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Astron. Astrophys. 352, 587-599 (1999)

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2.4-197 µm spectroscopy of OH/IR stars: the IR characteristics of circumstellar dust in O-rich environments *

R.J. Sylvester 1, F. Kemper 2, M.J. Barlow 1, T. de Jong 2,3, L.B.F.M. Waters 2,4, A.G.G.M. Tielens 5,6 and A. Omont 7

1 Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT
2 Astronomical Institute, University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands
3 SRON Laboratory for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands
4 Instituut voor Sterrenkunde, Katholieke Universiteit Leuven, Celestijnenlaan 200B, 3001 Heverlee, Belgium
5 Kapteijn Institute, University of Groningen, P.O. Box 800, 9700 AV Groningen, The Netherlands
6 SRON Laboratory for Space Research, P.O. Box 800, 9700 AV Groningen, The Netherlands
7 Institut d'Astrophysique de Paris. C.N.R.S., 98b bd. Arago, 75014 Paris, France

Received 30 July 1999 / Accepted 4 October 1999

Abstract

Infrared spectra of a number of evolved O-rich stars have been obtained with the Short- and Long- Wavelength spectrometers on board the Infrared Space Observatory. The very broad wavelength coverage (2.4-197 µm) obtained by combining observations made with the two spectrometers includes practically all of the flux emitted by the sources, and allows us to determine the emission and absorption features of the dense circumstellar dust shells. Agreement between the fluxes obtained by the two instruments is generally very good; the largest discrepancies are probably due to source variability. Our sample of oxygen-rich AGB stars exhibits a wealth of spectral features due to crystalline silicates and crystalline water ice in emission and absorption. In this study a qualitative overview of all features due to crystalline silicates and water ice in these high mass loss rate objects is presented. It seems that there is a certain onset value for the mass loss rate above which these features appear in the spectrum. Moreover, crystalline silicate emission features have been detected for the first time at wavelengths where the amorphous silicates are still in absorption, implying different spatial distributions for the two materials. A spherically symmetric and an axi-symmetric geometry are proposed.

Key words: stars: AGB and post-AGB – stars: circumstellar matter – stars: mass-loss – infrared: stars

* Based on observations with ISO, an ESA project with instruments funded by ESA Member States (especially the PI countries: France, Germany, the Netherlands and the United Kingdom) with the participation of ISAS and NASA.

Send offprint requests to: F. Kemper (ciska@astro.uva.nl)

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© European Southern Observatory (ESO) 1999

Online publication: December 2, 1999
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