On a Rapid Lithium Enrichment and Depletion of K Giant Stars

A model scenario has recently been introduced to explain the presence of verystrong Li lines in the spectra of some low mass K giant stars (de la Reza etal. 1996). In this scenario all ordinary, Li poor, K giants become Li richduring a short time ($\sim 10^5 yr$) when compared to the red giant phase of 510^7 yr. In this ``Li period'', a large part of the stars are associated withan expanding thin circumstellar shell supposedly triggered by an abruptinternal mixing mechanism resulting in a surface new ^7Li enrichment. Thisletter presents near 40 Li rich K giants known up to now. The distribution ofthese Li rich giants, along with other 41 observed K giants that have shell,but are not Li rich, in a color-color IRAS diagram confirms this scenario,indicating, also as a new result, that a rapid Li depletion takes place on atime scale of between $\sim 10^3$ and 10^5 yr. This model explains the problemof the presence of K giants with far infrared excesses presented by Zuckermanet al. (1995). Other present and future tests of this scenario are brieflydiscussed.Comment: 14 pages, 2 figure