paper (ps, preprint, 2.7M)
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 micron. 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-micron
C2H22
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-micron 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
theSiC 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 compare to the Galaxy.