Aluminum oxide and the opacity of oxygen-rich circumstellar dust in the 12-17-#mu#m range

Amorphous alumina (Al_2O_3) 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#-Al_2O_3. At T>1273 K crystalline #alpha#-Al_2O_3 (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 LRS spectra of bright Mira stars and semiregular variables. Its shape, which is satifactorily represented by a Lorentz profile, can be reproduced by Mie calculations with the data of #alpha#-Al_2O_3, but not with those of #gamma#-Al_2O_3. The calculations show that the 13-#mu#m band profile of #alpha#-Al_2O_3 is sensitive to grain shape. If #alpha#-Al_2O_3 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. (orig.)SIGLEAvailable from TIB Hannover: RR 8257(63) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekBundesministerium fuer Bildung, Wissenschaft, Forschung und Technologie, Bonn (Germany)DEGerman