when helium is exhausted in its core When a star exhausts the supply of
hydrogen by
nuclear fusion processes in its core, the core contracts and its temperature increases, causing the outer layers of the star to expand and cool. The star becomes a red giant, following a track towards the upper-right hand corner of the HR diagram. Eventually, once the
temperature in the core has reached approximately ,
helium burning (fusion of
helium nuclei) begins. The onset of helium burning in the core halts the star's cooling and increase in luminosity, and the star instead moves down and leftwards in the HR diagram. This is the
horizontal branch (for
population II stars) or a
blue loop for stars more massive than about .
AGB stage The AGB phase is divided into two parts, the early AGB (E-AGB) and the thermally pulsing AGB (TP-AGB). 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. During this phase, the star swells up to giant proportions to become a red giant again. The star's radius may become as large as one
astronomical unit (). 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. Because of this dredge-up, AGB stars may show
S-process elements in their spectra and strong dredge-ups can lead to the formation of
carbon stars. All dredge-ups following thermal pulses are referred to as third dredge-ups, after the first dredge-up, which occurs on the red-giant branch, and the second dredge up, which occurs during the E-AGB. In some cases there may not be a second dredge-up but dredge-ups following thermal pulses will still be called a third 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. AGB stars are typically
long-period variables, and suffer
mass loss in the form of a
stellar wind. For M-type AGB stars, the stellar winds are most efficiently driven by micron-sized grains. Thermal pulses produce periods of even higher mass loss and may result in detached shells of circumstellar material. A star may lose 50 to 70% of its mass during the AGB phase. The mass-loss rates typically range between and /year, and can even reach as high as /year; while wind velocities are typically between 5 and 30 km/s.
Circumstellar envelopes of AGB stars 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 , its maximum radius can be estimated to be roughly (30
light years). This is a maximum value since the wind material will start to mix with the
interstellar medium at very large radii, and it also assumes that there is no velocity difference between the star and the
interstellar gas. These envelopes have a dynamic and interesting
chemistry, much of which is difficult to reproduce in a laboratory environment because of the low densities involved. The nature of the chemical reactions in the envelope changes as the material moves away from the star, expands and cools. Near the star the envelope density is high enough that reactions approach thermodynamic equilibrium. As the material passes beyond about the density falls to the point where
kinetics, rather than thermodynamics, becomes the dominant feature. Some energetically favorable reactions can no longer take place in the gas, because the
reaction mechanism requires a third body to remove the energy released when a chemical bond is formed. In this region many of the reactions that do take place involve
radicals such as
OH (in oxygen rich envelopes) or
CN (in the envelopes surrounding carbon stars). In the outermost region of the envelope, beyond about , the density drops to the point where the dust no longer completely shields the envelope from interstellar
UV radiation and the gas becomes partially ionized. These ions then participate in reactions with neutral atoms and molecules. Finally as the envelope merges with the interstellar medium, most of the molecules are destroyed by UV radiation. 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 –. Chemical peculiarities of an AGB CSE outwards include: • Photosphere:
Local thermodynamic equilibrium chemistry • Pulsating stellar envelope: Shock chemistry • Dust formation zone • Chemically quiet • Interstellar ultraviolet radiation and
photodissociation of
molecules – complex chemistry 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. In the dust formation zone,
refractory elements and compounds (
Fe,
Si,
MgO, etc.) are removed from the gas phase and end up in
dust grains. The newly formed dust will immediately assist in
surface catalyzed reactions. The stellar winds from AGB stars are sites of
cosmic dust formation, and are believed to be the main production sites of dust in the universe. The stellar winds of AGB stars (
Mira variables and
OH/IR stars) are also often the site of
maser emission. The molecules that account for this are
SiO,
H2O,
OH,
HCN, and
SiS. SiO, H2O, and OH masers are typically found in oxygen-rich M-type AGB stars such as
R Cassiopeiae and
U Orionis, while HCN and SiS masers are generally found in carbon stars such as
IRC +10216.
S-type stars with masers are uncommon.
Physical samples Physical samples, known as presolar grains, of mineral grains from AGB stars are available for laboratory analysis in the form of individual refractory
presolar grains. These formed in the circumstellar dust envelopes and were transported to the early
Solar System by
stellar wind. A majority of presolar
silicon carbide grains have their origin in carbon stars in the late thermally-pulsing AGB phase of their stellar evolution.
Late thermal pulse As many as a quarter of all post-AGB stars undergo what is dubbed a "born-again" episode. The carbon–oxygen core is now surrounded by helium with an outer shell of hydrogen. 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. The outer atmosphere of the born-again star develops a stellar wind and the star once more follows an
evolutionary track across the
Hertzsprung–Russell diagram. However, this phase is very brief, lasting only about 200 years before the star again heads toward the
white dwarf stage. Observationally, this late thermal pulse phase appears almost identical to a
Wolf–Rayet star in the midst of its own
planetary nebula. Stars such as
Sakurai's Object and
FG Sagittae are being observed as they rapidly evolve through this phase. Mapping the circumstellar magnetic fields of thermal-pulsating (TP-) AGB stars has recently been reported using the so-called
Goldreich-Kylafis effect.
Super-AGB stars Stars close to the upper mass limit to still qualify as AGB stars show some peculiar properties and have been dubbed super-AGB stars. They have masses above and up to 9 or (or more). They represent a transition to the more massive supergiant stars that undergo full fusion of elements heavier than helium. During the
triple-alpha process, some elements heavier than carbon are also produced: mostly oxygen, but also some magnesium, neon, and even heavier elements. Super-AGB stars develop partially degenerate carbon–oxygen cores that are large enough to ignite carbon in a flash analogous to the earlier helium flash. 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. The size of the thermal pulses and third dredge-ups are reduced compared to lower-mass stars, while the frequency of the thermal pulses increases dramatically. Some super-AGB stars may explode as an electron capture supernova, but most will end as oxygen–neon white dwarfs. Since these stars are much more common than higher-mass supergiants, they could form a high proportion of observed supernovae. Detecting examples of these supernovae would provide valuable confirmation of models that are highly dependent on assumptions. ==See also==