AGB stars

All stars with initial masses between 0.5 and 8Mo end their lives on the Asymptotic Giant Branch. (More massive stars end as supernovae.)
→ circumstellar envelopes, dust, planetary nebula PN, white dwarf.

The majority of all stars that have ever left the main sequence have or soon will pass through the AGB.

An AGB star consists of a degenerate C/O core above which hydrogen and helium alternately burn in shells, with the helium burning phase being a thermonuclear runaway (Thermal Pulse TP). This is surrounded by a very extended convective atmosphere enriched by (S-) processed material "dredged up" from the pulse regions and from which mass is lost via a dense and dusty outflow at rates of 10-8 to 10-4Mo/yr and expansion velocities of 5-30 km/s. The mass-loss mechanism in AGB stars (dM/dt>10-5Mo/yr) is poorly understood. It is believed to be related to the slow pulsations and the formation of dust, which is subsequently pushed out by radiation pressure.

Fundamental mode log(P)=-2.07+1.94log(R)-0.9log(M)
99% of the matter leaving the AGB star is molecular, CO, SiO, ...

The final phase involves a superwind that forms an ~0.1 Mo planetary nebula (PN).

Improving our understanding of the physical mechanisms that drive this process is important because mass loss dominates AGB evolution and also because AGB stars play an important role in the chemical evolution of galaxies by returning gas and dust to the ISM.

The most massive AGBs have progenitors ~ 6Mo and are extremely luminous, Mbol~ -7.2 to -7.5

The lack of high luminosity carbon stars above Mbol~-6 may be due to nuclear processing at the bottom of the convective mantle of massive AGB's (hot bottom burning, HBB, initial mass > 3.5Mo) which cycles dredged up carbon to nitrogen leaving the atmospheres oxygen rich. This idea is supported by the fact that almost all luminous AGB's are lithium rich (Cameron & Fowler mechanism if T6>40).

If O>C get H20, SO2, OH, SiO, MgO, TiO, ZrO, ...
If C>O get HCN, C2H2, C2, SiC, TiC, ...

0.5-3Mo He (core) flash (degenerate core, thermal runaway)
3 - 8Mo He core ignition is "smooth"

Post-AGB stars B-K I,II,III not O or M.
R CrB, RV Tau

s-process in zero-metal AGB stars

Nucleosynthesis in AGB Wasserburg et al.

Mass loss in AGB stars

Questions that may be addressed are:

(i) Where does the dust form in the extended atmosphere? If it forms too far away from the photosphere the mass loss rate resulting from radiation pressure on the dust grains is insufficient to explain the observations.
(ii) What is the role of the pulsations in the mass loss process? Is the star distorted due to the large convective motions in the envelope?
(iii) What molecules are depleted in the dust formation region?
(iv) How does dust formation depend on the phase of the pulsation and on the chemical composition of the star?