We have observed five carbon-rich 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 ] \sim - 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 \times 10 ^ { -6 } M _ { \sun } yr ^ { -1 } .
The other two stars have mass-loss rates below 1.3 \times 10 ^ { -6 } M _ { \sun } 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 C _ { 2 } H _ { 2 } 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 of 2 \times 10 ^ { -5 } M _ { \sun } yr ^ { -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 processes in low metallicity galaxies .