Crystalline silicate dust around evolved stars ^*,**

I. The sample stars

¹ Astronomical Institute “Anton Pannekoek”, University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands
² School of Materials Science and Engineering, Georgia Tech, Atlanta, GA 30332-0245, USA
³ Instituut voor Sterrenkunde, Katholieke Universiteit Leuven, Celestijnenlaan 200B, 3001 Heverlee, Belgium
⁴ SRON Laboratory for Space Research Groningen, PO Box 800, 9700 AV Groningen, The Netherlands
⁵ Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, UK

Corresponding author: F. J. Molster, fmolster@so.estec.esa.nl

Received: 3 May 2001
Accepted: 5 November 2001

Abstract

This is the first paper in a series of three where we present the first comprehensive inventory of solid state emission bands observed in a sample of 17 oxygen-rich circumstellar dust shells surrounding evolved stars. The data were taken with the Short and Long Wavelength Spectrographs on board of the Infrared Space Observatory (ISO) and cover the 2.4 to 195 μm wavelength range. The spectra show the presence of broad 10 and 18 μm bands that can be attributed to amorphous silicates. In addition, at least 49 narrow bands are found whose position and width indicate they can be attributed to crystalline silicates. Almost all of these bands were not known before ISO. The incredible richness of the crystalline silicate spectra observed by ISO allows detailed studies of the mineralogy of these dust shells, and is a telltale about the origin and evolution of the dust. We have measured the peak positions, widths and strengths of the individual, continuum subtracted bands. Based on these measurements, we were able to order the spectra in sequence of decreasing crystalline silicate band strength. We found that the strength of the emission bands correlates with the geometry of the circumstellar shell, as derived from direct imaging or inferred from the shape of the spectral energy distribution. This naturally divides the sample into objects that show a disk-like geometry (strong crystalline silicate bands), and objects whose dust shell is characteristic of an outflow (weak crystalline silicate bands). All stars with the 33.6 μm forsterite band stronger than 20 percent over continuum are disk sources. We define spectral regions (called complexes) where a concentration of emission bands is evident, at 10, 18, 23, 28, 33, 40 and 60 μm. We derive average shapes for these complexes and compare these to the individual band shapes of the programme stars. In an Appendix, we provide detailed comments on the measured band positions and strengths of individual sources.