Lithium Enrichment–Mass-Loss Connection in K Giant Stars

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Abstract
Based on observed far-infrared properties of K giant stars, we propose a scenario linking the high Li abundances of some of these stars to the evolution of circumstellar shells. In this model, every K giant with masses between 1.0 and 2.5 M☉ become Li rich during the red giant branch stage, and the internal mechanism responsible for the Li enrichment will initiate a prompt mass-loss event. The evolutionary paths of the detached shells are compatible with observations for low expansion velocities of the order of 2 km s-1 and mass loss of (2-5) × 10-8 M☉ yr-1. This modest mass loss is, however, 2 orders of magnitude larger than those of normal, Li-poor K giants. A "Li time" of the order of 80,000 yr or somewhat larger is possible. This Li phase is, nevertheless, not related to the 12C/13C ratio which appears to evolve, for these low-mass stars, over a much longer time. This model requires a rapid internal process of Li enrichment and depletion. New arguments appearing in the literature concerning this internal process are discussed. Reference is also made to the importance of these stars to the Galactic Li evolution.