Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
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 Teff > 4.7 (but ill defined - photosphere Ýat different radii and is M 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 Wolf-Rayet Stars • Have lost H envelope - M > 40 M or binary with envelope ejection • WNL WNWCWO 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 offcenter 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 ≈1.1x107T98 erg 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 = 1 at T9 = 0.62; S /R = 0.1 at T9 = 0.46 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? cooling • several paths for neutrino creation e e 1020 of e e 2 • • • • 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 cooling 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 dE dP emiss dt dt dE dV dS dY P T N A i i dt dt dt dt i A T dS dY emiss N A i i dt dt i A • If A = 0 entropy decreases & there is cooling • A = 0 if there is no composition change cooling • If composition is changing dYZ dYe dY dY Z 1 dt dt dt dt i ui mic 2 chemical potential • for e- capture and decay w/ energy release Q N i A i dYi dY uZ uZ 1 ue Q Z dt dt affinity • 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 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) T dS Y emiss N A ui i T(v S) N A ui (v Yi ) dt t i i advectionof entropy& material cooling T dS Y emiss N A ui i T(v S) N A ui (v Yi ) dt t i i advectionof entropy& material • • • • 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 net dYZ (uZ uZ 1 ue Q) Yc d mec 2 c d dt 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