PDF《INTRODUCTION》

THE ASTROPHYSICAL JOURNAL, 476:199?208,

1997 February

10 1997.

The American Astronomical Society. All rights reserved. Printed in U.S.A. ( ALUMINUM OXIDE AND THE OPACITY OF OXYGEN-RICH CIRCUMSTELLAR DUST IN THE 12?17 MICRON RANGE B. J. TH. AND H. BEGEMANN, DORSCHNER, HENNING, MUTSCHKE Max-Planck-Gesellschaft, Arbeitsgruppe Staub in Sternentstehungsgebieten,?? 3, D-07745 Jena, Germany Schillerga ? ?chen J. AND C. GU ? RTLER KO ? MPE Universita ? ts-Sternwarte Jena, 2, D-07745 Jena, Germany Schillerga ? ?chen AND R. NASS Institut fu ? r Neue Materialien, Im Stadtwald, Geb. 43, D-66123 Saarbru ? cken, Germany Received

1996 February 26;

accepted

1996 September

3 ABSTRACT Amorphous alumina was produced by a sol-gel technique in order to make available its (Al

2 O

3 ) optical constants for possible astrophysical applications. Gradual annealing showed that the X-ray amorphousness of alumina ended somewhere between

723 and

873 K. Above this transition point, the structure changes into disordered At T [

1273 K, crystalline (corundum) is formed. c-Al

2 O

3 . a-Al

2 O

3 Mie calculations show that amorphous alumina exhibits a wide Al-O vibrational band, peaking at 11.5?11.8 km and having a steep blue ?? and an extended red ?? wing. It may be an important contrib- utor to the continuous opacity between the silicate bands in oxygen-rich circumstellar envelopes, whereas it is ruled out for the explanation of the

13 km band. An average

13 km band pro?le was derived from

51 IRAS low-resolution spectra of bright Mira stars and semiregular variables. Its shape, which is satisfactorily represented by a Lorentz pro?le, can be reproduced by Mie calculations with the data of but not with those of The calcu- a-Al

2 O

3 , c-Al

2 O

3 . lations show that the

13 km band pro?le of is sensitive to grain shape. If is the a-Al

2 O

3 a-Al

2 O

3 absorber, a second band should be present at

21 km. A close correlation was found between the strengths of the

13 km band and the

10 km silicate band. It suggests that the

13 km band carrier could also be somehow connected with silicate dust. Experimental arguments supporting this attribution are presented. Subject headings: circumstellar matter ? dust, extinction ? infrared: ISM: lines and bands ? stars: late-type 1. INTRODUCTION The mid-infrared spectra of oxygen-rich circumstellar envelopes around many evolved stars are dominated by the silicate emission bands at about

10 and

19 km. The IRAS low-resolution spectra (LRS) catalog (Olnon et al. 1986) presents the spectra of more than

1800 such sources. The wide range of widths and shapes and ?ne-structure details visible especially in the pro?le of the stronger

10 km feature have formed the base for classi?cation schemes, evolution- ary characterization of the envelopes, and suggestions about the chemical and mineralogical properties of the sili- catic dust de Jong, &

Willems Little- (Vardya, 1986;

Marenin &

Little Henning, &

1988, 1990;

Ossenkopf, Mathias &

Price Parallel 1992;

Simpson 1991;

Sloan 1995). to these observationally based analyses, much laboratory work has been done on analogs that promise to reproduce the observed silicate band pro?les, especially the

10 km band pro?le Henning, &

Dor- (Kra ? tschmer 1988;

Gu ? rtler, schner &

Tsuchiyama Shibai, &

1989;

Koike 1991;

Koike, Tsuchiyama et al. et al. 1993;

Ja ? ger 1994;

Dorschner 1995). Much less attention has been paid to the spectral region between the silicate bands. In this very range, which we will refer to as the trough ?? (12?17 km), at least two points wait for their explanation and, therefore, deserve greater atten- tion: 1. The opacity on the deepest point of the trough is sur- prisingly high, and the