Evolution of massive close binary systems Jelena Petrovic Utrecht Universiteit

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Evolution of massive close binary systemsJelena
PetrovicUtrecht Universiteit

• Binary systems
• Wolf-Rayet stars
• Gamma ray bursts

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Stellar evolution overview
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The evolutionary code I

• Code developed on the basis of hydrodynamic
  stellar structure equations for single stars by
  Langer (1991,1998)
• (?m/?r)t 4?r2? - conservation of mass
• (?r/?t)m v - radial velocity
  
• (?P/?m)t (Gm/r2 - (?v/?t)m)/4?r2 -
  hyydrostatic equlibrium inertia term
• (?L/?m)t ? - energy generation
• (?T/?m)t -?GmT/(4?r4P)(1 r2/(Gm)(?v/?t)m) -
  energy transport
• Compositional mixing included as a diffusive
  process (convection, semiconvection,
  thermohaline, rotationally induced mixing)

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The evolutionary code II

• It calculates simultaneous evolution of two
  stellar components
• Mass transfer rate is given by Ritter (1988)
  
• Changes of chemical composition nuclear network
  including pp chains, CNO-cycle and the major
  helium, carbon, neon and oxygen burning reactions
  
• Stellar wind during main sequence by Kudritzki et
  al. (1989), during WR phase by Hamman et al.
  (1995)
• Synchronization due to tidal spin-orbit coupling
  is included with a time scale given by Zahn
  (1977)
• Rotationally enhanced mass loss by Bjorkman
  Cassinelli (1993) Mloss
  Mloss,0 / (1-?)?
  
  
  where ? vrot / vcrit
  , vcrit (GM(1-?)/R)1/2 and ? L/Ledd

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Rotation instabilities

• Rotation by Kippenhahn Thomas (1970) and
  including rotational mixing processes
• dynamical shear removal of chemical
  inhomogeneities on equipotential surfaces
• secular shear also radiative movement of matter
• Eddington-Sweet circulation due to different
  temperature and density gradients on the pole
  and equator
• Solberg-Hoiland instability matter being pushed
  up, because of angular momentum difference with
  the surroundings
• Goldreich-Schubert-Fricke instability oscilatory
  instability due to temperature and density
  gradient

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Magnetic field

• Spruit 2002
• The generation of a magnetic field in a star
  requires a sufficiently powerful differential
  rotation --gt magnetic field amplification process
  by streching field lines (dynamo)
• Magnetic field creates instabilities --gt magnetic
  viscosity --gt influences transport of angular
  momentum through stellar interior
• Magnetic torque tries to maintain a state of
  nearly uniform rotation

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Binary system with mass transfer
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Binary stellar evolution overview

• Roche surface gravitational equipotential
  surface around two stars
• If the star fills its Roche lobe --gt mass
  transfer starts
• Case A mass transfer during core hydrogen
  burning phase of mass losing star
• Case B during shell hydrogen burning
• Case AB both during core and shell hydrogen
  burning

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Case A Case AB evolution

• OO binary system (two main sequence stars)
• The primary evolves faster and fills its Roche
  lobe during core hydrogen burning phase --gt Case
  A mass transfer
• The primary fills its Roche lobe again during
  shell hydrogen burning --gt Case AB
• The primary loses hydrogen envelope during mass
  transfer and system becomes WRO binary

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Wolf-Rayet stars

• Hot, massive helium core burning stars
• High mass loss rate
• Emission lines of nitrogen, carbon and oxygen
• Thick atmosphere --gt radius ?

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Wolf-Rayet star
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Evolution after WRO phase

• The primary finishes He, C, O,..., Si burning in
  the core and explodes as a supernova
• The secondary evolves further as a single star
  (disrupted system --gt no influence of tidal
  forces )
• Red supergiant phase with an iron core
• SN explosion --gt if angular momentum is high
  enough to form an accretion disk and hydrogen is
  lost from the envelope --gt GRB

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Gamma-ray bursts overview

• GRBs are intense and short (0.1-100 seconds)
  bursts of gamma-ray radiation
• Occur all over the sky approximately once per day
• Uniformly distributed across the sky
• Release extreme amount of energy like 1000 Suns
  during their entire lifetime!!!

