ON A RAPID LITHIUM ENRICHMENT AND DEPLETION OF K-GIANT-STARS

A model scenario has recently been introduced by de la Reza and collea gues to explain the presence of very strong Li lines in the spectra of some low-mass K giant stars. In this scenario all ordinary, Li-poor, K giants become Li rich during a short time (similar to 10(5) yr) when compared to the red giant phase of 5 x 10(7) yr. In this ''Li period, '' a large number of the stars are associated with an expanding thin c ircumstellar shell supposedly triggered by an abrupt internal mixing m echanism resulting in a surface new Li-7 enrichment. This Letter prese nts nearly 40 Li-rich K giants known up to now. The distribution of th ese Li-rich giants, along with 41 other observed K giants that have sh ells but are not Li rich, in a color-color IRAS diagram confirms this scenario, which indicates, also as a new result, that a rapid Li deple tion takes place on a timescale of between similar to 10(3) and 10(5) yr. This model explains the problem of the presence of K giants with f ar-infrared excesses presented by Zuckerman and colleagues. Other pres ent and future tests of this scenario are briefly discussed.