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Issue A&A
Volume 393, Number 1, October I 2002
Page(s) L7 - L10
Section Letters
DOI http://dx.doi.org/10.1051/0004-6361:20021127



A&A 393, L7-L10 (2002)
DOI: 10.1051/0004-6361:20021127

Letter

On the origin of the 19.5 $\mathsf{\mu}$m feature

Identifying circumstellar Mg-Fe-oxides
Th. Posch1, F. Kerschbaum1, H. Mutschke2, J. Dorschner2 and C. Jäger2

1  Institut für Astronomie, Türkenschanzstraße 17, 1180 Wien, Austria
2  Astrophysikalisches Institut und Universitätssternwarte, Schillergässchen 3, 07745 Jena, Germany

(Received 10 July 2002 / Accepted 2 August 2002 )

Abstract
We report the detection of a broad, prominent 19.5 $\mu$m dust emission feature in ISO-SWS spectra. It is especially conspicuous in the spectra of low mass-loss AGB stars belonging to the variability classes SRb and Lb with relatively high photospheric temperatures. The feature carrier is proposed to be Mg 0.1Fe 0.9O, a solid solution of MgO (periclase) and FeO (wustite). This dust species has cubic crystal symmetry like spinel, the carrier of the 13, 16.8 and 31.8 $\mu$m spectral features, with which the 19.5 $\mu$m feature is being observed. A broad emission plateau between 11 and 15 $\mu$m, which is attributable to amorphous Al 2O 3, is also detected together with the 19.5 $\mu$m and the spinel features. As a consequence of our discovery, we postulate the existence of a distinct class of circumstellar shells, dominated by oxide and not by silicate dust.


Key words: stars: circumstellar matter -- stars: formation -- stars: AGB and post-AGB -- solar system: formation -- methods: laboratory

Offprint request: Th. Posch, posch@astro.univie.ac.at

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© ESO 2002


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