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Asymptotic Giant Branch

Asymptotic Giant Branch. Learning outcomes. Evolution and internal structure of low mass stars from the core He burning phase to the tip of the AGB Nucleosynthesis and dredge up on the AGB Basic understanding of variability as observed on the AGB. Pagel, 1997. RGB phase. Pagel, 1997.

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Asymptotic Giant Branch

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  1. Asymptotic Giant Branch

  2. Learning outcomes • Evolution and internal structure of low mass stars from the core He burning phase to the tip of the AGB • Nucleosynthesis and dredge up on the AGB • Basic understanding of variability as observed on the AGB

  3. Pagel, 1997

  4. RGB phase

  5. Pagel, 1997

  6. He-flash and core He-burning

  7. Early AGB • Lower part of Asymptotic Giant Branch • He shell provides most of the energy • L increases, Teff decreases • M>4.5 Msun: 2nd dredge up phaseincrease of 14N, decrease of 16O • Re-ignition of H shell begin of thermal pulses (TP)

  8. Internal structure

  9. Thermal Pulses • Quiet phase, H shell provides luminosity, T increase in He shell • He shell ignition (shell flash), expansion, H shell off • Cooling of He shell, reduction of energy production • Convective envelope reaches burning layers, third dredge up • Recovery of H-burning shell, quiet phase

  10. PDCZ...Pulse driven convection zone

  11. Thermal Pulses continuous line...surface luminosity dashed line...H-burning luminosity dotted line...He-burning luminosity Wood & Zarro 1981

  12. Probability for observing an AGB star at a given luminosity during a thermal pulse. Boothroyd & Sackmann 1988

  13. Vassiliadis & Wood 1993

  14. Wood & Zarro 1981

  15. Nucleosynthesis on the AGB • H, He burning: He, C, O, N, F(?) • Slow neutron capture (s-process): various nuclei from Sr to Bi • Hot bottom burning (HBB): N, Li, Al(?)only for M≥4 Msun

  16. Neutron capture Sneden & Cowen 2003

  17. Pagel 1997

  18. Sneden & Cowen 2003

  19. weak component (A<90) main component (A<208) strong component (Pb, Bi) Busso et al. 1999

  20. 13C pocket 13C (α,n) 16O Production of 13C from 12C (p capture) The solid and dashed lines are from theoretical models calculated for a 1.5 solar mass star with varying mass of the 13C pocket. The solid line corresponds to ⅔ of the standard mass (which is 4×10−6 solar masses). The upper and lower dashed curve represent the envelope of a set of calculations where the 13C pocket mass varied from 1/24 to twice the standard mass (figure taken from Busso et al. 2001)

  21. Hot Bottom Burning (HBB) • Motivation: Carbon Star Mystery – Missing of very luminous C-stars • Solution:Bottom of the convective envelope is hot enough for running the CNO-cycle: 12C13C 14N(only in stars with M≥4 Msun)

  22. Lattanzio & Forestini 1999

  23. HBB Li production • Normaly Li destroyed through p capture • Cameron/Fowler mechanism (1971):3He (a,g) 7Be mixed to cooler layers 7Be(e-,n)7Li • Explains existence of super Li-rich stars

  24. Indicators for 3rd dredge up • existence & frequency of C-stars • C/O, 12C/13C • Isotopic ratios of O • Abundances of s-process elements in the photosphere (e.g. ZrO-bands, Tc, S-type stars) • Dependent on core mass, envelope mass, metallicity

  25. Typical AGB star characteristics • Radius: 200 - 600 Rsun • Teff: 2000 - 3500 K • L: up to Mbol = -7.5 • Mass loss rates: 10-8 to 10-4 Msun/yr • Variability period: 30 - 2800 days

  26. Summary of 1 Msun evolution Approximate timescales Phase  (yrs) Main-sequence 9 x109 Subgiant 3 x109 Redgiant Branch 1 x109 Red clump 1 x 108 AGB evolution ~5x106 PNe ~1x105 WD cooling >8x109

  27. Contributions to the ISM Sedlmayr 1994

  28. Pulsation mechanisms

  29. Motivation • Most AGB stars (see later) and obviously also a large fraction of the RGB stars are variable • Variations in brightness, colour, velocity and extension observed • Possibility to „look“ into the stellar interior

  30. Reasons for variability(single star) • Pulsation • Star spots, convective cells, asymmetries • Variable dust extinction

  31. Pulsation (background) • Radial oscillations of a pulsating star are result of sound waves resonating in the star‘s interior • Estimating the typical period from crossing time of a sound wave through the star

  32. adiabatic sound speed hydrostatic equilibrium integration with P=0 at the surface

  33. Pulsation constant Typical periods for AGB stars: a few 100 days

  34. Pulsation modes Radial modes = standing waves R R R 0 0 0 fundamental first overtone second overtone mode

  35. Driving pulsations • To support a standing wave the driving layer must absorb heat (opacity has to increase) during maximum compression • Normally opacity decreases with increasing T (i.e. increasing P) • Solution: partially ionized zones  compression produces further ionization

  36. mechanism(opacity mechanism) Expansion:Energy released by recombination in part. ionization zone Compression:Energy stored by increasing ionization in part. ionization zone In AGB stars: hydrogen ionization zone as driving layer

  37. Spots, convective cells & asymmetries • Expect only a few large convective cells on the surface of a red giant • Convective cell: hot matter moving upwards brighter than cold matter moving downwards  No averaging for cell size ≈ surface size  small amplitude light variations

  38. Simulation Bernd Freytag

  39. Asymmetries Kiss et al. 2000

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