The carrier of the 13μ m feature in the spectra of O-rich AGB stars

Details The carrier of the 13μ m feature in the spectra of O-rich AGB stars The carrier of the 13μ m feature in the spectra of O-rich AGB stars

Dec 2003

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003aas...20313702m&link_type=abstract

American Astronomical Society Meeting 203, #137.02; Bulletin of the American Astronomical Society, Vol. 36, p.584

Astronomy and Astrophysics

Astronomy

Scientific paper

Asymptotic Giant Branch (AGB) stars are able to pulsate and very gently lose mass by expelling atoms out of their atmospheres. Atoms are also pushed away by radiation pressure from the star. Then, once the atoms reach a certain distance from the star where the temperature is low enough (< 2000K) they are able to condense into dust grains and form a circumstellar dust shell. Eventually this dust and gas is pushed away into the interstellar medium and consequently incorporated into new star formation regions and planets. AGB stars are responsible for more than 90% of the chemical enrichment in a galaxy.
These circumstellar shells are either carbon or oxygen rich (C-rich or O-rich). Dust grains have particular signature IR spectrum. For O-rich AGB stars, attempts to identify a sequence of features for their evolution have been made by classifying the observed dust emission lines according to their shape and position. This sequence of features is believed to represent the dust evolution. However, one problem with this evolutionary sequence for dust grains in O-rich stars is that it does not incorporate a weak but distinctive emission feature around 13μ m that has been observed in 40-50% of the O-rich AGB stars by IRAS LRS. Semiregular variables (SRs) are much more likely to show the 13μ m feature than Miras or Red Supergiants. SRs possess less dense circumstellar shells than Miras or Red Supergiants, indicating that the 13μ m emission band is characteristic of the dust shell's density. Candidate species that have been proposed as the carrier of this enigmatic 13μ m include Al2O3, MgAl2O4 and SiO2.
We present the pros and cons for each candidate carrier species and suggest that SiO2 is the most likely carrier.

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