of Hydrogen

envelope Rapid rotating core (enough to



a BH) Removal of Hydrogen

envelope Rapid rotating core (enough to


ROTATING MASSIVE STARS

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