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 Chieffi Istituto Nazionale di AstroFisica (Istituto di Astrofisica Spaziale e Fisica Cosmica) & Centre for Stellar and Planetary Astrophysics – Monash University - Australia - PowerPoint PPT PresentationTRANSCRIPT
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
The basic idea was that of checking the “stability” of a (burning) shell
He
Log(T)
Log()
Log(P) d ln P d ln M
= − G M 2
41
r 4P
P∝ 1
r4
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...
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()
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 !
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 !
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 !
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 !
Which are the typical physical conditions for a stable burning?
He
Log(T)
Log()
3 MO
5 MO
10 MO
t
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)
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
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
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
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:
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.
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...
10 MO - Z = Z
O
