Research Note

Towards the identification of circumstellar hibonite

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

Corresponding author: H. Mutschke, mutschke@astro.uni-jena.de

Received: 15 February 2002
Accepted: 20 June 2002

Abstract

We have performed IR reflectance measurements on crystalline CaAl₁₂O₁₉ (hibonite) and derived optical constants for both principal directions of the electric field vector relative to the crystallographic c-axis. Based on the resulting dielectric functions, we calculated small particle spectra and found six maxima of the absorption cross section for spherical dust grains at the wavelengths 12.3, 15.9, 21.1, 25.1, 34.4, and 78.7 μm. For nonspherical particles, strong shifting and broadening of the two strongest resonances at 12.3 and 15.9 μm occurs. The signal-to-noise-ratio of ISO spectra of oxygen-rich AGB stars is not sufficient for a definite identification of these bands, even though there is some weak indication of their presence. We propose additional observations to clarify this point.