• Zijlstra, Albert
• Matsuura, M.
• Bernard-Salas, J.
• Loon, J. Th Van
• Harris, G. J.
• Cioni, M. R. L.
• Sloan, G. C.
• Wood, P. R.
• Feast, M. W.
• Groenewegen, M. A. T.
• Lagadec, E.
• Menzies, J. W.
• Whitelock, P. A.

Abstract

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. © 2007 American Institute of Physics.

Topics

• impedance spectroscopy
• amorphous
• Carbon
• grain
• phase