lecture 17 post-ms evolution ii. review review review
TRANSCRIPT
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Lecture 17Lecture 17
Post-ms evolution II
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ReviewReview
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ReviewReview
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ReviewReview
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Second dredge-up: He-shell burningSecond dredge-up: He-shell burning
•A Helium-burning shell ignites around a C,O core. Similar to the H-shell burning phase
•Again, the envelope expands and cools, becoming convective and causing a second dredge-up.
Instability strip
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Review: Horizontal branchReview: Horizontal branch
•H-burning shell is compressed, increasing the luminosity it produces
•He-shell burning: CO core collapses, while envelope expands
Start of HBEnd of HB
He →C,O burning
Convection
H →He burning
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Asymptotic giant branchAsymptotic giant branch
As the envelope cools it eventually reaches the Hayashi track and bends upward. This is the asymptotic giant branch.
He-burning dominates the luminosity
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Thermal pulsesThermal pulses
•He ash is dumped on the slightly degenerate He-burning shell, causing shell flashes
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Further nucleosynthesisFurther nucleosynthesis
For stars with 4<M/MSun<8, nuclear reactions can continue:
pNa
nMg
HeNe
HeO
CC
2311
2312
42
2010
42
168
126
126
2
nS
pP
HeSi
HeMg
OO
3116
3115
42
2814
42
2412
168
168
2
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AGB starsAGB stars
•High mass-loss rates, and cool effective temperatures (~3000 K)
•A dust shell hides most of the stellar luminosity and so the stars are seen only in the infrared.
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Mass lossMass loss
•Mass loss driven by the high luminosity and thermal pulses.
As mass decreases, and luminosity increases, the mass loss rate increases.
High-resolution radio image of mass-loss from an AGB star, TX Cam
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BreakBreak
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Post-AGB phasePost-AGB phase
• The cloud expands and becomes optically thin• Exposes the hotter interior
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Planetary nebulae: Fate of low mass Planetary nebulae: Fate of low mass starsstars
• The hot core lights up the expanding envelope, for about 20,000 years.
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Planetary NebulaePlanetary Nebulae
•Bluish-green colour due to [OIII] forbidden lines•Reddish colour from ionized hydrogen and nitrogen
Cat’s eye nebula
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The Helix NebulaThe Helix Nebula
•Looking along the rotation axis Gas is being ejected in “rings” preferentially along the
equator
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Outflow velocitiesOutflow velocities
•Typically shell expands at 10-30 km/s•Some are much faster: the Ant nebula has an outflow velocity of about 1000 km/s
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Fate of planetary nebulaeFate of planetary nebulae
•Release the envelope into the ISM, on a timescale of ~10000 years.
•There are probably about 15,000 in the Milky Way today ISM is being enriched at a rate of about 1 Msun/year.
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Planetary nebulae and white dwarfsPlanetary nebulae and white dwarfs
• When the helium and hydrogen shells are extinguished, the luminosity drops abruptly
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White dwarfsWhite dwarfs
•When the envelope has dispersed, only the hot, dense, small core is left