1. Naomi Pequette

2.  Goals:  Use Hansen’s Stellar Evolution Demo to follow the sun on its post-main sequence evolutionary track  Better understand the physics behind the evolution through his animations  Cool animation:  http://rainman.astro.illinois.e du/ddr/stellar/archive/suntra ckson.mpg http://rainman.astro.illinois.e du/ddr/stellar/archive/suntra ckson.mpg

3. 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

4.  Star starts @ ZAMS and evolves off after burns all core H  Movie: H mass fraction vs mass in interior of star as evolves off main sequence  H burned via pp chains  Not hot enough for CNO cycle  Phase ends when central H in exhausted

5.  At center: H -> He  Whatever H is burned appears in He (mass- fraction wise)  Movie shows conservation of mass  Raises average mass per particle (μ)  Pressure constant P=ρRT/μ  ρ and T must increase since μ is increasing

6.  After all H-> He-4 in core, formation of H-burning shell  Moves outwards in mass as star evolves  Star quickly moves toward giant branch  No nuclear energy generation-luminosity and thus grad(T) constant  Nearly isothermal core  Gravitational collapse makes not fully isothermal

7.  Ascending giant branch  Growing in radius =>outer envelope cools  Innermost edge of convective envelope moves inward in mass  Reach into region where was prev. H-burning and “dredges- up” products of earlier H burning  Also been N-14->C-12  Dredge up increases envelope abundance of N-14 and decreases C-12

8.  Convection retreats  In H shell: more H than required to maintain shell structure  Hydrostatic readjustment of structure  Decrease in luminosity briefly before continuing up giant branch  Means more stars seen in this region of HR diagram than just above or below=> Bump in Luminosity Function of Clusters

9.  Dramatic increase in radius of star  As radius increases material less tightly bound gravitationally to star  Increase in Stars luminosity  Links grains in envelope and gas comprising star  Mass loss rate increases  Seen in movie

10.  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!

11.  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

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

13.  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”

14.  Small jump in L @ 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 10 10 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

15. 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

16.  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

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

18.  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

19.  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

20.  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

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

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

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

24. 1.Early AGB 2.Thermally Pulsing AGB 3.Envelope Ejection 4.Planetary Nebula Phase 5.Final Configuration

25.  He core exhaustion, He shell established  High L generated-star expands and H shell extinguished  Outer layers cool— opacity increases  Convection dominates  2 nd dredge up  Convection cell goes through H shell and He shell moves outward  Get very close as shown in image

26.  CO Core:  He initially produces C-12 from triple-alpha reactions  Once some C-12 present: start producing O-16  He Shell:  As C-12 builds up, little He left to form O-16  Convective core keeps mixing in fresh He, but radiative shell does not  Thus, product of shell burning in C-12 NOT O-16

27. High rise in luminosity during phase Line color coded to abundances Note no region of variable H abundance after 2 nd dredge up— cyan disappears – Then H-shell reignited and provides most of star’s luminosity

28.  H and He shells very close together and geometrically thin  High Temp dependence for He burning  2 shells thermally unstable  Output from shell begins to oscillate  Then genuine thermal pulse develops  He luminosity reaches 10 5 L sun  H shell propelled outward— almost extinguished  Surface L barely changes

29.  Red line: center of H shell (eq to H exhausted core)  Moves outward during interpulse phase and stationary during pulse  Green line: edge of CO- core/He-shell  He-shell moves outward primarily during pulse

30.  H and He profiles during early pulses  Note alternate movement:  H-shell moves outward mostly between pulses (when star powered by H-burning)  He-shell is active and He- shell position moves outward during pulses

31.  Top: density profiles of shells  See evidence for expansion by decrease in density  Bottom: radius profiles  He shell closer to center  See dramatic expansion of H-shell  Propelled outward in radius by factor of 2

32.  Instability continues to recur and grow in strength  He luminosity grows to 10 8 L sun very quickly

33.  H and He-shells and convective envelope during same time as previous picture  w/ each pulse: expansion gets stronger  Convection reaches beyond (now extinct) H- shell  3 rd Dredge up!

34.  Convective envelope (green) moves into red line mixing C-12 to surface and pushes inward H/He discontinuity  When H-shell re- ignites, does so @ smaller mass-value than before  Depth of dredge-up grows with each pulse  Measured by “dredge-up parameter lambda”

35.  4 th pulse: final pulse: blows off last of envelope @ 100,000 yrs  C-O core revealed  About size of Earth  T~120.000K but then cools off as looses heat to surrounding space  Brightness~3500 L sun then fades rapidly as cools off

36.  UV photons from core ionize and illuminate ejected envelope  Envelope/Planetary Nebula expands and disperses in 10,000 years Image taken with Tzec Maun Mak-Newt 7min exposure taken Oct. 2007

37.  C-O core: mass 0.54M sun cools to White Dwarf size of Earth  Shines only by leftover heat  Cools off VERY slowly  Surrounded by Planetary Nebula  Left: Planetary nebula NGC 6543 courtesy HST

39. Hansen Stellar Evolution Demo for information and animations: http://web.maths.monash.edu.au/~johnl/StellarEvolnDemo/m1z0 2evoln.html http://web.maths.monash.edu.au/~johnl/StellarEvolnDemo/m1z0 2evoln.html H-R Diagram image: museumofflight.org “The Once and Future Sun” – http://www.astronomy.ohio- state.edu/~pogge/Lectures/vistas97.html http://www.astronomy.ohio- state.edu/~pogge/Lectures/vistas97.html