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What are progenitors of GRBs

• Two types of GRBs short (hard) and long (soft)
• Short GRBs merger in a binary system consisting
  of a two compact objects
• Long GRBs collapse of a massive star into a
  black hole --gt COLLAPSAR model

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Collapsar model overview

• Core of a rotating massive star collapses into a
  black hole
• Material far from the rotation axis forms
  accretion disk around BH
• Rapid accretion (0.1Ms/sec) releases huge amount
  of energy (heat)
• The heated gas at the poles (low density region)
  expands in a highly relativistic jet
• Shock wave accelerates charged particles--gt
  produce gamma-ray emission in the direction of
  rotating axis

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The progenitor star

• Initial mass gt 25Ms
• It goes through H, He, C, O and Si core burning
  --gt red supergiant with the iron core gt 2.5Ms --gt
  BH --gt SN
• Rotating, angular momentum large enough so an
  accretion disk forms around the BH
• Should lose all hydrogen from the envelope (GRBs
  lt--gt SN without hydrogen)

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The evolution of the internal stellar structure
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Conclusions

• Evolution of a massive close binary system can be
  highly non-conservative due to rotation close to
  critical
• WR stars radii can be influenced by chemical
  composition and stellar wind
• Rotating massive single or binary stars have
  enough specific angular momentum in their core to
  form a collapsar and a GRB
• Magnetic tries to maintain solid body rotation of
  the star and the stellar core spins down
  significantly during the evolution

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Velocity profiles of outer layers of 24Ms WR star
for different mass loss rates
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Single star as the progenitor previous results

• Collapsing core should have specific angular
  momentum gt31016cm2/s so an accretion disk would
  form around BH(Macfadyen Woosley 1999)
• Heger et al. 1999 showed that 20Ms star with the
  initial surface rotational velocity 200km/s
  doesn't fulfill this condition

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20Ms single star new results

• Initially solid body
• Star loses mass and angular mometum from the
  surface due to stellar wind
• Gradient in chemical abundances ( ?-gradient)
  inhibits rotational mixing
• The core and the envelope are separated by the
  region with large ?-gradient
• The core doesn't lose significant angular
  momentum during the evolution

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42Ms single star as a GRB progenitor

• More massive stars have initially larger angular
  momentum in the core (for the same initial
  surface rotational velocity)
• They also lose more mass by stellar wind
• Anyway, the final core has a specific angular
  momentum 1017cm2/s
• If the hydrogen is gone from the envelope the
  star forms a collapsar and a GRB!!!

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42Ms star with a magnetic field

• Diffusion coefficient due to magnetic field
  (Spruit 2002) is few orders of magnitude larger
  that the one due to rotation
• Gradient in chemical abundance can not inhibit
  mixing due to magnetic field
• The core loses significant amount of angular
  momentum during the evolution
• The final core doesn't have enough angular
  momentum to form a collapsar

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The evolutionary track of the secondary
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Binary system as a GRB progenitor

• The secondary star in a binary system
  synchronizes its spin with orbital period --gt
  angular momentum loss
• It also accretes matter during mass transfer
  Surface angular momentum of the secondary
  increases --gt transport inwards due to rotational
  mixing
• Final core can form a collapsar!!!

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Binary system with a magnetic field

• Due to the magnetic diffusion, the stellar core
  loses huge amount of angular momentum, and it can
  not form a collapsar

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Influence of stellar rotation on accretion
efficiency

• Before the mass transfer starts, periods of
  rotation of both components synchronize with the
  orbital period (tidal forces)
• When mass transfer starts, the primary starts
  losing matter and transfers it to the secondary
• This matter carries certain angular momentum and
  spins-up the surface layers of the secondary star
• Mass loss rate increases due to rapid rotation
  removing the matter and angular momentum from the
  secondary
• Also, tidal forces try to synchronize stellar
  rotation with the orbital motion

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Influence of rotation on an accretion efficiency

• Fast Case A Mtr 10-3 Ms/yr, ? 15
• Slow Case AMtr 10-5 Ms/yr, ? 90

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Models versus observations

• models
• M1M2Ms q pd --gt MWRMO
  q p
• 4120 2.05 6 --gt 1124
  0.46 9.8
• 5633 1.70 6 --gt 14.839
  0.38 8.5
• 6035 1.71 6 --gt 14.942
  0.35 7.6
• observations
• Name MWRMO
  q p
• HD186943 1736
  0.47 9.5
• HD90657 1937
  0.52 8.4
• GP Cep
  1527 0.54 6.7