Lithium during the AGB evolution in young open clusters of the Large Magellanic Cloud^*

¹ INAF, Osservatorio Astronomico di Roma , via Frascati 33, 00040 Monteporzio C. (RM), Italy e-mail: surname@coma.mporzio.astro.it
² GRAAL, Université Montpellier II, 34095 Montpellier Cedex 5, France
³ ISO Data Centre, Science Operations and Data Systems Division, Research and Scientific Support Department of ESA, VILSPA, 28080 Madrid, Spain

Corresponding author: C. Maceroni, maceroni@coma.mporzio.astro.it

Received: 29 July 2002
Accepted: 27 August 2002

Abstract

We present the results of mid-resolution spectroscopy in the 6708 Å spectral region of Asymptotic Giant Branch (AGB) stars belonging to young open clusters of the Large Magellanic Cloud. Most stars belong to the clusters NGC 1866 and NGC 2031, which have an age of 150 Myr. Lithium lines of different strength are detected in the spectra of stars evolving along the AGB, in agreement with theoretical predictions. According to stellar evolutionary models, at the start of the AGB the stars should all show a low residual lithium abundance as a consequence of dilution during the previous evolutionary phases. The most luminous and cooler thermally pulsating AGB stars, if they are massive enough, once in the AGB go first through a phase of Li destruction, which is followed by a phase of strong lithium production and further destruction. The production of lithium on the AGB is in particular explained by the onset of the “Hot Bottom Burning" (HBB) process. Our most conclusive results are obtained for the populous cluster NGC 1866 where: the “early–AGB” stars show a weak Li line, which can be attributed to the dilution of the initial abundance; one of the two luminous stars seem to have completely depleted lithium, as no line is detected; the second one shows a deep lithium line, whose strength can be explained by lithium production. The bolometric magnitude of these stars are consistent with the values predicted by the theory, for the mass evolving on the AGB of NGC 1866, at which lithium is first destroyed and then produced (M_bol). We also analyze the infrared luminosities (ISOCAM data) of these stars, to discuss if their evolutionary phase precedes or follows the lithium production stage. More intriguing and less clear results are obtained for the most luminous stars in NGC 2031: the brightest star seems as well to have destroyed lithium, while the second one shows a strong lithium line. However its bolometric luminosity – derived from the near–IR photometry, is much lower (M_bol) than that expected from HBB models. Although low luminosity lithium rich AGB stars are also known, whose appearance is attributed to non–canonical mixing processes, it is not clear why two almost coeval clusters show such a different behaviour. It is also possible that this star does not belong to NGC 2031. Finally we suggest the observational tests that could shed further light on this matter.