On the origin of the 13 μm feature

Author(s)

Thomas Posch, Franz Kerschbaum, Harald Mutschke, D. Fabian, Johann M. Dorschner, Josef Hron

Abstract

We have derived a mean profile of the 13

μm emission feature from 11 ISO spectra of oxygen-rich AGB stars and

present an overview of the mineral species that could account

for it. Our results can be summarized as follows: i) Spherical particles of

α-Al₂O₃

(corundum) have a sharp emissivity

maximum peaking at 12.7 μm. The difference of 0.3 μm in the

peak position compared to the mean observed band profile is

a serious obstacle to assigning the 13

μm dust feature to this

dust species. ii) From a continuous distribution of ellipsoidal

α-Al₂O₃

-particles, a broad emissivity profile peaking at 13–14 μm

emerges. It is difficult to reconcile the large width of this profile

with the width of the observed 13

μm band. iii) The most prominent emissivity maximum of TiO₂ (rutile) is located at 13.5

μm;

its width is larger and its strength three times smaller than the

emissivity maximum of corundum. iv) Core-mantle-grains composed of rutile and corundum or of corundum and amorphous

olivine can both produce, in a certain domain of core volume

fractions, a spectral signature very similar to the observed 13

μm

feature. However, the necessity to assume rather artificial distribution functions of the core volume fractions makes this scenario improbable. v) We consider MgAl₂O₄

(spinel) to be the

most promising candidate for the carrier of the 13

μm feature

since its emissivity peaks are located at 12.95

μm and at 16.8

μm

for spherical particles and since at

both

positions features are

present in most of the spectra of our sample. In view of the

relevance of this substance for circumstellar shells, laboratory

spectra of spinel particles embedded in KBr are also presented.

Organisation(s)

Department of Astrophysics

External organisation(s)

Friedrich-Schiller-Universität Jena

Journal

Astronomy & Astrophysics

Volume

352

Pages

609 - 618

No. of pages

10

ISSN

0004-6361

Publication date

1999

Peer reviewed

Yes

Austrian Fields of Science 2012

103003 Astronomy, 103004 Astrophysics

Portal url

https://ucrisportal.univie.ac.at/en/publications/bbc73d63-9bb3-4683-8992-c418685297a0

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