1 / 18

Lecture 16

Lecture 16. Post-ms evolution. Overview: evolution. 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. . Subgiant Branch.

damali
Download Presentation

Lecture 16

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  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.

More Related