Aluminum oxide and the opacity of oxygen-rich circumstellar dust in the 12-17 micron range

Amorphous alumina (Al2O3) was produced by a sol-gel technique in order to make available its 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 gamma-Al2O3. At T > 1273 K, crystalline alpha-Al2O3 (corundum) is formed. Mie calculations show that amorphous alumina exhibits a wide Al-O vibrational band, peaking at 11.5-11.8 mu m and having a steep ''blue'' and an extended ''red'' wing. It may be an important contributor to the continuous opacity between the silicate bands in oxygen-rich circumstellar envelopes, whereas it is ruled out for the explanation of the 13 mu m band. An average 13 mu m band profile was derived from 51 IRAS low-resolution spectra of bright Mira stars and semiregular variables. Its shape, which is satisfactorily represented by a Lorentz profile, can be reproduced by Mie calculations with the data of alpha-Al2O3, but not with those of gamma-Al2O3. The calculations show that the 13 mu m band profile of alpha-Al2O3 is sensitive to grain shape. If alpha-Al2O3 is the absorber, a second band should be present at 21 mu m. A close correlation was found between the strengths of the 13 mu m band and the 10 mu m silicate band. It suggests that the 13 mu m band carrier could also be somehow connected with silicate dust. Experimental arguments supporting this attribution are presented.