Evolu

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Evolução Estelar II

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Early-AGB
Once the He core is exhausted the star has a
burning He shell and a burning H shell around a
CO core As the core is contracting and releasing
gravitational energy the He burning reaction rate
increases and causes the layers above to expand
and cool.
Therefore the star moves upwards and to the right
on the HR diagram in a track close to the RGB
called the Asymptotic Giant Branch.
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Early-AGB H shell dormant
As the layers above the He burning shell expand
and cool, the energy generation rate of the H
burning shell decreases the H burning shell
becomes dormant. Just as it happened earlier
during the first dredge-up, this expansion and
cooling of the outer layers makes the envelope
become convective and the base of the convection
zone moves inwards. When convection reaches into
deeper layers it mixes material from these layers
to the surface, and is called the second
dredge-up. You will be able to investigate how,
as the star ascends the early part of the AGB,
the inner edge of the convective envelope keeps
moving inwards. You will also be able to find
which products are brought up to the surface.
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As we have just seen, at the end of the early-AGB
phase the H-burning and He-burning shells are
thin and close together, which combined with the
high temperature dependence of the He burning
reactions, makes the situation thermally
unstable.
As the star ascends the AGB the burning of H and
He will take turns in a process called thermal
pulsing. However, in very massive stars (those
with M gt 8 M?) the temperature in the core will
be high enough for C to ignite before the thermal
pulses, so they do not go through this phase!
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This is an rough outline of the thermal pulsing
mechanism
1. The H-burning shell is building up a He shell
underneath, until ...
2. The He shell has enough P and T for He fusion
to start, and then ...
3. The sudden massive burst of energy expands the
stars outer shells, and they cool down, so
...
4. H burning ceases and the He is gradually
burnt, so the star shrinks until the H has
enough P and T, and then ...
its back to H shell burning ...
This is just a very basic picture of what is
actually happening, for example the onset of
convection zones plays a very important role in
the thermal pulse mechanism.
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Pulsating some tips
But here are some tips (you will find more
suggestions in the Guided Lesson Plan page)

• Study the evolution of the contribution to the
  total luminosity from the H and He burning
  shells you will see that initially the H
  shell produces most of the energy, but as
  instability develops and a pulse takes place
  the luminosity from the He shell has a
  dramatic increase and during pulses the H shell
  is extinguished. However, the surface
  luminosity remains almost unchanged!

• Study the position (in radius) of the shells
  with time you should see the expansion of the
  H shell after a pulse, this means that the H
  shell is pushed to cooler regions which will
  turn off H burning for a while.

• Study the position in mass fraction of the H and
  He shells.

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The thermal instability grows in strength during
the AGB evolution and the He shell luminosity
becomes larger after each successive pulse.
This graphic (for a 5M? star) shows how the
luminosity generated by the He shell grows after
each pulse, reaching up to 108 L?.
The time between pulses depends on the mass of
the star, with more massive stars having shorter
pulsating periods. For example a 0.6 M? star has
a pulse period of hundred of thousands of years,
while a 5 M? star has a period of a few thousand
years.
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A CO core is building up during the active He
burning stages. You will be able to study the
evolution of the He, C and O profiles during the
early pulses and see how carbon and oxygen are
produced during each pulse. The abundance profile
of C and O will show you the development of the
CO core.
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Third Dredge-up
As the amplitude of the pulse grows, the
expansion gets stronger and the envelope
convection zone gets deeper. Eventually the
convective envelope reaches the H shell. But
also, during each pulse (as more energy is
released than can be carried by radiation) an
intershell convection zone is established between
the H and He shells. The combination of these two
effects produces the third dredge up the
intershell convection zone brings 12C up to the H
shell and then the envelope convective zone
brings it to the surface of the star.
You will be able to study this by looking at the
evolution of the He, C and O profiles during two
consecutive pulses (youll see the C abundance
increasing in the surface, and an intershell
convection region developing (remember that
convection regions are horizontal lines in
abundance profile graphics).
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Many of the nuclei found in the Universe are
produced in stars on the AGB stage. We have
briefly discussed the production of 12C and how
it reaches the envelope of the star, but there is
much more nucleosynthesis taking place in AGB
stars! Thermal pulses are the main sites of
nucleosynthesis but heavy nuclei are also
generated in a phenomenon called Hot Bottom
Burning (HBB), which occurs in more massive AGB
stars. Lets first briefly discuss
nucleosynthesis during thermal pulses
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The formation of heavy nuclei during thermal
pulses takes place through s-process reactions,
which are a type of neutron-capture
If this reaction is slow (i.e. longer half-life)
compared to beta-decay reactions, then the
neutron-capture is called a slow process or
s-process. So how does this happen in the AGB
star? Where are the neutrons involved in the
s-process reaction coming from?
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The first step involves some of the H from the
envelope mixing downwards into the intershell
region.
This can happen after the dredge-up, when the
envelope convective zone and the intershell
convective zone merge
The intershell region is rich in He and 12C, so
when H gets to this region it can combine with
12C to produce 13C.
This 13C will be the source for the neutrons that
are needed in the s-process reactions.
The neutrons are produced when 13C interacts with
the abundant He4 nuclei in the intershell region
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Now the s-process can take place the neutrons
are captured by Fe and other heavy elements to
produce more heavier nuclei
For example 56Fe - 57Fe - 58Fe - 59Fe,
followed by 59Fe - 59Co (beta decay). This
process produces heavy elements up to 208Pb and
209Bi.
The newly processed elements will be brought to
the surface of the star at the next pulse.
AGB stars are also thought to be the main source
of 19F. Ne, Al and Mg are also produced in the
AGB pulsating phase.
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This is a phenomenon that occurs in stars of
masses greater than 4 or 5 M?, where the inner
edge of the convective envelope penetrates the
top of the H shell.
The temperature at the bottom of the convective
envelope can be very high (108 K), and this can
initiate H burning and nucleosynthesis in the
convective envelope itself!
7Li is produced in AGB stars through HBB and the
so called Cameron-Fowler Beryllium Transport
Mechanism, which we dont get into detail here as
the Li abundance it is not included as a variable
for you to study in the stellar evolution Module.
However, the effect of HBB is included in the
models and you can see its effect in the N and C
abundances, as we explain now
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HBB CNO Cycle
Hot Bottom Burning will also allow H burning
through the CNO cycle, which will convert 12C
into 13C and 14N. Remember that with each pulse
more 12C is added to the envelope, which would
produce a carbon star. However if HBB is taking
place the 12C transported to the convective
envelope will be turned into 14N and the star
wont be a carbon star.
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This graphic shows how the surface abundance of
12C decreases after HBB starts to operate, and of
course at the same time the 14N and 13C surface
abundances increase.
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Enriching the ISM

