The Effects of Main-Sequence Mass Loss on Surface C/N Abundance Ratios During the Ascent of the First Giant Branch

Abstract

The surface C/N abundance ratios of many cluster and field G and K giants following the 1st dredge-up phase are much lower than predicted from standard stellar evolution modeling. The occurrence of substantial mass loss, either during or immediately after the main-sequence phase would both reduce the mass fraction of the unprocessed envelope necessary to contaminate with CN-cycle products, as well as allow CN-processing of a greater amount of core material during the earlier high-mass phase. Willson, Bowen and Struck-Marcell (1987) have proposed that a combination of pulsation and rapid rotation could drive substantial mass loss in main-sequence stars of initial mass 1-3 MΘ. We evolved a grid of 16 mass-losing models from the zero-age main sequence through 1st dredge-up. The models have initial masses of 1.25, 1.5, 1.75 and 2.0 MΘ, and exponentially decreasing mass-loss rates with e-folding times 0.2, 0.4, 1.0 and 2.0 Gyr; all models evolve toward a final mass of 1.0 M". Since the mass-loss epoch is short-lived, most of the models reach 1.0 M0 rapidly, and follow the evolutionary track of a standard 1 MΘ model redward away from the main sequence and up the 1st giant branch. The convecuve envelope deepens during 1st dredge-up to homogenize the outer 3/4 of the star's final mass.