A post-AGB star in the Small Magellanic Cloud observed with the Spitzer Infrared Spectrograph

We have observed an evolved star with a rare combination of spectral features, MSX SMC 029, in the Small Magellanic Cloud (SMC) using the low-resolution modules of the Infrared Spectrograph on the Spitzer Space Telescope. A cool dust continuum dominates the spectrum of MSX SMC 029. The spectrum also shows both emission from polycyclic aromatic hydrocarbons (PAHs) and absorption at 13.7 mu m from C2H2, a juxtaposition 2 2 seen in only two other sources, AFGL 2688 and IRAS 13416-6243, both post-asymptotic giant branch (AGB) objects. As in these sources, the PAH spectrum has the unusual trait that the peak emission in the 7-9 mu m complex lies beyond 8.0 mu m. In addition, the 8.6 mu m feature has an intensity as strong as the C-C modes that normally peak between 7.7 and 7.9 mu m. The relative flux of the feature at 11.3 mu m to that at 8 mu m suggests that the PAHs in MSX SMC 029 either have a low-ionization fraction or are largely unprocessed. The 13-16 mu m wavelength region shows strong absorption features similar to those observed in the post-AGB objects AFGL 618 and SMP LMC 11. This broad absorption may arise from the same molecules that have been identified in those sources:C2H2, C4H2, HC3N, and C6H6. The similarities between MSX SMC 029, AFGL 2688, and AFGL 618 lead us to conclude that MSX SMC 029 has evolved off the AGB in only the past few hundred years, making it the third post-AGB object identified in the SMC.