Dust features in the 10-mu m infrared spectra of oxygen-rich evolved stars

We have analyzed the 8 - 13.5 mum UKIRT CGS3 spectra of 142 M-type stars in cluding 80 oxygen-rich AGB star's and 62 red supergiants, with a view to un derstanding the differences and similarities between the dust features of t hese stars. We have classified the spectra into groups according to the obs erved appearance of the infrared features. In each case the normalized cont inuum-subtracted spectrum has been compared to those of the other stars to find similarities and form groups. The dust Features of the AGE stars are c lassified into six groups: broad AGB, where the feature extends from 8 mum to about 12.5 mum with little structure; broad+sil AGB, which consists of a broad feature with an emerging 9.7 mum silicate bump; anti four silicate A GB groups in which a "classic" 9.7 mum silicate feature gets progressively narrower. Likewise, the supergiant spectra have also been classified into g roups, however these do not all coincide with the AGB star groups. In the s upergiant case we again have six groups: featureless, where there is little or no emission above the continuum; broad Super, where the Feature ex tend s from about 9 mum to about 13 mum; and four silicate Super groups, which a gain show a progression towards the narrowest "classic" 9.7 Irm silicate fe ature. We compare the mean spectrum for each group, which yields two main r esults. Firstly, while the "classic" silicate feature is essentially identi cal for both AGE stars and red supergiants, the broad features observed for these two stellar types are quite different. We suggest that the dust in t hese two environments follows different evolutionary paths, with the dust: around Mira stars, whose broad feature spectra can be lit by a combination of alumina (Al2O3) and magnesium silicate, progressing from this com positi on to dust dominated by magnesium silicate only, while the dust around supe rgiants, whose broad feature can be fit by a combination of Ca-Al-rich sili cate and Al2O3, progresses from this initial composition to one eventually also dominated by magnesium silicate. The reason for the difference in the respective broad features is not clear as yet, but could be influenced by l ower C/O ratios and chromospheric UV radiation fields in supergiant outflow environments. The second result concerns the 12.5 - 13.0 mum feature disco vered in IRAS LRS spectra and widely attributed to Al2O3 This feature is se en predominantly in the spectra of semiregular variables, sometime in Miras and only once (so far) in supergiant spectra. We argue that it is unlikely that this feature is due to Al2O3 or, as has more recently been suggested, spinel (MgAl2O4): but could be associated with silicon dioxide or highly p olymerized silicates (not pyroxenes or olivines).