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

Abstract

Abstract.We have analyzed the 8 − 13.5 µm UKIRT CGS3 spectra of 142 M-type stars including 80 oxygenrich 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 continuumsubtracted 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 (Al 2 O 3 ) 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 Al 2 O 3 , progresses Send offprint requests to: A.K. Speck 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 Al 2 O 3 . 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 Al 2 O 3 or, as has more recently been suggested, spinel (MgAl 2 O 4 ), but could be associated with silicon dioxide or highly polymerized silicates (not pyroxenes or olivines).

Finding the most variable stars in the Orion Belt with the All Sky Automated Survey

“…It is an M8-9-type Mira Ceti variable found by Wolf (1904) with P = 424.75±1.77 d (Templeton et al 2005) and silicate dust emission (Sloan & Price 1998;Speck et al 2000). In particular, the 9.7 and 18 µm silicate dust feature originates from the circumstellar envelopes of oxygenrich asymptotic giant branch stars (Kwok et al 1997).…”

Optical constants of refractory oxides at high temperatures

“…In fact, all three dust species discussed in this paper have been proposed as carrier of the 13 μm band. Speck et al (2000) propose (amorphous) SiO 2 as feature carrier, Posch et al (1999) and Fabian et al (2001) favor MgAl 2 O 4 , Sloan et al (2003) and DePew et al (2006) have strengthened the case for α-Al 2 O 3 . It should be noted that this dust feature has also been detected in AGB stars outside our galaxy (e.g.…”

Near- and mid-infrared colors of evolved stars in the Galactic plane. TheQ1 andQ2 parameters

“…When comparing the distribution in colors of the 119 RSG stars in clusters with all RSGs listed by Skiff et al, however, no significant differences are seen. Verhoelst et al (2009) studied the dust properties of O-rich RSGs envelopes and found a dust condensation sequence resembling that of AGBs (Kemper et al 2001;Speck et al 2000;Sylvester et al 1999). Stars with low mass-loss rates are rich in oxides and alumina, while high mass-loss stars show olivine, and crystalline silicates (Waters 2010).…”