Spitzer Space Telescope spectra of post-AGB stars in the Large
  Magellanic Cloud ---polycyclic aromatic hydrocarbons at low metallicities
  This paper reports variations of polycyclic aromatic hydrocarbons (PAHs)
features that were found in Spitzer Space Telescope spectra of carbon-rich
post-asymptotic giant branch (post-AGB) stars in the Large Magellanic Cloud
(LMC). The paper consists of two parts. The first part describes our Spitzer
spectral observing programme of 24 stars including post-AGB candidates. The
latter half of this paper presents the analysis of PAH features in 20
carbon-rich post-AGB stars in the LMC, assembled from the Spitzer archive as
well as from our own programme. We found that five post-AGB stars showed a
broad feature with a peak at 7.7 micron, that had not been classified before.
Further, the 10--13 micron PAH spectra were classified into four classes, one
of which has three broad peaks at 11.3, 12.3 and 13.3 micron rather than two
distinct sharp peaks at 11.3 and 12.7 micron, as commonly found in HII regions.
Our studies suggest that PAHs are gradually processed while the central stars
evolve from post-AGB phase to PNe, changing their composition before PAHs are
incorporated into the interstellar medium. Although some metallicity dependence
of PAH spectra exists, the evolutionary state of an object is more significant
than its metallicity in determining the spectral characteristics of PAHs for
LMC and Galactic post-AGB stars.
