Matsuura et al. 2006, MNRAS, ... acetylene bands ... in the LMC

Spitzer observations of acetylene bands in carbon-rich asymptotic giant branch stars in the Large Magellanic Cloud

M. Matsuura, M. (Univ. of Manchester, National Astronomical Obs. of Japan), P.R. Wood (Australian National Univ.), G.C. Sloan (Cornell), A.A. Zijlstra (Univ. of Manchester), J. Th. van Loon (Keele Univ.), M.A.T. Groenewegen, J.A.D.L. Blommaert (K.U. Leuven), M.-R. Cioni (Univ. of Edinburgh), M.W. Feast (Univ. of Cape Town), H.J. Habing (Univ. of Leiden), S. Hony (Saclay), E. Lagadec (Univ. of Manchester), C. Loup (Institut d'Astrophysique de Paris), J.W. Menzies (SAAO), L.B.F.M. Waters (Univ. of Amsterdam), P.A. Whitelock (Univ. of Cape Town and SAAO)

2006, MNRAS, 371, 415

Full manuscript available from astro-ph (0606296) or locally (PDF).

We investigate the molecular bands in carbon-rich asymptotic giant branch (AGB) stars in the Large Magellanic Cloud (LMC), using the Infrared Spectrograph (IRS) onboard the Spitzer Space Telescope (SST) over the 5-38 µm range. All 26 low-resolution spectra show acetylene (C₂H₂) bands at 7 and 14 µm. The hydrogen cyanide (HCN) bands at these wavelengths are very weak or absent. This is consistent with low nitrogen abundances in the LMC. The observed 14 µm C₂H₂ band is reasonably reproduced by an excitation temperature of 500 K. There is no clear dilution of the 14 µm C₂H₂ band by circumstellar dust emission. This 14-µm band originates from molecular gas in the circumstellar envelope in these high mass-loss rate stars, in agreement with previous findings for Galactic stars. The C₂H₂ column density, derived from the 13.7 µm band, shows a gas mass-loss rate in the range 3 x 10^-6 to 5 x 10^-5 M_solar yr^-1. This is comparable with the total mass-loss rate of these stars estimated from the spectral energy distribution. Additionally, we compare the line strengths of the 13.7 µm C₂H₂ band of our LMC sample with those of a Galactic sample. Despite the low metallicity of the LMC, there is no clear difference in the C₂H₂ abundance among LMC and Galactic stars. This reflects the effect of the third dredge-up bringing self-produced carbon to the surface, leading to high carbon-to-oxygen ratio at low metallicity.

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