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

We present a Spitzer Space Telescope spectroscopic survey of mass-losing carbon stars (and one oxygen-rich star) in the Large Magellanic Cloud (LMC). The stars represent the superwind phase on the asymptotic giant branch (AGB), which forms a major source of dust for the interstellar medium (ISM) in galaxies. Bolometric magnitudes indicate progenitor masses of 1.5-2.5 M-circle dot. The spectra cover the wavelength range 5-38 mu m. They show varying combinations of dust continuum, dust emission features (SiC, MgS) and molecular absorption bands (C2H2, HCN). A 10-mu m absorption feature is attributed to C-3. A weak band at 5.8 mu m is suggestive of carbonyl. The circumstellar 7.5-mu m C2H2 band is found to be stronger at lower metallicity, explained by higher C/O ratios at low metallicity. The J - K versus K - A colours, used to select the sample, are shown to be relatively insensitive in separating carbon versus oxygen-rich AGB stars. The predominance of carbon stars therefore indicates that in the range 1.5-2.5 M-circle dot, LMC AGB stars become carbon-rich before onset of the superwind. A set of four narrow bands, dubbed the Manchester system, is used to define the infrared continuum for dusty carbon stars. We investigate the strength and central wavelength of the SiC and MgS dust bands as a 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. Metal-poor carbon stars can form amorphous carbon dust from self-produced carbon. The formation efficiency of oxygen-rich dust depends more strongly on metallicity. In lower-metallicity environments, the dust input into the ISM by AGB stars may be strongly biased towards carbonaceous dust.