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Evolved Massive Stars

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Title: Evolved Massive Stars


1
Evolved Massive Stars
2
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

3
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

4
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

5
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

6
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

7
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

8
? 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?

9
? 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

10
? cooling
11
? cooling
12
? 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

13
? 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

14
? 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)

15
? 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
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