PDF《INTRODUCTION》

19 km band pro?le is rather shallow if compared with laboratory data of silicates (see Simpson et al. 1991;

Dorschner 1995). 2. A weak but distinct emission band centered at about

13 km was detected in the IRAS LRS spectra of Mira and other evolved giant stars et al. Little- (Vardya 1986;

Marenin &

Little LeVan, &

Little- 1988, 1990;

Sloan, Marenin Up to now, no satisfactory explanation has 1996). been put forward. Possibly, both points are closely related in that the

13 km band is only the visible tip of the iceberg ?? and the contin- uum in the trough stems from the merging of a greater number of vibrational bands of solids condensing in the O-rich circumstellar environment. As promising candidates contributing to the emission in the 12?15 km spectral range, suggested Al-containing framework silicates, Tielens (1990) aluminates, and metal oxides, among the latter alu- SiO

2 , minum oxide alumina), spinel alumin- (Al

2 O

3 , (MgAl

2 O

4 ), ous cements, and rutile However, most of these (TiO

2 ). compounds have not yet been investigated seriously in an astrophysical context. Since the

13 km band has not been observed in carbon- rich stars, it was natural to attribute this feature to vibra- tions of an oxide, and de Jong, &

Willems Onaka, (1989) tentatively identi?ed it with the Al?O vibrational band of alumina grains. suggested that sapphire (a Glaccum (1995) terrestrial gem variety of could account for the

13 Al

2 O

3 ) km band. However, an ultimate proof of these identi?cation proposals has been handicapped by the lack of a reliable

199 200 BEGEMANN ET AL. Vol.

476 observationally based band pro?le. Alumina (its rhombohedral modi?cation is called corun- dum and designated as is considered as an early a-Al

2 O

3 ) condensate in a cooling gas of solar composition. Numer- ous theoretical studies have been devoted to the sequence of condensation processes taking place in such environments as the early solar system, expanding envelopes around evolved stars, and supernova shells &

Larimer (Grossman Schramm, &

Grossman &

1974;

Lattimer, 1978;

Kozasa Hasagawa Hasegawa, &

1987;

Sedlmayr 1989;

Kozasa, Nomoto In his conception of the chemical memory 1991). of the matter, ?? stressed the important role Clayton (1982) that may be played by corundum and spinel crystallites. Because the are the innermost cores of oxidic supernova condensates (SUNOCONs) protected by a silicate mantle, they may transport the condensed SN-aluminum bonded within highly refractory compounds through the interstellar space. As a matter of fact, Al-enriched inclusions with iso- topic anomalies (for Ca-Al inclusions, see McPherson, Wark, &

Armstrong as well as micron-sized presolar 1988) corundum and spinel grains have been detected in primitive meteorites (Nittler et al. et al. 1994, 1995;

Huss 1994;

et al. The isotope signature points to a red Hutcheon 1994). giant and asymptotic giant branch (AGB) star origin of the corundum grains et al. This lends additional (Nittler 1995). support to the idea that alumina grains contribute to cir- cumstellar dust opacity, even if the content of detectable corundum grains in the meteorites seems to be extremely small (about 0.5 ppm). There could be a large number of much smaller grains that escaped detection. Optical constants of single crystals and microcrystalline particles of have been measured by many authors a-Al

2 O

3 Smith, &

Morgan (Barker 1963;

Loewenstein, 1973;

Toon, Pollack, &