Lecture 17Lecture 17

Post-ms evolution II

ReviewReview

ReviewReview

ReviewReview

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

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

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

Thermal pulsesThermal pulses

•He ash is dumped on the slightly degenerate He-burning shell, causing shell flashes

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

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.

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

BreakBreak

Post-AGB phasePost-AGB phase

• The cloud expands and becomes optically thin• Exposes the hotter interior

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.

Planetary NebulaePlanetary Nebulae

•Bluish-green colour due to [OIII] forbidden lines•Reddish colour from ionized hydrogen and nitrogen

Cat’s eye nebula

The Helix NebulaThe Helix Nebula

•Looking along the rotation axis Gas is being ejected in “rings” preferentially along the

equator

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

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.

Planetary nebulae and white dwarfsPlanetary nebulae and white dwarfs

• When the helium and hydrogen shells are extinguished, the luminosity drops abruptly

White dwarfsWhite dwarfs

•When the envelope has dispersed, only the hot, dense, small core is left