Program: GS-2009B-Q-97

Recent Spitzer spectroscopy of possible post-AGB stars in the Large and Small Magellanic Clouds has detected the unidentified 21 micron emission feature in nine objects. These infrared spectra show a number of unusual properties compared to the 16 previously known 21 micron objects in the Galaxy, with a trend to much stronger aromatic (PAH) features and generally weaker 21 micron features, along with broad-band colours that suggest that the dust is significantly hotter in these objects than in Galactic analogues. The hotter dust implies that the objects have had less time to evolve off the AGB than the Galactic analogues, while the stronger aromatic bands suggests a stronger UV radiation field which would require that the stars are hotter (more evolved) than the Galactic 21 micron sources. If the stars are both hotter and have left the AGB recently they must have more massive cores than similar objects in the Galaxy. I request optical spectroscopy of several brighter 21 micron sources in the Clouds, which can be done in poor weather, to determine the stellar types and see whether they are different than the Galactic objects. These observations will also allow the objects to be placed on the HR diagram with reasonable accuracy for the first time, because unlike Galactic objects the distance to the objects is known.