Modelling the alumina abundance of oxygen-rich evolved stars in the Large Magellanic Cloud

O.C. Jones (Manchester), F. Kemper (ASIAA), S. Srinivasan (ASIAA), I. McDonald (Manchester), G.C. Sloan (Cornell), and A.A. Zijlstra (Manchester) T.L. Herter (Cornell), V. Charmandaris (Univ. of Crete),

2014, MNRAS, 440, 631

Full manuscript available locally (PDF) or from astro-ph (1402.2485)

In order to determine the composition of the dust in the circumstellar envelopes of oxygen-rich asymptotic giant branch (AGB) stars we have computed a grid of MODUST radiative-transfer models for a range of dust compositions, mass-loss rates, dust shell inner radii and stellar parameters. We compare the resulting colours with the observed oxygen-rich AGB stars from the SAGE-Spec Large Magellanic Cloud (LMC) sample, finding good overall agreement for stars with a mid-infrared excess . We use these models to fit a sample of 37 O-rich AGB stars in the LMC with optically thin circumstellar envelopes, for which 5-35 µm Spitzer Infrared Spectrograph (IRS) spectra and broadband photometry from the optical to the mid-infrared are available. From the modelling, we find mass-loss rates in the range ~8 x 10^-8 to 5 x 10^-6 M_sun yr^-1, and we show that a grain mixture consisting primarily of amorphous silicates, with contributions from amorphous alumina and metallic iron provides a good fit to the observed spectra. Furthermore, we show from dust models that the AKARI [11]-[15] versus [3.2]-[7] colour-colour diagram, is able to determine the fractional abundance of alumina in O-rich AGB stars.

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Last modified 23 April, 2014. © Gregory C. Sloan and others.