Letter to the Editor

On the origin of the 19.5 m feature^*

Identifying circumstellar Mg-Fe-oxides

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

Corresponding author: Th. Posch, posch@astro.univie.ac.at

Received: 10 July 2002
Accepted: 2 August 2002

Abstract

We report the detection of a broad, prominent 19.5 μm 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 μm spectral features, with which the 19.5 μm feature is being observed. A broad emission plateau between 11 and 15 μm, which is attributable to amorphous Al₂O₃, is also detected together with the 19.5 μm 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.