LUMINOUS ASYMPTOTIC GIANT BRANCH STARS IN THE LARGE MAGELLANIC CLOUD

We present the results of a search for optically obscured asymptotic giant branch (AGB) stars in the Large Magellanic Cloud (LMC), combining data obtained using the IRAS satellite with near-infrared photographic plate material of a 15 deg2 region in the northern LMC. Of the 156 IRAS sources that are detected either in separate cross-scans or in more than one passband, 63 have [12 - 25] colors consistent with their being either stellar photospheres or circumstellar dust shells. Seventeen of these we identify with bright (Ic < 9) foreground stars in our own Galaxy, while a further 17 are associated with red supergiants in the LMC. Of the remaining stars, no more than five are likely to be optically visible AGB stars, while the rest have no obvious optical counterpart. This immediately rules out the presence of sufficient high-luminosity "cocoon" stars to explain the observed deficit of several hundred luminous (Mbol < -6) AGB stars between the predictions of standard models of AGB evolution and the observed luminosity function. It remains possible that most of the unidentifid sources are dusty AGB stars, evolving through a phase of enhanced mass loss toward becoming planetary nebulae. We infer bolometric magnitudes as low as Mbol ∼ -5 for these sources and suggest that this phase can be triggered at low luminosities, truncating AGB evolution and leading to the observed scarcity of asymptotic giant branch stars with bolometric magnitudes brighter than -6.0 mag.