Evolution from AGB to planetary nebula in the MSX survey ^*,**

¹ Departamento de Física, UFES, Av. Fernando Ferrari 514, 29075-910, Vitória, ES, Brazil e-mail: ortiz@cce.ufes.br
² GEMAC, Depto. de Astronomia, OV/UFRJ, Ladeira Pedro Antonio 43, 20080-090, Rio de Janeiro, RJ, Brazil
³ Departamento de Astronomia, IAG/USP, Rua do Matão, 1226, Cidade Universitária, 05508-900, São Paulo, SP, Brazil

Received: 6 March 2004
Accepted: 27 August 2004

Abstract

We investigate the evolution of oxygen- and carbon-rich AGB stars, post-AGB objects, and planetary nebulae using data collected mainly from the MSX catalogue. Magnitudes and colour indices are compared with those calculated from a grid of synthetic spectra that describe the post-AGB evolution beginning at the onset of the superwind. We find that carbon stars and OH/IR objects form two distinct sequences in the (K-[8.3][8.3]-[14.7]) MSX colour diagram. OH/IR objects are distributed in two groups: the bluest ones are crowded near [14.7]-[ 21.3 and [8.3]-[ 14.7, and a second, redder group is spread over a large area in the diagram, where post-AGB objects and planetary nebulae are also found. High mass-loss rate OH/IR objects, post-AGB stars, and planetary nebulae share the same region in the (K-[8.3][8.3]-[14.7]) and [14.7]-[ 21.3[8.3]-[14.7]) colour-colour diagrams. This region in the diagram is clearly separated from a bluer one where most OH/IR stars are found. We use a grid of models of post-AGB evolution, which are compared with the data. The gap in the colour-colour diagrams is interpreted as the result of the rapid trajectory in the diagram of the stars that have just left the AGB.