  We present an analysis and comparison of the 30 micron dust features seen in
the Spitzer Space Telescope spectra of 207 carbon-rich asymptotic giant branch
(AGB) stars, post-AGB objects, and planetary nebulae located in the Milky Way,
the Magellanic Clouds (MCs), or the Sagittarius dwarf spheroidal galaxy (Sgr
dSph), which are characterised by different average metallicities. We
investigated whether the formation of the 30 micron feature carrier may be a
function of the metallicity. Through this study we expect to better understand
the late stages of stellar evolution of carbon-rich stars in these galaxies.
Our analysis uses the `Manchester method' as a basis for estimating the
temperature of dust for the carbon-rich AGB stars and the planetary nebulae in
our sample. We used a black-body function with a single temperature deduced
from the Manchester method or its modification to approximate the continuum
under the 30 micron feature. The most important conclusion of our work is the
fact that the formation of the 30 micron feature is affected by metallicity.
Specifically that, as opposed to more metal-poor samples of AGB stars in the
MCs, the feature is seen at lower mass-loss rates, higher temperatures, and has
seen to be more prominent in Galactic carbon stars. The averaged feature
(profile) in the AGB, post-AGB objects, and PNe seems unaffected by metallicity
at least between a fifth and solar metallicity, but in the case of PNe it is
shifted to significantly longer wavelengths.
