On the Nature of the Thermal Pulses on the Asymptotic Giant Branch Alessandro Chieffi
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On the Nature of the Thermal Pulses on the
Asymptotic Giant Branch
Alessandro ChieffiIstituto Nazionale di AstroFisica (Istituto di Astrofisica Spaziale e Fisica Cosmica)
&
Centre for Stellar and Planetary Astrophysics – Monash University - Australia
Email: [email protected]
Marco LimongiIstituto Nazionale di AstroFisica (Osservatorio Astronomico di Roma)
&
Centre for Stellar and Planetary Astrophysics – Monash University - Australia
Email: [email protected]
Xth Torino Workshop on AGB Nucleosynthesis:from Rutherford to Beatrice Tinsley and beyond
Christchurch, New ZealandJanuary 25-29, 2010
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The basic idea was that of checking the “stability” of a (burning) shell
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He
Log(T)
Log()
Log(P) d ln P d ln M
= − G M 2
41
r 4P
P∝ 1
r4
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Understanding what triggers a Thermal Pulse is important...
Let's take a step back
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!
...but it is not the whole story...
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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 3 cross section is the same in both cases
He
Log(T)
Log()
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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 3 cross section is the same in both cases
He
Log(T)
Log()
He burning (any burning) starts in an inert environment:there is no shell at all !
![Page 7: On the Nature of the Thermal Pulses on the Asymptotic Giant Branch Alessandro Chieffi](https://reader036.fdocuments.us/reader036/viewer/2022062804/56814c2b550346895db93062/html5/thumbnails/7.jpg)
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 3 cross section is the same in both cases
He
Log(T)
Log()
He burning (any burning) starts in an inert environment:there is no shell at all !
![Page 8: On the Nature of the Thermal Pulses on the Asymptotic Giant Branch Alessandro Chieffi](https://reader036.fdocuments.us/reader036/viewer/2022062804/56814c2b550346895db93062/html5/thumbnails/8.jpg)
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 3 cross section is the same in both cases
He
Log(T)
Log()
He burning (any burning) starts in an inert environment:there is no shell at all !
![Page 9: On the Nature of the Thermal Pulses on the Asymptotic Giant Branch Alessandro Chieffi](https://reader036.fdocuments.us/reader036/viewer/2022062804/56814c2b550346895db93062/html5/thumbnails/9.jpg)
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 3 cross section is the same in both cases
He burning (any burning) starts in an inert environment:there is no shell at all !
![Page 10: On the Nature of the Thermal Pulses on the Asymptotic Giant Branch Alessandro Chieffi](https://reader036.fdocuments.us/reader036/viewer/2022062804/56814c2b550346895db93062/html5/thumbnails/10.jpg)
Which are the typical physical conditions for a stable burning?
He
Log(T)
Log()
3 MO
5 MO
10 MO
t
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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)
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The situation changes drastically on the AGB:
Also in this case the T- profile of the region where the He burning will ignite is very different from the one “typical” of a quiescent burning
3 MO
5 MO
10 MO
t
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The situation changes drastically on the AGB:
Also in this case the T- profile of the region where the He burning will ignite is very different from the one “typical” of a quiescent burning
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The situation changes drastically on the AGB:
An enormous amount of energy must be provided by the He burning to turn an accretion shaped T- profile
into a burning controlled T- profile
3 MO
5 MO
10 MO
tAlso in this case the T- profile of the region where the He burning will ignite is very different from the one “typical” of a quiescent burning
(bin
ding e
nergy) =
10 4
8 erg
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Summarizing:
The huge amount of energy produced by a Thermal Pulse is determined by the binding energy difference required to turn from a T- 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?
NOT AT ALL!
Question:
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Red line – standard case
Blue line – test case: the cross section of the 3 substituted by the 14N(p,) cross section shifted to provide the same value at 200 MK.
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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” considerationsbut
we also started a plan to follow a super-AGB starup to the beginning of the electron captures on Ne and Mg...
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10 MO - Z = Z
O
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