Mixing at young ages: beryllium abundances in cool main-sequence stars of the open clusters IC 2391 and IC 2602

Context. Determination of lithium abundances in stars of young clusters has shown that they deplete Li by different degrees during their pre-main sequence phase. Beryllium abundances are complementary to the Li ones, and can help tracing the mixing processes in the stellar interiors. Aims. Our aim is to derive Be abundances in a sample of G- and K-type stars of two young pre-main sequence open clusters, IC 2391 and IC 2602. The Be abundances are used to investigate the mixing of internal material in these stars. The reliability of the Be lines as abundance indicators in low temperatures is also investigated in detail. Methods. We derived Be abundances from high-resolution, high signal-to-noise UVES/VLT spectra using spectrum synthesis and model atmospheres. Atmospheric parameters and other elemental abundances are adopted from a previous work. Results. The sample stars have masses in the range between 0.80 ≤ M/M⊙ ≤ 1.20. They have been shown to differ in Li abundance by about 0.60 dex, with lower A(Li) in cooler and lower mass stars. Here, we find that all the stars have the same Be abundance within the uncertainties. These observations show that the Be abundance is not affected by the mixing events in the pre-main sequence, in this mass range, in agreement with the expectation of evolutionary models. A comparison with Be abundances in older clusters shows that, contrary to the models, cool stars deplete Be during their main-sequence lifetime, confirming what has been previously suggested in the literature