Comparing Carbon Dust Grain Types from the Milky Way and the Large Magellanic Cloud

Details Comparing Carbon Dust Grain Types from the Milky Way and the Large Magellanic Cloud Comparing Carbon Dust Grain Types from the Milky Way and the Large Magellanic Cloud

Jan 2010

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010aas...21542809m&link_type=abstract

American Astronomical Society, AAS Meeting #215, #428.09; Bulletin of the American Astronomical Society, Vol. 42, p.346

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Scientific paper

Intermediate mass stars eventually evolve into asymptotic giant branch (AGB) stars. During this phase carbon is produced in the helium-burning shell and dredged up to the stellar surface by the convection current. If the star's carbon-to-oxygen ratio exceeds one, it becomes a carbon star. AGB stars experience mass loss, leading to the formation of a circumstellar dust shell around the star. Some carbon stars have such high mass-loss rates that the dust shell it forms completely obscures starlight; these are called extreme carbon stars, and they are major contributors of carbon to the ISM.
Carbons stars commonly show a spectral feature at 11 microns, attributed to silicon carbide (SiC). This feature is usually seen in emission, but in some extreme carbon stars the feature is seen in absorption. Such stars in our own galaxy have been well studied, but similar objects in other galaxies have only recently been discovered. Here we present a study of extreme carbon stars in the Large Magellanic Cloud (LMC). We show that while extreme carbon stars in both galaxies are qualitatively similar, the dust properties seem to be different.
Using radiative transfer modeling we show that the LMC stars are consistent with dust shells dominated by amorphous carbon, while their Milky Way counterparts are better modeled with graphite. However, it is possible to model the Milky Way objects using AmC, but this requires a significantly larger dust shell. Our models suggest that either there is a metallicity effect on the crystallinity of the carbon grains, or that the LMC objects have much smaller dust shells than their Galactic counterpart. We suggest mechanisms by which metallicity might affect the crystal structure of dust forming around otherwise similar stars.

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