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

Abstract

We have analyzed the 8 - 13.5 μm UKIRT CGS3 spectra of 142 M-type stars including 80 oxygen-rich AGB stars and 62 red supergiants, with a view to understanding the differences and similarities between the dust features of these stars. We have classified the spectra into groups according to the observed appearance of the infrared features. In each case the normalized continuum-subtracted spectrum has been compared to those of the other stars to find similarities and form groups. The dust features of the AGB stars are classified into six groups: broad AGB, where the feature extends from 8 μm to about 12.5 μm with little structure; broad+sil AGB, which consists of a broad feature with an emerging 9.7 μm silicate bump; and four silicate AGB groups in which a "classic" 9.7 μm silicate feature gets progressively narrower. Likewise, the supergiant spectra have also been classified into groups, however these do not all coincide with the AGB star groups. In the supergiant case we again have six groups: featureless, where there is little or no emission above the continuum; broad Super, where the feature extends from about 9 μm to about 13 μm; and four silicate Super groups, which again show a progression towards the narrowest "classic" 9.7 μm silicate feature. We compare the mean spectrum for each group, which yields two main results. Firstly, while the "classic" silicate feature is essentially identical for both AGB stars and red supergiants, the broad features observed for these two stellar types are quite different. We suggest that the dust in these two environments follows different evolutionary paths, with the dust around Mira stars, whose broad feature spectra can be fit by a combination of alumina (Al2O3) and magnesium silicate, progressing from this composition to dust dominated by magnesium silicate only, while the dust around supergiants, whose broad feature can be fit by a combination of Ca-Al-rich silicate 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 lower C/O ratios and chromospheric UV radiation fields in supergiant outflow environments. The second result concerns the 12.5-13.0 μm feature discovered in IRAS LRS spectra and widely attributed to Al2O3. This feature is seen 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 polymerized silicates (not pyroxenes or olivines).