Matsuura et al. 2007, MNRAS, ... Spitzer ... observations of AGB stars in ... Fornax ...

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

M. Matsuura (National Astronomical Obs. of Japan, Univ. Coll. London), A.A. Zijlstra (Univ. of Manchester), J. Bernard-Salas (Cornell), J.W. Menzies (SAAO), G.C. Sloan (Cornell), P.A. Whitelock (Univ. of Cape Town and SAAO), P.R. Wood, (Australian National Univ.), M.-R.L. Cioni (Univ. of Edinburgh), M.W. Feast (Univ. of Cape Town), E. Lagadec (Univ. of Manchester), J.Th. van Loon (Univ. of Keele), M.A.T. Groenewegen (K.U. Leuven), and G.J. Harris (Univ. Coll. London)

2007, MNRAS, 382, 1889

We have observed five carbon-rich AGB stars in the Fornax dwarf spheroidal (dSph) galaxy, using the Infrared Spectrometer on the Spitzer Space Telescope. The stars were selected from a near-infrared survey of Fornax and include the three reddest stars, with presumabely 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 x 10^-6 M_sunyr^-1. The other two stars have mass loss rates below 1.3 x 10^-6 M_sunyr^-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 of 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 of this galaxy is of the order 2 x 10^-5 M_sunyr^-1. This is much lower than that of the dwarf irregular galaxy 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 dwarf spheroidal galaxies. Our study will be useful to constrain gas and dust recycling process in low metallicity galaxies.

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