AGB and post-AGB stars

Abstract

Intermediate mass stars (1-8 solar masses) evolve along the Asymptotic Giant Branch after completion of hydrogen and helium core burning. At the tip they lose for several ten to hundred thousand years copious amounts of mass and exhibit various forms of variability, as for example large-amplitude variations with periods of one to five years. In the oxygen-rich circumstellar envelopes powerful OH, H2O and SiO masers may operate. Part of the AGB population is converted to carbon stars. During the latest phases of AGB evolution the mass loss rates approach several 10e-5 solar masses/year, so that the stars become invisible in the optical and in part in the near infrared. On departure from the AGB a fundamental transition in the mass loss process is taking place changing from a spherically symmetric outflow on the AGB to axi-symmetric or point-symmetric geometries. While the mass loss rates decrease to ~10e-8 solar masses/year, the velocities are strongly increasing. Stars are now in their "post-AGB" phase. Mounting evidence is gathered that during the very latest phase of AGB evolution and during the post-AGB phase, evolution proceeds on very short timescales, which in extreme cases is comparable to the working life of an astronomer.