Mon. Not. R. Astron. Soc., 427, 3209-3229 (2012/December-3)
On the metallicity dependence of crystalline silicates in oxygen-rich asymptotic giant branch stars and red supergiants.
JONES O.C., KEMPER F., SARGENT B.A., McDONALD I., GIELEN C., WOODS P.M., SLOAN G.C., BOYER M.L., ZIJLSTRA A.A., CLAYTON G.C., KRAEMER K.E., SRINIVASAN S. and RUFFLE P.M.E.
Abstract (from CDS):
We investigate the occurrence of crystalline silicates in oxygen-rich evolved stars across a range of metallicities and mass-loss rates. It has been suggested that the crystalline silicate feature strength increases with increasing mass-loss rate, implying a correlation between lattice structure and wind density. To test this, we analyse Spitzer Infrared Spectrograph and Infrared Space Observatory Short Wavelength Spectrometer spectra of 217 oxygen-rich asymptotic giant branch and 98 red supergiants in the Milky Way, the Large and Small Magellanic Clouds, and Galactic globular clusters. These encompass a range of spectral morphologies from the spectrally rich which exhibit a wealth of crystalline and amorphous silicate features to `naked' (dust-free) stars. We combine spectroscopic and photometric observations with the GRAMS grid of radiative transfer models to derive (dust) mass-loss rates and temperature. We then measure the strength of the crystalline silicate bands at 23, 28 and 33µm. We detect crystalline silicates in stars with dust mass-loss rates which span over 3dex, down to rates of ∼ 10–9M☉/yr. Detections of crystalline silicates are more prevalent in higher mass-loss rate objects, though the highest mass-loss rate objects do not show the 23-µm feature, possibly due to the low temperature of the forsterite grains or it may indicate that the 23-µm band is going into absorption due to high column density. Furthermore, we detect a change in the crystalline silicate mineralogy with metallicity, with enstatite seen increasingly at low metallicity.
© 2012 The Authors Monthly Notices of the Royal Astronomical Society © 2012 RAS
radiative transfer - stars: AGB and post-AGB - circumstellar matter - dust, extinction - Magellanic Clouds - infrared: stars
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<Available at CDS (J/MNRAS/427/3209): table2.dat table3.dat table4.dat table5.dat table6.dat>
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