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Post-Main Sequence Evolution: Helium Shell Burning and Planetary Nebula Formation

This lecture reviews the post-main sequence evolution of stars, focusing on the helium-burning shell around carbon-oxygen cores. It covers phases like horizontal branch stability, asymptotic giant branch expansion, thermal pulses, and the eventual formation of planetary nebulae. The discussion includes the impact of mass loss and the resulting transformation of stars into white dwarfs. Observational characteristics, such as the bluish-green color from [OIII] lines and morphological features of nebulae, are highlighted, emphasizing their role in enriching the interstellar medium.

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Post-Main Sequence Evolution: Helium Shell Burning and Planetary Nebula Formation

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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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