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

Abstract

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.

ASTROPHYSICAL IMPACT OF THE UPDATED9Be(p,α)6Li AND10B(p,α)7Be REACTION RATES AS DEDUCED BY THM

“…In agreement with theoretical predictions, these stars do not show any pre-MS 9 Be surface depletion and thus they are only marginally useful to further constrain convection efficiency during this evolutionary phase (see, e.g., Smiljanic et al 2011). On the contrary, smaller stars with a deeper and hotter bottom of the convective envelope, are expected to burn 9 Be in pre-MS, thus representing a more useful benchmark for testing convective models.…”

“…Up to now, 9 Be can be safely measured only in stars with temperatures higher than about 5000-5200 K (see, e.g., Garcia Lopez et al 1995;Randich et al 2007;Smiljanic et al 2011;Delgado Mena et al 2012). In agreement with theoretical predictions, these stars do not show any pre-MS 9 Be surface depletion and thus they are only marginally useful to further constrain convection efficiency during this evolutionary phase (see, e.g., Smiljanic et al 2011).…”

“…For less massive mid-aged stars (with an effective temperature of T 5600 eff  K), observations reveal an increasing 9 Be depletion while T eff decreases, in disagreement with standard theoretical calculations (no significant rotation effects, no magnetic fields, etc., see, e.g., Santos et al 2004;Randich et al 2007;Delgado Mena et al 2012). Such a depletion, which has not yet been seen in young clusters (e.g., Smiljanic et al 2011), seems to indicate a possible cause acting on the MS timescale. The level of discrepancy between predictions and observations, or the difference between surface Be abundance in depleted and undepleted stars is significantly larger than observational uncertainties and even larger than the initial 9 Be abundances (solar or meteoritic) adopted by different authors.…”

The Be-test in the Li-rich star #1657 of NGC 6397: evidence for Li-flash in RGB stars?

“…2, with the synthetic spectrum overimposed, computed as in Smiljanic et al (2011). The stellar spectrum has a low S /N ∼ 10 and shows no clear detection of the Be lines.…”

CUBES: cassegrain U-band Brazil-ESO spectrograph

“…Many different physical mechanisms have been proposed to explain these observations: atomic diffusion, mass loss, magnetic fields, rotation-induced mixing, mixing by internal gravity waves, or combinations of some of these processes. To help constrain the nature and description of the transport mechanisms of chemicals and angular momentum inside low-mass stars it is important to combine Li data with Be abundances (Smiljanic et al 2010(Smiljanic et al , 2011.…”