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ROTATING MASSIVE STARS

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as Long Gamma-Ray Burst progenitors Matteo Cantiello - Sterrekundig Instituut Utrecht What s this talk about? Rotation and Massive Stars Chemically Homogeneous ... – PowerPoint PPT presentation

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Title: ROTATING MASSIVE STARS


1
ROTATING MASSIVE STARS
  • as
  • Long Gamma-Ray Burst progenitors
  • Matteo Cantiello - Sterrekundig Instituut Utrecht

2
Whats this talk about?
  • Rotation and Massive Stars
  • Chemically Homogeneous Evolution
  • Long GRB progenitors

What !?
3
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
4
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)

5
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
6
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)

7
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

8
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

9
Thank you!
10
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)

11
Chemically Homogeneous Evolution
12
Final angular momentum
13
Normal Evolution vs CHES
14
A Bifurcation in the HR diagram
15
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)
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