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 log T eff > 4.7 (but ill defined - photosphere is at different radii and T eff for different ) ~ M  yr -1 v wind ~ 1-4x10 3 km s -1 ~ 1/2 of kinetic energy in ISM within 3 kpc of sun is from WR winds Wind energy comparable to SN

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

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

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

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  ≈1.1x10 7 T 9 8 erg g -1 s -1 for T 9 < 6 &  < 3x10 5 g cm -3 –   = L/M ~ 3.1x10 4 S  /R erg g -1 s -1 after H burning –set  =   –rates equal for S  /R = 1 at T 9 = 0.62; S  /R = 0.1 at T 9 = 0.46

cooling photons must diffuse, so rate of energy loss   2 T –  ’s must traverse star, interacting with and depositing energy in material –   ~ R 2 N  /c ~  1/3 M 2/3 ’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?

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

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

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 & dY Z /dt is negative - generates entropy if affinity is negative,  decay is driven to completion & dY Z /dt is positive - also generates entropy

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)

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