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

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

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.

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).

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

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.

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

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).

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

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

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

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

Where is TDUP? TDUP 14th pulse 15th pulse Time (108 yrs) Mesh points Time step During pulse 2000 80-90 hours Main sequence 800-900 103-4 years Black regions: convective White regions: radiative

At maximum penetration of the envelope

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).