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On the Nature of the Thermal Pulses on the Asymptotic Giant Branch Alessandro Chieffi

On the Nature of the Thermal Pulses on the Asymptotic Giant Branch Alessandro Chieffi Istituto Nazionale di AstroFisica (Istituto di Astrofisica Spaziale e Fisica Cosmica) & Centre for Stellar and Planetary Astrophysics – Monash University - Australia

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On the Nature of the Thermal Pulses on the Asymptotic Giant Branch Alessandro Chieffi

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  1. On the Nature of the Thermal Pulses on the Asymptotic Giant Branch Alessandro Chieffi Istituto Nazionale di AstroFisica (Istituto di Astrofisica Spaziale e Fisica Cosmica) & Centre for Stellar and Planetary Astrophysics – Monash University - Australia Email: alessandro.chieffi@iasf-roma.inaf.it Marco Limongi Istituto Nazionale di AstroFisica (Osservatorio Astronomico di Roma) & Centre for Stellar and Planetary Astrophysics – Monash University - Australia Email: marco@oa-roma.inaf.it Xth Torino Workshop on AGB Nucleosynthesis: from Rutherford to Beatrice Tinsley and beyond Christchurch, New Zealand January 25-29, 2010

  2. The basic idea was that of checking the “stability” of a (burning) shell

  3. Log(T) Log(P) Log(r) He

  4. Understanding what triggers a Thermal Pulse is important... ...but it is not the whole story... It is also important to understand why so much energy is produced by a TP. It's this energy the ultimate responsible for: the freezing of the H-burning shell the occurrence of the 3rd dredge-up. In principle a TP could generate a “minor” fraction of energy! Let's take a step back

  5. He burning (any burning) starts in an inert environment: there is no shell at all ! Why the birth of a burning shell is unstable in the AGB while it is stable in (most of) the other cases ? the steepness of the 3a cross section is the same in both cases He Log(T) Log(r)

  6. He burning (any burning) starts in an inert environment: there is no shell at all ! Why the birth of a burning shell is unstable in the AGB while it is stable in (most of) the other cases ? the steepness of the 3a cross section is the same in both cases He Log(T) Log(r)

  7. He burning (any burning) starts in an inert environment: there is no shell at all ! Why the birth of a burning shell is unstable in the AGB while it is stable in (most of) the other cases ? the steepness of the 3a cross section is the same in both cases He Log(T) Log(r)

  8. He burning (any burning) starts in an inert environment: there is no shell at all ! Why the birth of a burning shell is unstable in the AGB while it is stable in (most of) the other cases ? the steepness of the 3a cross section is the same in both cases He Log(T) Log(r)

  9. He burning (any burning) starts in an inert environment: there is no shell at all ! Why the birth of a burning shell is unstable in the AGB while it is stable in (most of) the other cases ? the steepness of the 3a cross section is the same in both cases

  10. He 5 MO t Log(T) Log(r) 10 MO 3 MO Which are the typical physical conditions for a stable burning?

  11. Question: how much energy must be provided by the burning shell to “shape” the incoming matter in the proper “stable burning” configuration? Answer:none (because each layer increases its binding energy while it contracts, flows, towards the burning region) (in other words the work is done by the gravitational field)

  12. 5 MO t 10 MO 3 MO The situation changes drastically on the AGB: Also in this case the T-r profile of the region where the He burning will ignite is very different from the one “typical” of a quiescent burning

  13. The situation changes drastically on the AGB: Also in this case the T-r profile of the region where the He burning will ignite is very different from the one “typical” of a quiescent burning

  14. 5 MO t 10 MO 3 MO The situation changes drastically on the AGB: Also in this case the T-r profile of the region where the He burning will ignite is very different from the one “typical” of a quiescent burning d(binding energy) = 10 4 8 erg An enormous amount of energy must be provided by the He burning to turn an accretion shaped T-r profile into a burning controlled T-r profile

  15. Summarizing: The huge amount of energy produced by a Thermal Pulse is determined by the binding energy difference required to turn from a T-r profile modeled in the intershell by the quiescent H burning and the one required by the quiescent He burning. Is this huge energy demand a necessary and sufficient condition to explain the existence of the Thermal Pulses? Question: NOT AT ALL!

  16. Red line – standard case Blue line – test case: the cross section of the 3a substituted by the 14N(p,g) cross section shifted to provide the same value at 200 MK.

  17. Conclusions: The huge energy demand to turn the physical structure in a typical He burning configuration is a necessary condition otherwise there would not be enough energy to expand and freeze the H burning Also a rapid energy injection, granted by the thermal instability, is a necessary condition otherwise the transition occurs “more gently” and the H burning region does not switch off We do not spend our spare time only in these “philosophical” considerations but we also started a plan to follow a super-AGB star up to the beginning of the electron captures on Ne and Mg...

  18. 10 MO - Z = ZO

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