The spectral evolution of post-AGB stars
P.A.M. van Hoof, R.D. Oudmaijer, L.B.F.M. Waters
published in: MNRAS 289, 371 (1997)
A parameter study of the spectral evolution of a typical post-AGB star, with
particular emphasis on the evolution of the infrared colours, is presented. The
models are based on the latest evolutionary tracks for hydrogen 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 photo-ionization code Cloudy, including dust in the radiative transfer.
Different assumptions for the dust properties and dust formation are considered.
It is shown that by varying these parameters in a reasonable way, entirely
different paths are followed in the IRAS colour-colour diagram. First of all,
the effects of the evolution of the central star on the expanding dust shell can
not be neglected. Also the dust properties and the definition of the end of the
AGB phase have an important effect. The model tracks show that objects
occupying the same location in the IRAS colour-colour diagram can have a
different evolutionary past, and therefore the position in the IRAS
colour-colour diagram alone can not a priori give a unique determination of the
evolutionary status of an object. An alternative colour-colour diagram, the
K-[12] vs. [12]-[25] diagram, is presented. The tracks in this diagram seem less
affected by particulars of the grain emission. This diagram may be a valuable
additional tool for studying post-AGB evolution.
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Peter van Hoof
Royal Observatory of Belgium
Ringlaan 3
1180 Brussel
Belgium
email: p DOT vanhoof AT oma DOT be
