PROBING THE AGB TIP - LUMINOUS CARBON STARS IN THE GALACTIC PLANE

To address the question of the progenitor mass range of carbon stars, we searched for CO emission from a sample of distant (d approximately 2-10 kpc), heavily obscured asymptotic giant branch (AGB) stars that a re confined to the galactic plane. We detected 47 stars for the first time, and have determined luminosities for approximately 40 of these v ia a kinematic technique. Based on IRAS low resolution spectra, detect ions of circumstellar molecules other than CO, IR colors, and/or CO li ne strength relative to IRAS flux, we have classified the bulk of the circumstellar envelopes as carbon- or oxygen-rich. We deduce similar a verage luminosities for carbon stars and oxygen-rich stars from 1.3 10 (4) to 2 10(4)L., depending on the method used to compute bolometric f luxes. These averages are larger than typically assumed or derived for carbon stars in the solar neighborhood, and are similar to the averag e luminosity of ''cold'' OH/IR stars. The results suggest that luminou s carbon- and oxygen-rich AGB stars near the galactic plane are descen ded from stellar populations of similar mean mass. This mean mass (> 2 M.) is probably larger than the mean progenitor mass of AGB stars in g eneral.