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Lecture 17

Lecture 17. Post-ms evolution II. Review. Review. Review. Second 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.

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Lecture 17

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  1. Lecture 17 Post-ms evolution II

  2. Review

  3. Review

  4. Review

  5. Second 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

  6. H →He burning He →C,O burning Convection Review: Horizontal branch • H-burning shell is compressed, increasing the luminosity it produces • He-shell burning: CO core collapses, while envelope expands End of HB Start of HB

  7. Asymptotic 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

  8. Thermal pulses • He ash is dumped on the slightly degenerate He-burning shell, causing shell flashes

  9. Further nucleosynthesis • For stars with 4<M/MSun<8, nuclear reactions can continue:

  10. AGB 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.

  11. Mass 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

  12. Break

  13. Post-AGB phase • The cloud expands and becomes optically thin • Exposes the hotter interior

  14. Planetary nebulae: Fate of low mass stars • The hot core lights up the expanding envelope, for about 20,000 years.

  15. Planetary Nebulae • Bluish-green colour due to [OIII] forbidden lines • Reddish colour from ionized hydrogen and nitrogen • Cat’s eye nebula

  16. The Helix Nebula • Looking along the rotation axis • Gas is being ejected in “rings” preferentially along the equator

  17. Outflow 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

  18. Fate 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.

  19. Planetary nebulae and white dwarfs • When the helium and hydrogen shells are extinguished, the luminosity drops abruptly

  20. White dwarfs • When the envelope has dispersed, only the hot, dense, small core is left

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