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LibraryE. Lagadec, A.A. Zijlstra (Univ. of Manchester), G.C. Sloan (Cornell), M. Matsuura (National Astronomical Obs. of Japan), P.R. Wood (Australian National Univ.), G.J. Harris (Univ. Coll. London), J. Th. van Loon (Keele Univ.), J.A.D.L. Blommaert (K.U. Leuven), S. Hony (Saclay), M.A.T. Groenewegen (K.U. Leuven), M.W. Feast (Univ. of Cape Town), P.A. Whitelock (Univ. of Cape Town and SAAO), J.W. Menzies (SAAO), M.-R. Cioni (Univ. of Edinburgh), L.B.F.M. Waters (Univ. of Amsterdam)
2007, MNRAS, 376, 1270
Full manuscript available from astro-ph (0611071) or locally (PDF).
We present Spitzer Space telescope spectroscopic observations of 14 carbon-rich AGB stars in the Small Magellanic Cloud. SiC dust is seen in most of the carbon-rich stars but it is weak compared to LMC stars. The SiC feature is strong only for stars with significant dust excess, opposite to what is observed for Galactic stars. We argue that in the SMC, SiC forms at lower temperature than graphite dust, whereas the reverse situation occurs in the Galaxy where SiC condenses at higher temperatures and forms first. Dust input into the interstellar medium by AGB stars consists mostly of carbonaceous dust, with little SiC or silicate dust. Only the two coolest stars show a 30 µm band due to MgS dust. We suggest that this is due to the fact that, in the SMC, mass-losing AGB stars generally have low circumstellar (dust) optical depth and therefore effective heating of dust by the central star does not allow temperatures below the 650 K necessary for MgS to exist as a solid. Gas phase C2H2 are stronger in the SMC than in the LMC or Galaxy. This is attributed to an increasing C/O ratio at low metallicity. We present a colour-colour diagram based on Spitzer IRAC and MIPS colours to discriminate between O- and C-rich stars. We show that AGB stars in the SMC become carbon stars early in the thermal-pulsing AGB evolution, and remain optically visible for ~6 x 105 yr. For the LMC, this lifetime is ~3 x 105 yr. The superwind phase traced with Spitzer lasts for ~104 yr. Spitzer spectra of a K supergiant and a compact HII region are also given.
Last modified 1 August, 2007. © Gregory C. Sloan and others.