Naomi Pequette

1.Core Hydrogen Burning 2.Shell Hydrogen Burning 3.First Dredge Up 4.The Bump in the Luminosity Function 5.Core Helium Flash 6.Core Helium Burning 7.Ascent to Asymptotic Giant Branch 8.Asymptotic Giant Branch Evolution

 He core contracting and gradually heating  Getting denser and more degenerate  Degenerate core=>Polytropic equation of state Pressure = Kρ γ  No Temperature Dependence  Temp high enough for He burning—extra energy released increases local temperature  Due to degeneracy—burning rate increases further =>Runaway process  This is the Helium Core Flash!

 High density of core—neutrino processes remove energy from core  Depends on density—more efficient at center  Causes center to cool more quickly than other regions  Max. temp moves away from center and slightly outward in mass  Helium flash ignited at point of max temp  Flash begins in shell

 Top: Position of max. temperature within the star  Bottom: Where in mass max. temperature occurs  Initially was at center, but moves away from center

 Temperature profiles during ignition of flash  Huge energy release drives convective zone which reaches all the way to the H shell  Temp of core region decreases  Energy released by flash mostly used in changing core from degeneracy to “nearly perfect-gas equation of state”

 Small jump in H buring shell  Dwarfed by energy produced by He flash  Flash does not last long, as seen in movie  Note: These are log plots!  L reaches L sun but surface value changes little  H L ->0: expansion causes H shell to be pushed outward and cool so much H burning stops

He burning produces C-12 Convection mixes C-12 throughout convective region Animation uses assumptions that are untrue: can ignore dynamic terms in EoM and mixing is instantaneous – Yet we see stars in the next phase that look like models

 There are a couple mini- flashes after main flash  Each removes degeneracy from core  Each successive flash occurs closer to center than one before it  Convective zone of flash settle down to being steadily burning

 Have 2 energy sources: He-burning core and H- burning shell  Movie: interior profiles of He as star continues up HR diagram

 He->C-12 burning increases opacity in core  Radiative gradient increases, and ratio of gradients increases  Other side of convective border: no change in composition since burning happening in very center  Discontinuity in composition and ratio of radiatvie to adiabatic temperature gradients

 Core has large discontinuity in ratio of gradients  Finite acceleration inside  No point where gradients are equal: thus positive acceleration outward and restoring force inward on opp. side  Seemingly core should grow

 But, ratio of gradients has local min.  As more He mixes into core, ratio of gradients will lower throughout core  As region falls below unity, convection disappears and radiation carries energy  1 st pt to become radiative is local min

 Growth of convective core driven by mixing of carbon rich material  Convection “pinched local min: inner region gradients convective, out region separated  Convective core grows till ratio of gradients unity (local min) then semi convection begins

 Semi convective zone grows as evolution proceeds until encompasses ½ as much matter as convective core  As shown in picture to the left

 Once substantial amount of C-12, star producing O-16  Left: The time dependence of the abundances  Spikes: Core breathing pulses

1.Early AGB Evolution 2.Thermally Pulsing AGB 3.Anatomy of a Thermal Pulse 4.Two Consecutive Thermal Pulses

Hansen Stellar Evolution Demo for information and animations: 2evoln.html 2evoln.html H-R Diagram image: museumofflight.org