• We have explained here the principles of the
  nucleosynthesis processes taking place in AGB
  stars.
• The variety of nuclei produced during the AGB
  phase will be released to the interstellar medium
  in the subsequent evolution of the star, through
  stellar winds and the expanding gas shell of
  planetary nebulae.
• The evolution after the AGB phase will be
  dramatically different for stars of different
  masses. We have seen that a carbon and oxygen
  core has been building up during the AGB phase,
  what happens next will depend on wether or not
  this core is massive enough to ignite nuclear
  burning.

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After the AGB
Since the stellar evolution data available for
your project includes the post AGB evolution for
the stellar model corresponding to the Sun, we
will briefly review how low to intermediate-mass
stars evolve after the AGB.
We have seen that stars become unstable as they
climb the AGB and thermal pulsation takes place,
while a carbon-oxygen core is developing. We will
now consider the subsequent evolution of stars
with zero-age-main-sequence (ZAMS) masses lt 4 M?
, for which the carbon-oxygen core is not massive
enough to ignite nuclear burning. The final
product of these stars will be a white dwarf,
lets see how the star evolves to that stage.
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Loosing Mass
During the AGB phase the star suffers substantial
mass loss due to thermal pulses and stellar winds.
The stars envelope has expanded to about 70
times larger than it was during most of the
stars lifetime and the surface material becomes
less tightly bound, which contributes to an
increase in the mass loss rate.
The resulting spectacular Planetary Nebulae (PNe)
is due to the interaction of the newly ejected
shell of gas with the ultraviolet light from the
central, hot stellar remnant which energises the
gas and causes it to fluoresce.
fluorescence emission of electromagnetic
radiation, stimulated in a substance by the
absorption of incident radiation and persisting
only as long as the stimulating radiation is
continued.
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Final da evolução estelar
Pequena massa (? Msol)
Grande massa (gt8 Msol)
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Final da evolução estelar
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White Dwarfs
White dwarf stars are much smaller than normal
stars, such that a white dwarf of the mass of the
Sun is only slightly larger than the Earth.
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Morte estelar (M lt 1 M0)

• Por que, após o flash do hélio, a pressão de
  degenerescência dos elétrons é removida?
• Fusão de hélio em carbono.
• Resfriamento e expansão do envelope.
• Núcleos e elétrons se recombinam.
• O envelope é ejetado nebulosa
  planetária!

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Nebulosa planetária...
Núcleo de Anã Branca
Sirius A e B
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Nebulosas (arquivos do Hubble)
Stingray
Cats Eye
Eskimo
Helix
Egg
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White Dwarf Evolution
Once a white dwarfs contracts to its final size,
it no longer has any nuclear fuel available to
burn. The white dwarf is still very hot and
cools by radiating its energy outward. The white
dwarf will eventually give up all its energy and
become a solid, crystal black dwarf.
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Supernova 1994D
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Supernova 1987A
Supernova 1987AOs dois anéis ainda não são
completamente compreendidos, embora pareçam estar
associados à supernova. Os anéis são resultado
de algum processo ocorrido antes da formação da
SN1987A, provavelmente associado aos fortes
ventos estelares esperados nestes objetos.
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Possibilidades para estrelas de grande massa
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Finais para resíduos de várias massas
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Summary
In this presentation we have reviewed how stars
of different masses live their lives.
We have seen how stars burn H into He in the Main
Sequence until H is exhausted in their cores. At
this stage, we have studied how a H burning shell
develops as stars ascends the RGB. The first
dredge up process, which takes place during the
RGB, has been also reviewed. We have discussed
why the cores of stars with masses below 2.2 M?
become degenerate, which leads to the He core
flash. We have studied how these stars move along
the Horizontal Branch as they burn He in their
cores. We also discussed why stars of masses
above 2.2 M? ignite the He in their cores
smoothly, and how He core burning continues as
the star moves along the Blue Loop.
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Créditos
IC 418 Nebulosa Espirográfica, cortesia de The
Hubble Heritage Team (STScI/AURA) R. Sahai (JPL)
e A. Hajian (USNO) NASA/HSThttp//hubble.stsci.ed
u/news_.and._views/data/2000/28/image.jpg Images
of M16, SN 1987A and Betelgeuse. Credit STScI
and NASA http//oposite.stsci.edu/pubinfo/pictures
.html Figuras e animações Swinburne Online
Education Computational Astrophysics
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