Evolved Massive Stars

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Evolved Massive Stars
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Wolf-Rayet Stars

• Classification
• WNL - weak H, strong He, NIII,IV
• WN2-9 - He, N III,IV,V earliest types have
  highest excitation
• WC4-9 - He, C II,III,IV, O III,IV,V
• WO1-4 - C III,IV O IV,V,VI
• WN most common, WO least

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Wolf-Rayet Stars

• log L/L? gt 5.5
• log Teff gt 4.7 (but ill defined - photosphere is
  at different radii and Teff for different ?)
• 10-6 - 10-4 M? yr-1
• vwind 1-4x103 km s-1
• 1/2 of kinetic energy in ISM within 3 kpc of
  sun is from WR winds
• Wind energy comparable to SN

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Wolf-Rayet Stars

• Have lost H envelope - M gt 40 M? or binary with
  envelope ejection
• WNL ?WN?WC?WO is an evolutionary sequence and a
  mass sequence
• Mass loss first exposes CNO burning products -
  mostly He,N
• Next partial 3? burning - He, C, some O
• finally CO rich material
• Lowest mass stars end as WN, only most massive
  become WO
• Surrounded by ionized, low density wind-blown
  bubble
• Metallicity dependence for occurrence of WRs
• in Galaxy observed min mass for WR 35 M?
• in SMC min mass 70 M?
• WOs found only in metal-rich systems

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Wolf-Rayet Stars

• High luminosities result in supereddington
  luminosities in opacity bumps produced by Fe peak
  elements at 70,000K and 250,000K
• Without H envelope these temperatures occur near
  surface
• Radiative acceleration out to sonic point of wind
  
• Wind driven by continuum opacity instead of line
  opacity
• Photosphere lies in optically thick wind

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Advanced Burning Stages

• No observations - these stages are so short that
  they are completed faster than the thermal
  adjustment time of the star - the stellar surface
  doesnt know whats happening in the interior
• Hydrodynamics may render the previous statement
  untrue
• For stars gt 8 M? C ignition occurs before
  thermal pulse-like double shell burning
• limits s-process to producing elements with A lt
  90
• C burning and later (T gt 5e8 K) dominated are
  neutrino cooled - energy carried by ?, not
  photons
• Near minimum mass C ignition is degenerate and
  often off-center since ? cooling starting in core
  - maximum T occurs outside core

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Advanced Burning Stages

• C burning and later (T gt 5e8 K) dominated are
  neutrino cooled - energy carried by ?, not
  photons
• When does ? cooling take over?
• at low T,? energy loss rate ??1.1x107T98 erg g-1
  s-1 for T9 lt 6 ? lt 3x105 g cm-3
• ?? L/M 3.1x104S?/R erg g-1 s-1 after H
  burning
• set ?? ??
• rates equal for S? /R 1 at T9 0.62 S? /R
  0.1 at T9 0.46

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? cooling

• photons must diffuse, so rate of energy loss ?
  ?2T
• ?s must traverse star, interacting with and
  depositing energy in material
• ?? R2N?/c ?1/3M2/3
• ?s are free streaming even in stellar
  material interaction cross sections are small
• cooling is local - ?s dont interact with star
  to depositi energy before escaping
• since ?s dont interact, they provide no
  pressure support
• Homework What does this imply about late burning
  stages?

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? cooling

• several paths for neutrino creation
• plasmon decay - plasma excitation decays into
  ? pair
• photoneutrino process - ? pair replaces ? in
  ?-e- interaction
• neutrino-nuclear bremsstrahlung - ?s of
  breaking radiation
• replaced by ? pairs
• At low T photoneutrino dominates, cooling/g
  independent of ?
• At higher T e-e annihilation dominates,
  suppressed w/ increasing ?
• At high ?, low T e- degeneracy inhibits pair
  formation plasmon rate dominates
• Overall rate increases w/ T

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? cooling
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? cooling
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? cooling

• The URCA process - generating changes in neutron
  excess and thereby heating cooling through mass
  movements of material undergoing weak
  interactions
• rate of emission of energy by escaping
  neutrinos/mole
• If A 0 entropy decreases there is cooling
• A 0 if there is no composition change

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? cooling

• If composition is changing
• for e- capture and ? decay w/ energy release Q
• if affinity is positive, e- capture (ec) is
  driven to completion dYZ/dt is negative -
  generates entropy
• if affinity is negative, ? decay is driven to
  completion dYZ/dt is positive - also generates
  entropy

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? cooling

• If conversion is slow, process is reversible and
  no heat generated
• If fast, degeneracy energy transferred into ?s
  inefficient heat generated
• depending on rate of ? cooling, heating or
  cooling can occur
• For fluid with mass motions (convection)

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? cooling

• affinity will change with T,? as fluid moves, as
  will S
• More complications from nuclear excited states
• De-excitation releases ?s which heat material
• In convection or waves ?s may be deposited in
  different place from capture or decay - net
  energy transport
• where the Urca pair are nuclei c d and ?c
  ?d are the rates of energy emission as
  antineutrinos from ? decay of c and as neutrinos
  from e- capture on d, respectively