Asymptotic giant branch
This is a period of stellar evolution undertaken by all low- to intermediate-mass stars (about 0.5 to 8 solar masses[citation needed]) late in their lives.
Observationally, an asymptotic-giant-branch star will appear as a bright red giant with a luminosity ranging up to thousands of times greater than the Sun.
[3] After the completion of helium burning in the core, the star again moves to the right and upwards on the diagram, cooling and expanding as its luminosity increases.
During the E-AGB phase, the main source of energy is helium fusion in a shell around a core consisting mostly of carbon and oxygen.
The power of the shell flash peaks at thousands of times the observed luminosity of the star, but decreases exponentially over just a few years.
[4] During the thermal pulses, which last only a few hundred years, material from the core region may be mixed into the outer layers, changing the surface composition, in a process referred to as dredge-up.
Thermal pulses increase rapidly in strength after the first few, so third dredge-ups are generally the deepest and most likely to circulate core material to the surface.
[5][6] AGB stars are typically long-period variables, and suffer mass loss in the form of a stellar wind.
[7] Thermal pulses produce periods of even higher mass loss and may result in detached shells of circumstellar material.
[10] The extensive mass loss of AGB stars means that they are surrounded by an extended circumstellar envelope (CSE).
Given a mean AGB lifetime of one Myr and an outer velocity of 10 km/s, its maximum radius can be estimated to be roughly 3×1014 km (30 light years).
[11][12] The temperature of the CSE is determined by heating and cooling properties of the gas and dust, but drops with radial distance from the photosphere of the stars which are 2,000–3,000 K. Chemical peculiarities of an AGB CSE outwards include:[13] The dichotomy between oxygen-rich and carbon-rich stars has an initial role in determining whether the first condensates are oxides or carbides, since the least abundant of these two elements will likely remain in the gas phase as COx.
A majority of presolar silicon carbide grains have their origin in 1–3 M☉carbon stars in the late thermally-pulsing AGB phase of their stellar evolution.
If the helium is re-ignited a thermal pulse occurs and the star quickly returns to the AGB, becoming a helium-burning, hydrogen-deficient stellar object.
Mapping the circumstellar magnetic fields of thermal-pulsating (TP-) AGB stars has recently been reported[26] using the so-called Goldreich-Kylafis effect.
The second dredge-up is very strong in this mass range and that keeps the core size below the level required for burning of neon as occurs in higher-mass supergiants.
