Transcript 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
He-flash and core He-burning
Early AGB
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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)
12C13C
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
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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
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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
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Simulation Bernd Freytag
Asymmetries
Kiss et al. 2000