AGB and POST-AGB STARS: an Overview

Abstract

In the last phase of their existence stars of low and intermediate main-sequence mass, Mmsbelow about 6 M⊘, become very luminous red giants; in the HR-diagram they populate the ``Asymptotic Giant Branch''. ``AGB'' stars are very rare: this phase is very brief and lasts perhaps 105 yr. Later the star becomes a ``post-AGB star'' and this phase is probably even shorter, perhaps a few times 104 yr; during this phase the object moves towards higher effective temperatures at constant luminosity and will become a planetary nebula. AGB stars have several properties that make them special: (1) the luminosity is the highest that a star of a given main-sequence mass will ever reach; (2) the luminosity varies periodically in time with a large amplitude and a long period (Mira variables, OH/IR stars, infrared carbon stars); (3) the stars experience regularly ``thermal pulses'', a short episode with a duration of the order of a few years during which the luminosity is even further increased and during which new elements, produced in the stellar interior, are being dredged up; (4) mass is lost at a high rate, of the order of 1 M⊘ in 106 to 104 yr leading to the formation of a sometimes optically very thick ``envelope''. All these properties make the stars easily detectable especially in the infrared.