Zijlstra et al. 2006, MNRAS, ... Carbon stars in the LMC

A Spitzer mid-infrared spectral survey of mass-losing carbon stars in the Large Magellanic Cloud

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

2006, MNRAS, 370, 1961

Full manuscript available from astro-ph (0602531) or locally (PDF).

We present a Spitzer Space Telescope spectroscopic survey of mass-losing carbon stars (and one oxygen-rich star) in the Large Magellanic Cloud. The spectra cover the wavelength range 5--38 µm. They show varying combinations of dust continuum, dust emission features (SiC, MgS) and molecular absorption bands (C₂H₂, HCN). A set of four narrow bands, dubbed the Manchester system, is used to define the infrared continuum for dusty carbon stars. The relations between the continuum colours and the strength of the dust and molecular features are studied, and are compared to Galactic stars of similar colours. The circumstellar 7-µm C₂H₂ band is found to be stronger at lower metallicity, from a comparison of stars in the Galaxy, the LMC and the SMC. This is explained by dredge-up of carbon, causing higher C/O ratios at low metallicity (less O). A possible 10-µm absorption feature seen in our spectra may be due to C₃. This band has also been identified with interstellar silicate or silicon-nitrite dust. The line-to-continuum ratio of the SiC and MgS bands shows some indication of being lower at low metallicity. The MgS band is only seen at dust temperatures below 600 K. We discuss the selection of carbon versus oxygen-rich AGB stars using the J-K vs. K-A colours, and show that these colours are relatively insensitive to chemical type. Metal-poor carbon stars form amorphous carbon dust from self-produced carbon. The formation efficiency of oxygen-rich dust depends more strongly on metallicity. We suggest that in lower-metallicity environments, the dust input into the Interstellar Medium by AGB stars is efficient but may be strongly biassed towards carbonaceous dust, as compared to the Galaxy.

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