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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. - PowerPoint PPT Presentation
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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 dashed line...H-burning luminositydotted line...He-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
Busso et al. 1999
weakcomponent(A<90)
main component(A<208)
strongcomponent(Pb, Bi)
13C pocket13C (α,n) 16OProduction 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: 12C13C 14N(only in stars with M≥4 Msun)
Latt
anzi
o &
For
estin
i 199
9
HBB Li production• Normaly Li destroyed through p capture• Cameron/Fowler mechanism (1971):
3He (,) 7Be mixed to cooler layers 7Be(e-,)7Li
• Explains existence of super Li-rich stars
6000 6500 7000 7500 80000
2000
4000
6000
8000
10000
12000
14000
Li
WZ CasLFO/OeFOSCOctober 2003
AD
U
wavelength [A]
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 evolutionApproximate 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
Contributions to the ISM
1
10
100
%
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
vs P
dP
dr 43G2r
P(r)2
3G2(R2 r2)
2 dr
vs0
R
32G
const.
adiabatic sound speed
hydrostatic equilibrium
integration with P=0at the surface
Q
sunPulsation constant
Typical periods for AGB stars: a few 100 days
Pulsation modes
Radial modes = standing waves
0
R
0
R
0
R
fundamental first overtone second overtonemode
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 recombinationin part. ionization zone
Compression:Energy stored by increasing ionizationin 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