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

• 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