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 He → C,O burning Convection H → He burning

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/M Sun <8, nuclear reactions can continue:

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 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 M sun /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