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Evolved Massive Stars

Evolved Massive Stars. 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. Wolf-Rayet Stars. log L/L  > 5.5

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Evolved Massive Stars

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  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 > 5.5 • log Teff > 4.7 (but ill defined - photosphere is at different radii and Tefffor different ) • ~ 10-6 - 10-4M 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 > 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 doesn’t know what’s happening in the interior • Hydrodynamics may render the previous statement untrue • For stars >~ 8 M C ignition occurs before thermal pulse-like double shell burning • limits s-process to producing elements with A < 90 • C burning and later (T > 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 > 5e8 K) dominated are neutrino cooled - energy carried by , not photons • When does  cooling take over? • at low T, energy loss rate ≈1.1x107T98erg g-1 s-1 for T9 < 6 &  < 3x105 g cm-3 •  = L/M ~ 3.1x104S/R erg g-1 s-1 after H burning • set  =  • rates equal for S /R = 1at T9 = 0.62; S /R = 0.1at 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 don’t interact with star to depositi energy before escaping • since ’s don’t 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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