Asymptotic Giant Branch stars

Asymptotic giant branch

The asymptotic giant branch is the region of the Hertzsprung-Russell diagram populated by evolving low to medium-mass stars. This is a period of stellar evolution undertaken by all low to intermediate mass stars (0.6-10 solar masses) late in their life.

Observationally, an asymptotic giant branch (AGB) star will appear as either a red giant or a red supergiant. Its interior structure is characterized by a central and inert core of carbon and oxygen, a shell where helium is undergoing fusion to form carbon (known as helium burning), another shell where hydrogen is undergoing fusion forming helium (known as hydrogen burning) and a very large envelope of material of composition similar to normal stars.

Stellar evolution

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's luminosity increases greatly, and it becomes a red giant, following a track leading into the upper-right hand corner of the HR diagram.

Eventually, once the temperature in the core has reached approximately 3x108K, helium burning begins. The onset of helium burning in the core halts the star's cooling and increase in luminosity, and the star instead moves back towards the left hand side of the HR diagram. This is the horizontal branch (for population II stars) or red clump (for population I stars). After the completion of helium burning in the core, the star again moves to the right and upwards on the diagram. Its path is almost aligned with its previous red giant track, hence the name asymptotic giant branch. Stars at this stage of stellar evolution are known as AGB stars.

The 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 may become as large as one astronomical unit. After the helium shell runs out of fuel, the TP-AGB starts. Now the star derives its energy from fusion of hydrogen in a thin shell, inside of which lies the now inactive helium shell. However, over periods of 10,000 to 100,000 years, the helium shell switches on again, and the hydrogen shell switches off, a process known as a helium shell flash. Due to these flashes, which only last a few thousand years, material from the core region is mixed into the outer layers, changing its composition, a process referred to as dredge-up. Because of this dredge-up AGB stars may show S-process elements in their spectra. Subsequent dredge-ups can lead to the formation of Carbon stars.

AGB stars are typically long period variables, and suffer large mass loss in the form of a stellar wind. A star may lose 50 to 70% of its mass during the AGB phase.

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 10 km/s, its maximum radius can be estimated as 1019 cm. 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. Dynamically most of the interesting action is quite close to the star, where the wind is launched and the mass loss rate is determined. However, the outer layers of the CSE show chemically interesting processes, and are due to size and lower optical depth easier to observe.

The temperature of the CSE is set by heating and cooling processes for the gas and the dust, but is dropping with radial distance from the photosphere of the star's of some 2200 K. A chemical picture of an AGB CSE outwards was suggested by Marwick (2000) something like this:

  1. Photosphere: Local thermodynamic equilibrium chemistry;
  2. Pulsating stellar envelope: Shock chemistry;
  3. Dust formation zone;
  4. Chemically quiet;
  5. Interstellar UV radiation: Photodissociation of molecules - complex chemistry

Here the dichotomy between oxygen-rich and carbon-rich stars will have an initial say.In the dust formation zone the so-called refractory elements (Fe, Si, Mg, ...) are removed from the gas phase and end up in dust grains. The newly formed dust will immediately assist in surficide 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 are also often the site of maser emission. The masering molecules are SiO, H2O, and OH.

After these stars have lost nearly all of their envelopes, and only the core regions remain, they evolve further into short lived preplanetary nebulae. The final fate of the AGB envelopes are represented by planetary nebulae (PNe).

References

  • H. J. Habing, Hans Olofsson; Asymptotic Giant Branch Stars, Springer (2004). ISBN 0387008802.

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