Carbon-rich AGB stars in our Galaxy and nearby galaxies as possible sources of PAHs

We have obtained infrared spectra of carbon-rich AGB stars in three nearby galaxies - the Large and Small Magellanic Clouds, and the Fornax dwarf spheroidal galaxy. Our primary aim is to investigate gas compositions and mass-loss rate of these stars as a function of metallicity, by comparing AGB stars in several galaxies with different metallicities. C₂H₂ are detectable from AGB stars, and possibly PAHs are subsequently formed from C₂H₂. Thus, it is worth investigating chemical processes at low metallicity. These stars were observed using the Infrared Spectrometer (IRS) onboard the Spitzer Space Telescope which covers 5-35 μm region, and the Infrared Spectrometer And Array Camera (ISAAC) on the Very Large Telescope which covers the 2.9-4.1 μm region. HCN, CH and C₂H₂ molecular bands, as well as SiC and MgS dust features are identified in the spectra. The equivalent width of C₂H₂ molecular bands is larger at lower metallicity, thus PAHs might be abundant in AGB stars at low metallicity. We find no evidence that mass-loss rates depend on metallicity. Chemistry of carbon stars is affected by carbon production during the AGB phase rather than the metallicities. We argue that lower detection rate of PAHs from the interstellar medium of lower metal galaxies is caused by destruction of PAHs in the ISM by stronger UV radiation field.