Spitzer spectroscopy of carbon stars in the Small Magellanic Cloud

doi:10.1111/j.1365-2966.2007.11517.x

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Monthly Notices of the Royal Astronomical Society

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Monthly Notices of the Royal Astronomical Society: Letters

Volume 376 Issue 3, Pages 1270 - 1284

Published Online: 7 Mar 2007

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Spitzer spectroscopy of carbon stars in the Small Magellanic Cloud

Eric Lagadec ^1★ , Albert A. Zijlstra ¹ , G. C. Sloan ² , Mikako Matsuura ^1,3 , Peter R. Wood ⁴ , Jacco Th. van Loon ⁵ , G. J. Harris ⁶ , J. A. D. L. Blommaert ⁷ , S. Hony ⁸ , M. A. T. Groenewegen ⁷ , M. W. Feast ⁹ , P. A. Whitelock ^9,10,11 , J. W. Menzies ⁹ and M.-R. Cioni ¹²

¹ University of Manchester, School of Physics & Astronomy, PO Box 88, Manchester M60 1QD ² Department of Astronomy, Cornell University, 108 Space Sciences Building, Ithaca, NY 14853-6801, USA ³ Division of Optical and IR Astronomy, National Astronomical Observatory of Japan, Osawa 2-21-1, Mitaka, Tokyo 181-8588, Japan ⁴ Research School of Astronomy and Astrophysics, Australian National University, Cotter Road, Weston Creek, ACT 2611, Australia ⁵ Astrophysics Group, School of Physical & Geographical Sciences, Keele University, Staffordshire ST5 5BG ⁶ Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT ⁷ Instituut voor Sterrenkunde, K.U. Leuven, Celestijnenlaan 200 D, B-3001 Leuven, Belgium ⁸ CEA, DSM, DAPNIA, Service d'Astrophysique, C.E. Saclay, F-91191 Gif-sur-Yvette Cedex, France ⁹ Department of Astronomy, University of Cape Town, 7701 Rondebosch, South Africa ¹⁰ South African Astronomical Observatory, PO Box 9, 7935 Observatory, South Africa ¹¹ NASSP, Department of Mathematics & Applied Mathematics, University of Cape Town, 7701 Rondebosch, South Africa ¹² Institute for Astronomy, University of Edinburgh, Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ

Correspondence to ^★ E-mail: eric.lagadec@manchester.ac.uk

KEYWORDS

stars: AGB and post-AGB • stars: carbon • circumstellar matter • stars: mass-loss • Magellanic Clouds • infrared: stars

ABSTRACT

We present Spitzer Space Telescope spectroscopic observations of 14 carbon-rich asymptotic giant branch (AGB) stars in the Small Magellanic Cloud (SMC). SiC dust is seen in most of the carbon-rich stars but it is weak compared to Large Magellanic Cloud (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 in the Galaxy SiC and graphite condensate at more comparable temperatures. 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 bands 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 InfraRed Array Camera (IRAC) and Multiband Imaging Photometer for Spitzer (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 × 10⁵ yr . For the LMC, this lifetime is ∼3 × 10⁵ yr . The superwind phase traced with Spitzer lasts for ∼10⁴ yr. Spitzer spectra of a K supergiant and a compact H ii region are also given.