The Magellanic zoo: Mid-infrared Spitzer spectroscopy of evolved stars and circumstellar dust in the Magellanic Clouds

摘要

We observed a sample of evolved stars in the Large and Small MagellanicClouds (LMC and SMC) with the Infrared Spectrograph on the Spitzer SpaceTelescope. Comparing samples from the SMC, LMC, and the Galaxy reveals that thedust-production rate depends on metallicity for oxygen-rich stars, but carbonstars with similar pulsation properties produce similar quantities of dust,regardless of their initial metallicity. Other properties of the oxygen-richstars also depend on metallicity. As the metallicity decreases, the fraction ofnaked (i.e. dust-free) stars increases, and among the naked stars, the strengthof the 8 um absorption band from SiO decreases. Our sample includes severalmassive stars in the LMC with long pulsation periods which produce significantamounts of dust, probably because they are young and relatively metal rich.Little alumina dust is seen in circumstellar shells in the SMC and LMC, unlikein Galactic samples. Three oxygen-rich sources also show emission frommagnesium-rich crystalline silicates. Many also show an emission feature at 14um. The one S star in our sample shows a newly detected emission featurecentered at 13.5 um. At lower metallicity, carbon stars with similar amounts ofamorphous carbon in their shells have stronger absorption from molecularacetylene (C_2H_2) and weaker emission from SiC and MgS dust, as discovered inprevious studies.