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 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

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

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 MSSGB First dredge-up He → C,O burning Convection H → He burning 5M Sun RGB

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

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 L sun released in a few seconds  Absorbed by envelope, and may drive mass loss

Break

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.

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

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

RR Lyrae stars in M3

Helium burning: the Horizontal branch Analogous to point just after leaving the main sequence:  the core contracts, while the envelope expands and cools. Crosses instability strip again Instability strip

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

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. Start of HB End of HB He → C,O burning Convection H → He burning

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

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 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.

Summary