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Astron. Astrophys. 358, L49-L52 (2000)

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1. Introduction

In low-mass red giants ([FORMULA]) the excursion of the base of the convective envelope (BCE) into the radiative zone separating it from the hydrogen-burning shell (HBS) initiates the first dredge-up episode during which the surface composition experiences modest changes. The best indicators of the 1st dredge-up are the ratio 12C/13C and the Li abundance, both decreasing. A decrease of the C and an increase of N are also predicted (Boothroyd & Sackmann 1999). In the overwhelming majority of the field red giants (K giants) the surface Li abundance is low. However, there are several percent of K giants which possess surprisingly high Li abundances, sometimes exceeding the initial value of [FORMULA] for Pop. I stars (Wallerstein & Sneden 1982; Hanni 1984; Brown et al. 1989; Berdyugina & Savanov 1994; Da Silva et al. 1995; De la Reza et al. 1996, 1997). It was not until recently when the evolutionary status of these Li-rich giants (LIRGs) became clearer. Owing to the more accurate luminosities estimated through the Hipparcos parallaxes Jasniewicz et al. (1999) have concluded that most of the LIRGs are past the 1st dredge-up red giant branch (RGB) stars (the low 12C/13C ratios observed in the LIRGs support this) and hence the initial abundance of Li in their atmospheres could not have been preserved.

De la Reza et al. (1997), Jasniewicz (1999) and other researchers argue convincingly for an internal mechanism of Li production in LIRGs. In both papers quoted the "7Be-transport" mechanism (Cameron & Fowler 1971) is considered as the most promising internal source of Li. This internal mechanism has recently received new support: Castilho et al. (1999) have reported that Be - as predicted - is very depleted (about 10 times) in both LIRGs they studied. In this Letter we propose a combined scenario of Li-enrichment: a giant planet (or brown dwarf) is engulfed by a red giant; this external source or event activates inside the giant the "7Be-mechanism" producing then Li internally. Before presenting our model in Sect. 3, we will discuss existing evidence for extra-mixing in red giants (Sect. 2), because this is crucial for scrutinizing any scenario. Sect. 4 concludes the paper.

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© European Southern Observatory (ESO) 2000

Online publication: June 8, 2000