K. DePew (University of Missouri-Columbia)
Asymptotic Giant Branch (AGB) stars lose material by a combination of stellar pulsation and radiation pressure. Once this gaseous material reaches a certain distance from the star where the temperature is low enough (< 2000K) dust grains can condense, so that the star is surrounded by a circumstellar shell of dust and gas. Eventually this dust and gas is pushed away into the interstellar medium and consequently incorporated into new star formation regions and planets.
AGB star circumstellar shells are either carbon or oxygen rich (C-rich or O-rich). The nature of the dust grains can be identified using IR spectroscopy. In 1986, a spectral feature peaking between 12.5 and 13.0 microns was discovered in IRAS LRS data. Since then, three major candidate species have been advanced and continuously investigated: alumina (Al2O3), spinel (MgAl2O4) and silica (SiO2). The feature has several enigmatic correlations. It has been shown that the 13 um feature is more often found in semi-regular variable stars than in Miras. The presence of the feature in supergiant spectra is rare (Speck et al. 2000). Furthermore, the 13um feature has been correlated with various other spectral features, although these correlations are not universally accepted. We present the pros and cons for each candidate carrier species and how they fit into the bigger picture of stardust formation, including their relation to evidence from theoretical, laboratory and meteoritic investigations.
Bulletin of the American Astronomical Society, 37 #2
© 2005. The American Astronomical Soceity.