Astron. Astrophys. 342, 831-838 (1999)

1. Introduction

About one percent of G-K giants show strong Li lines. Some of the Li-rich giants show Li abundances larger than that of the interstellar medium (log (Li) = 3.1, da Silva et al. 1995a,b). The overabundance of Li is unexpected from the standard first dredge-up evolutionary models, which predicts a strong Li depletion. Lithium is expected to be destroyed in all but the outermost layers (1 to 2 percent by mass) of a main-sequence star. On ascent of the red giant branch, a deepening convective envelope dilutes the remaining lithium and so reduces the photospheric lithium abundance by a large factor. G and K giants are therefore expected to have very low lithium abundances as a result of convective dilution. Iben's (1967a, b) models of solar metallicity predict the dilution at the tip of the red giant branch to amount to a factor of 60 for a 3 M star to a factor of 28 for a 1 M star. Therefore the red giants of solar metallicity are expected to have a maximum Li abundance of log (Li) 1.5, assuming that their main-sequence progenitors retained the log (Li) = 3.0. However, since many main-sequence stars show significant amount of dilution of Li, theoretical predictions refer to the maximum amount of Li abundance expected in a red giant. From observations of large sample of G and K giants it has been found that the average Li abundance in a typical G-K giant is of the order of log (Li) = 0.1 (Brown et al. 1989). This clearly indicates that there is extra Li dilution during the evolution from the zero age main-sequence phase to the tip of the red giant phase. However, a few (of the order of one percent) of the red giants have been observed to have lithium abundances in excess with the standard predictions (Wallerstein and Sneden 1982, Gratton and D'Antona 1989, Brown et al. 1989, Pilachowski et al. 1990). Some of these Li-rich G and K giants have been found to have Li abundance log (Li) = 3.0 and higher than the present interstellar medium abundance (da Silva et al. 1995a,b, de la Reza and da Silva 1995, de la Reza et al. 1996). Recently it has been found that several of these Li-rich red giants have infrared excesses, interpreted as associated circumstellar dust shells (Gregorio-Hetem et al. 1993, Castilho et al. 1998, de la Reza et al. 1997). Gregorio-Hetem et al. (1993) studied the IRAS colours of Li-rich G and K giants and concluded that the Li-rich giants define a locus on the IRAS color-color diagram (see also Castilho et al. 1998, de la Reza et al. 1997).

Recently, Zuckerman et al. (1995) from an analysis of IRAS point source catalogue have found that about one percent of G and K giants have circumstellar dust. From a similar analysis of the IRAS data Plets et al. (1997) also found several G and K giants with infrared excess. Since the post-main-sequence, first-ascent giants are not expected to show overabundance of Li and circumstellar dust, several theories have been proposed to explain the overabundance of Li and also to account for the circumstellar dust in G and K giants (Plets et al. 1997, de la Reza et al. 1996, 1997). De la Reza et al. (1996, 1997) proposed a scenario linking the high Li abundance of some of these stars to the evolution of circumstellar shells. In this model, every K giant becomes Li-rich during the red giant branch stage, and the internal mixing mechanism responsible for the Li enhancement will initiate a prompt mass-loss event. Recently, de la Reza et al. (1997) discovered several Li-rich giants with large infrared excesses. These Li-rich giants have not yet reached the AGB, and most of them are first-ascent low mass giants and have not experienced hot bottom burning. Therefore the overabundance of Li and circumstellar dust is a puzzle. In fact the study of Pilachowski et al. (1993) shows that there is extra lithium depletion by large factors occurring on the RGB subsequent to first dredge-up.

In order to further understand the connection between the infrared excess (circumstellar dust) and Li abundance in G and K giants we have obtained high resolution spectra of several G and K giants with circumstellar dust. The G and K giants with far-infrared emission excess were selected from the list published by Zuckerman et al. (1995). An analysis of the high resolution spectra of these stars in the Li line region is presented in this paper, first of a series. Other papers will follow with spectroscopic analysis of the same stars in other wavelength regions and with CORAVEL data.

© European Southern Observatory (ESO) 1999

Online publication: February 23, 1999
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