Astron. Astrophys. 348, 501-511 (1999)

Mid-infrared colors as a diagnostic tool of circumstellar envelopes in AGB stars ^*

M. Marengo ^1,2, M. Busso ³, G. Silvestro ⁴, P. Persi ⁵ and P.O. Lagage <sup>6

1</sup> SISSA - Int. School for Advanced Studies, Trieste, Italy
² Harvard-Smithsonian Center for Astrophysics, Cambridge, Massachussets, USA
³ Osservatorio Astronomico di Torino, Pino Torinese, Italy
⁴ Dipartimento di Fisica Generale, Università di Torino, Italy
⁵ Istituto di Astrofisica Spaziale CNR, Roma, Italy
⁶ DSM/DAPNIA/Service d'Astrophysique - CEA Saclay, Gif-sur-Yvette, France

Received 8 April 1999 / Accepted 16 June 1999

Abstract

New observations of AGB circumstellar envelopes made with the mid-IR imaging camera CAMIRAS are presented. Diagnostic tools based on mid-IR colors are proposed for the analysis of the observed objects, and tested against a larger sample of AGB sources derived from IRAS observations. Radiative transfer modelling is used to calibrate our diagnostic procedures, in order to investigate the physical and chemical characteristics of both samples. The different chemical signatures of C-rich and O-rich envelopes are put in evidence by mid-IR color-color diagrams, and correlations are found between the observed colors and a mass loss parameter derived by radio observations. Evidences for possible temporal variations in the mass loss rates are also suggested. The possibility to derive direct information on the spatial structure and symmetry of the envelopes is finally investigated, and the image of the O-rich star WX Psc, as an example of a spatially resolved axisymmetric envelope, is presented.

Key words: stars: AGB and post-AGB stars: circumstellar matter stars: individual: 0191-B2B infrared: stars

* Based on observations made at the Italian Infrared Telescope of Gornergrat (TIRGO)

Send offprint requests to: M. Marengo

© European Southern Observatory (ESO) 1999

Online publication: July 26, 1999

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