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

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5. Conclusions

From our analysis of the Li region spectra of 29 giants from the list of Zuckerman et al. (1995) with infrared excess, eight stars were found to show log [FORMULA](Li) [FORMULA] 1.0. In the remaining stars log [FORMULA](Li) [FORMULA] 0.0. There seems to be no correlation between Li abundance and infrared excess.

HD 219025 is found to be a Li-rich (log [FORMULA](Li) = 3.0), rapidly rotating, and dusty K giant. The absolute visual magnitude of HD 219025 derived from the Hipparcos parallax shows that it is a K giant and not a pre-main-sequence star.

HD 169689 and HD 176884 are also found to be rapidly rotating Li-rich giants. HD 205349 and HD 152786 are found to be Li-rich K supergiants.

The position of Li-rich K giants in the HR diagram shows that most of them are brighter and more evolved than the "clump giants". The overabundance of Li is not confined to narrow range of luminosities. The Li-abundances of Li-rich giants is not correlated with their luminosities, rotational velocities, infrared excesses, chromospheric activity and carbon isotope ratios.

There are Li-poor (normal) giants with circumstellar dust, Li-rich giants with dust and with no dust. The number of Li-rich and dusty giants is much smaller than the number of Li-poor dusty giants. This indicates that they destroy their freshly synthesized Li much faster than they disperse their circumstellar dust. However, the presence of Li-rich giants with no circumstellar dust is a puzzle. Rotation, binarity, mass and evolutionary age on the red giant branch, and the dependence of extra deep mixing and the associated cool bottom processing on these parametres need to be explored in order to understand the dust and Li problem in late-type giants.

Preserving the initial pre-main-sequence Li or accreting Li from external sources (such as engulfing planets or brown dwarfs during the giant phase) will not be able to explain the Li-rich K giant phenomenon. Li seems to have been created inside the star and brought to the surface. Recently Sackmann and Boothroyd (1998) demonstrated that Li can be created in low mass red giants via the Cameron-Fowler mechanism, due to extra deep mixing and the associated "cool bottom processing". It is not yet clear whether this will trigger mass loss. Also the effect of rapid rotation on deep circulation and cool bottom processing needs to be explored in order to understand the Li-rich red giants with rapid rotation and circumstellar dust.

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© European Southern Observatory (ESO) 1999

Online publication: February 23, 1999