Proceedings of the International Astronomical Union

Contributed Papers

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

M. Matsuuraa1, G. C. Sloana2, J. Bernard-Salasa2, A. A. Zijlstraa3, P. R. Wooda4, P. A. Whitelocka5a6a7, J. W. Menziesa5, M. Feasta6, E. Lagadeca3, M. A. T. Groenewegena8, M. R. Cionia9, J. Th. van Loona10 and G. Harrisa11

a1 National Astronomical Observatory of Japan, Osawa 2-21-1, Mitaka, Tokyo 181-8588, Japan email:

a2 Astronomy Department, Cornell University, 610 Space Sciences Building, Ithaca, NY 14853-6801, USA

a3 Jodrell Bank Centre for Astrophysics, School of Physics and Astronomy, The University of Manchester, Oxford Street, Manchester M13 9PL, UK

a4 Research School of Astronomy & Astrophysics, Mount Stromlo Observatory, Australian National University, Cotter Road, Weston ACT 2611, Australia

a5 South African Astronomical Observatory, P.O. Box 9, 7935 Observatory, South Africa

a6 Astronomy Department, University of Cape Town, 7701 Rondebosch, South Africa

a7 NASSP, Department of Mathematics and Applied Mathematics, University of Cape Town, 7701 Rondebosch, South Africa

a8 Instituut voor Sterrenkunde, KU Leuven, Celestijnenlaan 200B, 3001 Leuven, Belgium

a9 Centre for Astrophysics Research, University of Hertfordshire, Hatfield AL10 9AB, UK

a10 Astrophysics Group, School of Physical and Geographical Sciences, Keele University, Staffordshire ST5 5BG, UK

a11 Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, UK


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.


  • Stars: AGB and post-AGB;
  • ISM: molecules;
  • stars: late-type;
  • (stars:) circumstellar matter;
  • (galaxies:) Magellanic Clouds;
  • (galaxies:) Local Group