Journal Article

On the metallicity dependence of crystalline silicates in oxygen-rich asymptotic giant branch stars and red supergiants

O. C. Jones, F. Kemper, B. A. Sargent, I. McDonald, C. Gielen, Paul M. Woods, G. C. Sloan, M. L. Boyer, A. A. Zijlstra, G. C. Clayton, K. E. Kraemer, S. Srinivasan and P. M. E. Ruffle

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 427, issue 4, pages 3209-3229
Published in print December 2012 | ISSN: 0035-8711
Published online December 2012 | e-ISSN: 1365-2966 | DOI: http://dx.doi.org/10.1111/j.1365-2966.2012.21978.x
On the metallicity dependence of crystalline silicates in oxygen-rich asymptotic giant branch stars and red supergiants

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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 33 μm. We detect crystalline silicates in stars with dust mass-loss rates which span over 3 dex, down to rates of ∼10−9 M yr−1. 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.

Keywords: radiative transfer; stars: AGB and post-AGB; circumstellar matter; dust, extinction; Magellanic Clouds; infrared: stars

Journal Article.  13964 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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