  The Infrared Spectrograph on the Spitzer Space Telescope observed 184 carbon
stars in the Magellanic Clouds. This sample reveals that the dust-production
rate (DPR) from carbon stars generally increases with the pulsation period of
the star. The composition of the dust grains follows two condensation
sequences, with more SiC condensing before amorphous carbon in metal-rich
stars, and the order reversed in metal-poor stars. MgS dust condenses in
optically thicker dust shells, and its condensation is delayed in more
metal-poor stars. Metal-poor carbon stars also tend to have stronger absorption
from C2H2 at 7.5 um. The relation between DPR and pulsation period shows
significant apparent scatter, which results from the initial mass of the star,
with more massive stars occupying a sequence parallel to lower-mass stars, but
shifted to longer periods. Accounting for differences in the mass distribution
between the carbon stars observed in the Small and Large Magellanic Clouds
reveals a hint of a subtle decrease in the DPR at lower metallicities, but it
is not statistically significant. The most deeply embedded carbon stars have
lower variability amplitudes and show SiC in absorption. In some cases they
have bluer colors at shorter wavelengths, suggesting that the central star is
becoming visible. These deeply embedded stars may be evolving off of the AGB
and/or they may have non-spherical dust geometries.
