An ISO/SWS study of the dust composition around S stars^*

A novel view of S-star dust

¹ Laboratoire AIM, CEA/DSM – CNRS – Université Paris Diderot, DAPNIA/Service d'Astrophysique, Bât. 709, CEA-Saclay, 91191 Gif-sur-Yvette Cédex, France e-mail: sacha.hony@cea.fr
² Research and Scientific Support Department-ESA/ESTEC, PO Box 299, 2200 AG Noordwijk, The Netherlands
³ NOVA, PO Box 9513, 2300 RA Leiden, The Netherlands
⁴ Instituut voor Sterrenkunde, K.U. Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium

Received: 9 March 2009
Accepted: 22 April 2009

Abstract

Aims. We investigate the composition of the solid-state materials in the winds around S-type AGB stars. The S stars produce dust in their wind that bears a resemblance to the dust produced in some O-rich AGB stars. However, the reported resemblance is mostly based on IRAS/LRS spectra with limited spectral resolution, sensitivity, and wavelength coverage.

Methods. We investigate the dust composition around S stars using ISO/SWS data that surpass the previous studies in terms of spectral resolution and wavelength coverage. We selected the dust producing S stars in the ISO/SWS archive with enough signal to perform a detailed dust analysis, and then compare the dust spectra from the 9 sources with the O-rich AGB spectra and a subset of M super-giants. We constructed average dust emission spectra of the different categories.

Results. We report the discovery of several previously unreported dust emission features in the S star spectra. The long wavelength spectra of W Aql and  Gru exhibit the “30” μm feature attributed to MgS. Two sources exhibit a series of emission bands between 20 and 40 μm that we tentatively ascribe to Diopside. We show that the 10-20 μm spectra of the S stars are significantly different from the O-rich AGB stars. The O-rich stars exhibit a structured emission feature that is believed to arise from amorphous silicate and aluminium-oxide. The S stars lack the substructure found in the O-rich stars. Instead they show a smooth peak with a varying peak-position from source to source. We suggest that this feature is caused by a family of related materials, whose exact composition determines the peak position. The observed trend mimics the laboratory trend of non-stoichiometric silicates. In this scenario the degree of non-stoichiometry is related to the Mg to SiO₄ ratio, in other words, to the amount of free O available during the dust grain growth.