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, T eff decreases M>4.5 M sun : 2nd dredge up phase increase of 14 N, decrease of 16 O 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 luminositydashed line...H-burning luminosity dotted line...He-burning luminosityWood & 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 M sun

Neutron capture Sneden & Cowen 2003

Pagel 1997

Sneden & Cowen 2003

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

13 C pocket 13 C (α,n) 16 O Production of 13 C from 12 C (p capture) The solid and dashed lines are from theoretical models calculated for a 1.5 solar mass star with varying mass of the 13 C 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 13 C 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: 12 C  13 C  14 N (only in stars with M≥4 M sun )

Lattanzio & Forestini 1999

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

Indicators for 3 rd dredge up existence & frequency of C-stars C/O, 12 C/ 13 C 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: R sun T eff : K L: up to M bol = -7.5 Mass loss rates: to M sun /yr Variability period: days

Summary of 1 M sun evolution Approximate timescales Phase  (yrs) Main-sequence 9 x10 9 Subgiant 3 x10 9 Redgiant Branch 1 x10 9 Red clump 1 x 10 8 AGB evolution ~5x10 6 PNe ~1x10 5 WD cooling >8x10 9

Contributions to the ISM 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

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

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

Pulsation modes Radial modes = standing waves 0 R 0 R 0 R fundamental first overtonesecond overtone mode

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

Simulation Bernd Freytag

Asymmetries Kiss et al. 2000