The evolution of asymptotic giant branch stars in the Large Magellanic Cloud. II. Spectroscopy of a complete sample.

Spectra of 113 asymptotic giant branch (AGB) star candidates constituting six magnitude-limited, area-complete samples in the outer regions of the northern LMC have been obtained. Luminosity functions constructed from these data are well represented by an underlying intermediate-mass AGB population, the product of continuous star formation over the last 3-4 Gyr, supplemented in some areas by more massive red giant and supergiant stars, with a typical age of about 10 to the 8th yr. Most stars with Mbol between -5 and -5.5 show evidence for dredge-up of s-process elements, as do a smaller proportion in the range Mbol between -4.5 and -5. Only 10 percent of the sample, however, are C stars, and these are concentrated toward the Bar of the LMC. No evidence has been found for envelope burning in any of the stars in the sample.