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On the nature of lithium-rich giant stars. Constraints from beryllium abundances
We have derived beryllium abundances for 7 Li-rich giant (A(Li) >1.5) stars and 10 other Li-normal giants with the aim of investigatingthe origin of the lithium in the Li-rich giants. In particular, we testthe predictions of the engulfment scenario proposed by Siess & Livio(1999, MNRAS, 308, 1133), where the engulfment of a brown dwarf or oneor more giant planets would lead to simultaneous enrichment of7Li and 9Be. We show that regardless of theirnature, none of the stars studied in this paper were found to havedetectable beryllium. Using simple dilution arguments we show thatengulfment of an external object as the sole source of Li enrichment isruled out by the Li and Be abundance data. The present results favor theidea that Li has been produced in the interior of the stars by aCameron-Fowler process and brought up to the surface by an extra mixingmechanism.
| Beryllium abundance in lithium-rich giants
About 2% of the Population I red giants show lithium abundancessignificantly larger than expected by dilution due to mixing byclassical convection and, in some of them, the lithium abundance reachesvalues similar to (and even larger than) the Pop I value (meteoritic,open clusters etc.) around logN(Li) = 3.3. The classical convectionpredicts also a (smaller) dilution of beryllium and (an even smallerone) of boron. Two main interpretations of the Li-rich giants arepossible: the initial Li has been somehow preserved (perhaps by theinhibition of the classical mixing) on the contrary, a mixing, deeperthan the classical one, took place in some (or all?) giants, leading toa dilution of the superficial lithium but sometimes overcompensating forit by an internal production of lithium, with transport to the surface(by the Cameron-Fowler mechanism). In the first interpretation, Be (morerobust than Li) is a fortiori preserved, in the second one, a mixingdeeper than the classical one should dilute Be more than the classicalBe dilution. We have observed the Be II lambda 3130.420 and 3131.066Angstroms lines in two Li-rich giants and three reference stars: oneLi-poor giant and alpha Cen A and B. The observations were carried outat the ESO 3.6m telescope using the CASPEC spectrograph. By comparingthe observed spectra with spectrum synthesis calculations we show thatfor the three giant stars the Be abundance is low, suggesting that, inthese giants, Be is very depleted (>90%) from the initial Pop I value(logN(Be) ~ 1.4). This result implies that the original Li in thesestars must have been almost completely destroyed, and that the high Liabundances in the Li-rich red giants is most probably due to Liproduction in these stars. Observations collected at the EuropeanSouthern Observatory -- ESO, Chile
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