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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 He-flash and core He-burning 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 phase increase of 14N, decrease of 16O • Re-ignition of H shell begin of thermal pulses (TP) Internal structure Thermal Pulses 1. Quiet phase, H shell provides luminosity, T increase in He shell 2. He shell ignition (shell flash), expansion, H shell off 3. Cooling of He shell, reduction of energy production 4. Convective envelope reaches burning layers, third dredge up 5. Recovery of H-burning shell, quiet phase PDCZ...Pulse driven convection zone Thermal Pulses continuous line...surface luminosity dotted line...He-burning luminosity dashed line...H-burning luminosity Wood & Zarro 1981 Probability for observing an AGB star at a given luminosity during a thermal pulse. Boothroyd & Sackmann 1988 Vassiliadis & Wood 1993 Wood & Zarro 1981 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 Neutron capture Sneden & Cowen 2003 Pagel 1997 Sneden & Cowen 2003 weak component (A<90) main component (A<208) strong component (Pb, Bi) Busso et al. 1999 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) 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: 14N (only in stars with M≥4 Msun) 12C13C Lattanzio & Forestini 1999 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 14000 12000 WZ Cas LFO/OeFOSC October 2003 ADU 10000 8000 6000 4000 Li 2000 0 6000 6500 7000 wavelength [A] 7500 8000 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 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 Summary of 1 Msun evolution Approximate timescales Phase Main-sequence Subgiant Redgiant Branch Red clump AGB evolution PNe WD cooling (yrs) 9 x109 3 x109 1 x109 1 x 108 ~5x106 ~1x105 >8x109 Contributions to the ISM 100 % 10 1 TP-AGB SN RGB WR R,YSG E-AGB MS Sedlmayr 1994 Pulsation mechanisms 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 Reasons for variability (single star) • Pulsation • Star spots, convective cells, asymmetries • Variable dust extinction 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 gP vs dP 4 2 G r dr 3 2 2 2 2 P(r) G (R r ) 3 R dr 3 2 2gG 0 vs const. adiabatic sound speed hydrostatic equilibrium integration with P=0 at the surface Q sun Pulsation constant Typical periods for AGB stars: a few 100 days Pulsation modes Radial modes = standing waves R R 0 0 fundamental mode first overtone R 0 second overtone 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 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 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 Zur Anzeige wird der QuickTime™ Dekompressor „YUV420 codec“ benötigt. Simulation Bernd Freytag Asymmetries Kiss et al. 2000