An ISO/SWS study of the dust composition around S stars. A novel view of S-star dust

Details An ISO/SWS study of the dust composition around S stars. A novel view of S-star dust An ISO/SWS study of the dust composition around S stars. A novel view of S-star dust

Jul 2009

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009a%26a...501..609h&link_type=abstract

Astronomy and Astrophysics, Volume 501, Issue 2, 2009, pp.609-617

Astronomy and Astrophysics

Astrophysics

9

Stars: Agb And Post-Agb, Stars: Mass-Loss, Infrared: Stars, Circumstellar Matter, Supergiants

Scientific paper

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 π1 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.
based on observations obtained with ISO, an ESA project with instruments funded by ESA Member states (especially the PI countries: France, Germany, the Netherlands and the United Kingdom) and with the participation of ISAS and NASA.

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