AGB Stars and Massive Star...
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AGB Stars and Massive Star Evolution Sunday, May 1, 2011
Transcript of AGB Stars and Massive Star...
AGB Stars and Massive Star Evolution
Sunday, May 1, 2011
Iben
Sunday, May 1, 2011
The Helium Flash (< 2 Msun)
Helium burning
Once He fusion stars, temperature goes up, but core doesn’t expand immediately. This creates a brief burst of energy, much of which goes into inflating the core and star..
The central core becomes dense enough, that degeneracy pressure dominates over thermal pressure. Still, the core radiates energy and continues to contract, until Helium fusion occurs.
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From Bohm Vitense
Degeneracy occurs when there are not enough quantum states at low velocities, pushing electrons to higher momentum (i.e. velocity) states.
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Nuclear Burning: the triple α process
4He + 4He -> 8Be
8Be+4He -> 12Cq = k ρ2 T40
Releasing 7.275 Mev (compared 25 Mev for Hydrogen burning)
12CO + 4 He -> 16OWhen enough carbon accumulates:
Releasing 7.162 MevSunday, May 1, 2011
The core expands and nuclear reaction rates and luminosity may decrease. The star then enters a helium burning main sequence.
The Horizontal Branch
At this point the star is relatively steady for 108 years. However, pulsations may occur. Luminosities of 50-100 Lsun.
Helium burning
Hydrogen burning
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http://www.atlasoftheuniverse.com/hr.htmlhttp://stars.astro.illinois.edu/sow/hrd.html
Eddington Luminosity
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Piotto et al. 2002
More Envelope
Less Envelope
RR Lyrae gap
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Asymptotic Giant Branch
Helium burning
The star may become a supergiant. However, pulsations and dust formation in the envelope may lead to the ejection of the envelope, leaving a white dwarf.
Now the central carbon/oxgen core becomes unstable and starts to contract. There is now Helium and Hydrogen burning in shells.
Hydrogen burning
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MS
Horizontal Branch AGB
Sub-
gian
t t
o G
iant
PrialnikSunday, May 1, 2011
Thin Shell InstabilitiesIn Hydrostatic Equilibrium
PrialnikSunday, May 1, 2011
Imagine layers expands. If pressure from surrounding gas (i.e. hydrostatic equilibrium) drops faster than the pressure in expansion, layer continues to expand and cool. If not, temperature rises and we get thermal instability.
PrialnikSunday, May 1, 2011
Prialnik
Dredge Up
H -> Hein convective
region
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Schwarzchild & Harm (1967)
AGB stars pulsing
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The 9th Cycle
Schwarzchild & Harm (1967)Sunday, May 1, 2011
Carbon Burning12C + 12C -> 24Mg + γ
12C + 12C -> 23 Mg + n
12C + 12C -> 23Na + p
12C + 12C -> 20Ne + α
12C + 12C -> 16Ο + 4α
~13 ΜeV per reaction
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When does degeneracy happen?Prialnik
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Siess et al. 2006
Carbon Burning in Intermediate Mass AGB Stars
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Siess et al. 2006
Carbon Burning in Intermediate Mass AGB Stars
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Prumo & Siess 2007
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http://www.atlasoftheuniverse.com/hr.htmlhttp://stars.astro.illinois.edu/sow/hrd.html
The Supergiant Branch
Relatively Constant Luminosity
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PrialnikSunday, May 1, 2011
Contracting core and Hydrogen shell burning lead to large envelope and red colors.Helium burning in core implies expanded core and shell, smaller envelope and bluer colors.
The Blue Loop Excursions
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The Blue Loop Excursions
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Massive Star Evolution
Maeder et al. 1990 A&AS 84, 139 Sunday, May 1, 2011
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Semi-ConvectionOpacity per mass higher for Hydrogen then Helium
(more electrons per mass)
Imagine core of 40% Helium surrounded by an envelope layer of mostly 10% Helium. High opacity leads to convection, which mixes Helium into envelope, lowering opacity.
Thus, such regions may have slow convection just to keep material mixed.
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SummaryAsymptotic Giant Branch phase occurs after Helium core burning.
Thin shell instability in helium burning leads to pulsations.
These pulsations lead to significant change in luminosity. Helium and Hydrogen shell burning alternate.
Intermediate mass AGB stars may have carbon burning phase.
Massive stars shown mainly evolution in temperature (close to Eddington limit). Mass loss important.
Contains nested shells of nuclear burning, up to Iron
Semi-convection mixes hydrogen and helium in core,
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