Lecture 16

1. Lecture 16 Post-ms evolution

2. Overview: evolution

3. Subgiant branch • An inert, isothermal helium core grows, while H burns in a shell. When the Schönberg-Chandrasekhar limit is reached, the core begins to collapse on the Kelvin-Helmholtz timescale.

4. Subgiant Branch • Collapsing core releases gravitational energy on a short timescale, causing the envelope to expand and cool. • Hydrogen-burning shell narrows, and produces even more energy • This phase lasts about 2 million years

5. Red Giant Branch • Envelope cools, opacity increases • The star reaches the Hayashi track where efficient transport of energy by convection leads to increased luminosity, at constant T. • Lasts about 0.5 million years

6. 5MSun H →He burning First dredge-up He →C,O burning MS SGB RGB Convection First Dredge-up • The energy generated by the shell increases as the core collapses • This energy is partially absorbed by the envelope, which expands and cools. • The increased opacity creates a surface convection zone, which reaches into the inner regions and brings processed material to the surface

7. Helium ignition • Once the central temperature and density have reached a high enough level, the triple-alpha process can occur. • Core expands, pushing the H-burning shell outward and decreasing the total luminosity

8. Helium Core Flash • Lower mass stars have strongly electron-degenerate cores • Energy produced by helium ignition goes into lifting the degeneracy, rather than expanding the core • The release of energy is explosive • Generates 1011 Lsun released in a few seconds • Absorbed by envelope, and may drive mass loss

9. Break

10. Horizontal branch • He → C → O fusion occurs in the core • Hydrogen burning occurs in a shell • Effective temperature increases • He-analogue of the main-sequence phase, but only lasts about 10 million years.

11. H →He burning He →C,O burning Convection Helium burning: the Horizontal branch • The temperature-dependence of the triple-alpha process induces a convective core HB

12. Helium burning: the Horizontal branch • As the temperature increases, the star crosses instability strip • this leads to pulsations which allow a test of the theory. Instability strip

13. RR Lyrae stars in M3

14. 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

15. H →He burning He →C,O burning Convection Helium burning: the Horizontal branch • Core helium is quickly exhausted; inert C-O core forms • Helium-burning shell established (like subgiant branch) • H-burning shell expands, cools and turns off. End of HB Start of HB

16. Early Asymptotic Giant Branch • Helium-burning shell dominates the energy production • H-burning shell is almost inactive

17. H →He burning He →C,O burning Convection Second Dredge-up • 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. Start of AGB

18. Asymptotic giant branch • As the envelope cools it eventually reaches the Hayashi track and bends upward. This is the asymptotic giant branch.