1. The Adventures of a Thermally Pulsating AGB Star
6th European Summer School on Experimental Nuclear Astrophysics
Ghina Mahmoud
American University of Beirut
The Adventures of a Thermally Pulsating AGB Star

2. Thermally pulsating AGB
HR diagram for a , solar metallicity star.
Thermally pulsating AGB
Code described in

3. Thermal pulsations on the AGB (Periodic Shell Flashes): Why do they happen?
Instability, thermonuclear runaway. Convection takes over
radiation in the He-shell (Pulse-driven convective zone, PDCZ)
Thinness of the He shell
Double shell structure of the AGB star.

4. Game: Thermal Pulsation Players: Helium (Asterix) & Hydrogen (Obelix)
Elles sont fous 
ces étoiles!!
Helium (Asterix): Goes for dangerous or exotic missions (thermal flashes).
 Hydrogen (Obelix):
carries evolution on his shoulders most of the time during pulsation (just does quiescent burning, nothing fancy).

5. Thermal Pulsation: A Smart H-He luminosity Interplay
Color code:
H luminosity
He luminosity
Total luminosity

6. 4 Phases of a full pulse (1 pulse in detail)
Inter-pulse phase (off phase):
quiescent H-burning
provides the star’s luminosity
(5-10 * 104 years).
Pulse (on phase):
He-shell flashes with luminosity peaks,
a convective zone is formed (a few 100 years)
Power down: convection shuts off, H-shell almost extinguished)
Third dredge up: envelope extends inward.

7. Third (of 12C and s-process elements) CARBON 13 POCKET
A. I, Boothroyd, Science Vol 314 nb 5806, 2006

8. Why do I care about TDUP? 1. Responsible for C and s-process
enrichment of the envelope of AGB stars.
(Smith & Lambert 1986; Malaney & Lambert
1988; Busso et al. 1995, 2001; Abia et al. 2008).
2. Carbon enhancement causes a significant
increase in the mass loss rate (superwind).
3. Formation of the so called 13C pocket, which is
the neutron source for the main and the strong
components of the classical s-process (Iben
1981, Herwig et al. 1997, Gallino et al. 1998).

9. TDUP: How does it happen? The Schwarzschild boundary: Stable?
Bottom of convective envelope
Mass (solar units)
N. Mowlavi, Astron. Astrophys. 344, 617–631 (1999)

10. Standard convection recipe
Maximum penetration: H-discontinuity !!
C rich
He rich
H rich
N. Mowlavi, Astron. Astrophys. 344, 617–631 (1999)

11. Extra Mixing (overshooting)
Standard convection recipe
Extra Mixing (overshooting)

12. The Schwarzschild boundary: Stable? NO
Opacity
N. Mowlavi, Astron. Astrophys. 344, 617–631 (1999)

13. Where is TDUP? TDUP 14th pulse 15th pulse Time (108 yrs) Mesh points
Time step
During pulse
2000
80-90 hours
Main sequence
103-4 years
Black regions: convective
White regions: radiative

15. At maximum penetration of the envelope

16. Conclusions
Obtaining the 13C pocket is “not simple” and the situation (TDUP) is complex and not well-known” (quoting M. Busso).
TDUP doesn’t happen “naturally”. It is impossible to obtain in intermediate mass stars with solar metallicity without assuming some sort of extra mixing (TDUP more favored in metal-poor stars).
Extra-mixing in some cases causes convergence problems that need to be taken care of.
The extent of this extra mixing should be fine tuned (a very delicate situation due the instability of the convective boundary of the envelope) (In progress).