Spitzer observations of acetylene bands in carbon-rich asymptotic giant branch stars in the Large Magellanic Cloud |
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| Authors: | M Matsuura P R Wood G C Sloan A A Zijlstra J Th van Loon M A T Groenewegen J A D L Blommaert M-R L Cioni M W Feast H J Habing S Hony E Lagadec C Loup J W Menzies L B F M Waters P A Whitelock |
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| Institution: | School of Physics and Astronomy, University of Manchester, Sackville Street, PO Box 88, Manchester M60 1QD;APS Division, Department of Pure and Applied Physics, Queen's University Belfast, University Road, Belfast BT7 1NN;National Astronomical Observatory of Japan, Osawa 2-21-1, Mitaka, Tokyo 181-8588, Japan;Research School of Astronomy &Astrophysics, Mount Stromlo Observatory, Australian National University, Cotter Road, Weston ACT 2611, Australia;Astronomy Department, Cornell University, 610 Space Sciences Building, Ithaca, NY 14853-6801, USA;Astrophysics Group, School of Physical and Geographical Sciences, Keele University, Staffordshire ST5 5BG;Instituut voor Sterrenkunde, KU Leuven, Celestijnenlaan 200B, 3001 Leuven, Belgium;SUPA, School of Physics, University of Edinburgh, IfA, Blackford Hill, Edinburgh EH9 3HJ;Astronomy Department, University of Cape Town, 7701 Rondebosch, South Africa;Sterrewacht Leiden, Niels Bohrweg 2, 2333 RA Leiden, the Netherlands;Institut d'Astrophysique de Paris, CNRS, 98bis Boulevard Arago, 75014 Paris, France;South African Astronomical Observatory, PO Box 9, 7935 Observatory, South Africa;Astronomical Institute 'Anton Pannekoek', University of Amsterdam, Kruislaan 403, 1098 SJ, Amsterdam, the Netherlands;NASSP, Department of Mathematics and Applied Mathematics, University of Cape Town, 7701 Rondebosch, South Africa |
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| Abstract: | We investigate the molecular bands in carbon-rich asymptotic giant branch (AGB) stars in the Large Magellanic Cloud (LMC), using the Infrared Spectrograph (IRS) onboard the Spitzer Space Telescope ( SST ) over the 5–38 μm range. All 26 low-resolution spectra show acetylene (C2H2) bands at 7 and 14 μm. The hydrogen cyanide (HCN) bands at these wavelengths are very weak or absent. This is consistent with low nitrogen abundances in the LMC. The observed 14 μm C2H2 band is reasonably reproduced by an excitation temperature of 500 K. There is no clear dilution of the 14 μm C2H2 band by circumstellar dust emission. This 14-μm band originates from molecular gas in the circumstellar envelope in these high mass-loss rate stars, in agreement with previous findings for Galactic stars. The C2H2 column density, derived from the 13.7 μm band, shows a gas mass-loss rate in the range 3 × 10?6 to 5 × 10?5 M⊙ yr?1. This is comparable with the total mass-loss rate of these stars estimated from the spectral energy distribution. Additionally, we compare the line strengths of the 13.7 μm C2H2 band of our LMC sample with those of a Galactic sample. Despite the low metallicity of the LMC, there is no clear difference in the C2H2 abundance among LMC and Galactic stars. This reflects the effect of the third dredge-up bringing self-produced carbon to the surface, leading to high carbon-to-oxygen ratio at low metallicity. |
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| Keywords: | stars: AGB and post-AGB stars: atmospheres stars: carbon stars: massloss Magellanic Clouds |
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