Spitzer Space Telescope spectral observations of AGB stars in the Fornax dwarf spheroidal galaxy

doi:10.1111/j.1365-2966.2007.12501.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 382 Issue 4, Pages 1889 - 1900

Published Online: 16 Nov 2007

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Spitzer Space Telescope spectral observations of AGB stars in the Fornax dwarf spheroidal galaxy

M. Matsuura ^1,2★ , A. A. Zijlstra ³ , J. Bernard-Salas ⁴ , J. W. Menzies ⁵ , G. C. Sloan ⁴ , P. A. Whitelock ^5,6,7 , P. R. Wood ⁸ , M.-R. L. Cioni ⁹ , M. W. Feast ⁶ , E. Lagadec ³ , J. Th. van Loon ¹⁰ , M. A. T. Groenewegen ¹¹ and G. J. Harris ²

¹ National Astronomical Observatory of Japan, Osawa 2-21-1, Mitaka, Tokyo 181-8588, Japan ² Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT ³ School of Physics and Astronomy, University of Manchester, Sackville Street, PO Box 88, Manchester M60 1QD ⁴ Astronomy Department, Cornell University, 610 Space Sciences Building, Ithaca, NY 14853-6801, USA ⁵ South African Astronomical Observatory, PO Box 9, 7935 Observatory, South Africa ⁶ Astronomy Department, University of Cape Town, 7701 Rondebosch, South Africa ⁷ NASSP, Department of Mathematics and Applied Mathematics, University of Cape Town, 7701 Rondebosch, South Africa ⁸ Research School of Astronomy & Astrophysics, Mount Stromlo Observatory, Australian National University, Cotter Road, Weston, ACT 2611, Australia ⁹ SUPA, School of Physics, University of Edinburgh, IfA, Blackford Hill, Edinburgh EH9 3HJ ¹⁰ Instituut voor Sterrenkunde, KU Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium ¹¹ Astrophysics Group, School of Physical and Geographical Sciences, Keele University, Staffordshire ST5 5BG

Correspondence to ^★ E-mail: mikako@optik.mtk.nao.ac.jp

KEYWORDS

stars: AGB and post-AGB • stars: atmospheres • stars: carbon • stars: mass-loss

ABSTRACT

We have observed five carbon-rich asymptotic giant branch (AGB) stars in the Fornax dwarf spheroidal (dSph) galaxy, using the Infrared Spectrometer on board the Spitzer Space Telescope. The stars were selected from a near-infrared survey of Fornax and include the three reddest stars, with presumably the highest mass-loss rates, in that galaxy. Such carbon stars probably belong to the intermediate-age population (2–8 Gyr old and metallicity of [Fe/H]∼−1) of Fornax. The primary aim of this paper is to investigate mass-loss rate, as a function of luminosity and metallicity, by comparing AGB stars in several galaxies with different metallicities. The spectra of three stars are fitted with a radiative transfer model. We find that mass-loss rates of these three stars are 4–7 × 10⁻⁶ M_⊙ yr⁻¹ . The other two stars have mass-loss rates below 1.3 × 10⁻⁶ M_⊙ yr⁻¹ . We find no evidence that these rates depend on metallicity, although we do suggest that the gas-to-dust ratio could be higher than at solar metallicity, in the range 240 to 800. The C₂H₂ bands are stronger at lower metallicity because of the higher C/O ratio. In contrast, the SiC fraction is reduced at low metallicity due to low silicon abundance. The total mass-loss rate from all known carbon-rich AGB stars into the interstellar medium (ISM) of this galaxy is of the order of 2 × 10⁻⁵ M_⊙ yr⁻¹ . This is much lower than that of the dwarf irregular galaxy Wolf Lundmark Melotte (WLM), which has a similar visual luminosity and metallicity. The difference is attributed to the younger stellar population of WLM. The suppressed gas-return rate to the ISM accentuates the difference between the relatively gas-rich dwarf irregular and the gas-poor dSph galaxies. Our study will be useful to constrain gas and dust recycling processes in low-metallicity galaxies.