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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. Instability strip Review: Horizontal branch • H-burning shell is compressed, increasing the luminosity it produces • He-shell burning: CO core collapses, while envelope expands Start of HB End of HB H →He burning He →C,O burning Convection 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 Thermal pulses • He ash is dumped on the slightly degenerate He-burning shell, causing shell flashes Further nucleosynthesis For stars with 4<M/MSun<8, nuclear reactions can continue: O 2 He Ne He 12 12 6C 6C Mg n Na p 16 8 20 10 23 12 23 11 4 2 4 2 24 4 12 Mg 22 He 28 4 Si 14 16 16 2 He 8 O 8 O 31 15 P p 1631S n 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. 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 Break Post-AGB phase • The cloud expands and becomes optically thin • Exposes the hotter interior Planetary nebulae: Fate of low mass stars • The hot core lights up the expanding envelope, for about 20,000 years. Planetary Nebulae • Bluish-green colour due to [OIII] forbidden lines • Reddish colour from ionized hydrogen and nitrogen Cat’s eye nebula The Helix Nebula • Looking along the rotation axis Gas is being ejected in “rings” preferentially along the equator 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 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. Planetary nebulae and white dwarfs • When the helium and hydrogen shells are extinguished, the luminosity drops abruptly White dwarfs • When the envelope has dispersed, only the hot, dense, small core is left