ROTATING MASSIVE STARS

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ROTATING MASSIVE STARS

• as
• Long Gamma-Ray Burst progenitors
• Matteo Cantiello - Sterrekundig Instituut Utrecht

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Whats this talk about?

• Rotation and Massive Stars
• Chemically Homogeneous Evolution
• Long GRB progenitors

What !?
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Rotating Stars

• A couple of good reasons
• Observations just says stars are rotating, some
  of them pretty fast (Fukuda, 1982 - Mokiem et
  al., 2005)
• At low Z stars are expected to be rotating faster
  because of weaker stellar winds
• (See talks from I.Brott and L. Muijres
  )

• And what we expect from rotation ?

MIXING
Rotational Instabilities
Rotation
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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

• Its 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)

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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 RSG phase). This is because the
  envelope and the core are mixed by the meridional
  circulation -gt no Hydrogen envelope
• Because the star is not experiencing the RSG
  phase it retains an higher angular momentum in
  the core (Yoon Langer, 2005)

R1 Rsun
R1000 Rsun
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Gamma Ray Bursts

• Short Gamma Ray Bursts (lt2s) Coalescence of
  compact objects
• Long Gamma Ray Bursts (gt2s) 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)

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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

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Conclusions

• 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!
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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 its 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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Chemically Homogeneous Evolution
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Final angular momentum
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Normal Evolution vs CHES
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A Bifurcation in the HR diagram
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The Collapsar Model

• Collaspar Scenario for Long GRB (3
  ingredients)
• Massive core (enough to produce a BH)
• Removal of Hydrogen envelope
• Rapid rotating core (enough to produce an
  accretion disk)
• (Woosley ,1993)