Molecules and Dust Grains in AGB Stars in Nearby Galaxies—the Influence of Metallicities

We have obtained infrared spectra of carbon stars in four nearby galaxies—the Large and Small Magellanic Clouds, the Sagittarius dwarf spheroidal galaxy, and the Fornax dwarf spheroidal galaxy. Our primary aim is to investigate mass-loss rate and molecular bands of these stars as a function of metallicity, by comparing AGB stars in several galaxies with different metallicities. 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. We find no evidence that mass-loss rates depend on metallicity. Carbon stars are strongly affected by carbon production during the AGB phase; primarily mass-loss of carbon-rich stars are driven by amorphous carbon dust grains, which explains the little metallicity dependence of mass-loss rate for carbon-rich stars. We found that C2H2 bands are prominent features at 3–15 µm among extragalactic carbon stars, which is not always the case for Galactic carbon stars. We argue that the difference is caused by systematically high C/O ratios in low-metallicity environments.