THE SPECTRAL EVOLUTION OF POSTASYMPTOTIC GIANT BRANCH STARS

A parameter study of the spectral evolution of a typical post-AGB star , with particular emphasis on the evolution of the IR colours, is pres ented. The models are based on the latest evolutionary tracks for hydr ogen-burning post-AGB stars. For such tracks the evolutionary rate is very dependent on the assumed mass-loss rate as a function of time. We investigate this effect by modifying the mass-loss prescription. The newly calculated evolutionary rates and density distributions are used to model the spectral evolution of a post-AGB star with the photoioni zation code CLOUDY, including dust in the radiative transfer, Differen t assumptions for the dust properties and dust formation are considere d. It is shown that by varying these parameters in a reasonable way, e ntirely different paths are followed in the IRAS colour-colour diagram . First of all, the effects of the evolution of the central star on th e expanding dust shell cannot be neglected. Also the dust properties a nd the definition of the end of the AGB phase have an important effect , The model tracks show that objects occupying the same location in th e IRAS colour-colour diagram can have a different evolutionary past, a nd therefore the position in the IRAS colour-colour diagram alone cann ot a priori give a unique determination of the evolutionary status of an object. An alternative colour-colour diagram, the K-[12] versus [12 ]-[25] diagram, is presented. The tracks in this diagram seem less aff ected by particulars of the grain emission. This diagram may be a valu able additional tool for studying post-AGB evolution.