Volume 501, Number 2, July II 2009
|Page(s)||609 - 617|
|Section||Interstellar and circumstellar matter|
|Published online||13 May 2009|
An ISO/SWS study of the dust composition around S stars*
A novel view of S-star dust
Laboratoire AIM, CEA/DSM – CNRS – Université Paris Diderot, DAPNIA/Service d'Astrophysique, Bât. 709, CEA-Saclay, 91191 Gif-sur-Yvette Cédex, France e-mail: firstname.lastname@example.org
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
Accepted: 22 April 2009
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.
Key words: stars: AGB and post-AGB / stars: mass-loss / infrared: stars / circumstellar matter / supergiants
© ESO, 2009
Current usage metrics show cumulative count of Article Views (full-text article views including HTML views, PDF and ePub downloads, according to the available data) and Abstracts Views on Vision4Press platform.
Data correspond to usage on the plateform after 2015. The current usage metrics is available 48-96 hours after online publication and is updated daily on week days.
Initial download of the metrics may take a while.