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

We present a Spitzer Space Telescopespectroscopic survey of mass-losing carbon stars (and one oxygen-rich star) in the Large Magellanic Cloud. The stars represent the superwind phase on the Asymptotic Giant Branch, which forms a major source of dust for the interstellar medium in galaxies. The spectra cover the wavelength range 5‐38� m. 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 r elations 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-� m 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-� m absorption feature seen in our spectra may be due to C3. This band has also been identified with interstellar silicate or silicon-nitr ite dust. We investigate the strength and central wavelength of the SiC and MgS dust bands as function of colour and metallicity. The line-to-continuum ratio of these 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. This type of dust forms more readily in the presence of a higher C/O ratio. Low metallicity carbon dust may contain a smaller fraction of SiC and MgS constituents, which do depend on metallicity. 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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