⁷Li surface abundance in pre-main sequence stars

Abstract

Context.The disagreement between theoretical predictions and observations for surface lithium abundance in stars is a long-standing problem, which indicates that the adopted physical treatment is still lacking in some points. However, thanks to the recent improvements in both models and observations, it is interesting to analyse the situation to evaluate present uncertainties. Aims. We present a consistent and quantitative analysis of the theoretical uncertainties affecting surface lithium abundance in the current generation of models. Methods. By means of an up-to-date and well tested evolutionary code, FRANEC, theoretical errors on surface 7 Li abundance predictions, during the pre-main sequence (pre-MS) and main sequence (MS) phases, are discussed in detail. Then, the predicted surface 7 Li abundance was tested against observational data for five open clusters, namely Ic 2602, α Per, Blanco1, Pleiades, and NGC 2516, and for four detached double-lined eclipsing binary systems. Stellar models for the aforementioned clusters were computed by adopting suitable chemical composition, age, and mixing length parameter for MS stars determined from the analysis of the colour−magnitude diagram of each cluster. We restricted our analysis to young clusters, to avoid additional uncertainty sources such as diffusion and/or radiative levitation efficiency. Results. We confirm the disagreement, within present uncertainties, between theoretical predictions and 7 Li observations for standard models. However, we notice that a satisfactory agreement with observations for 7 Li abundance in both young open clusters and binary systems can be achieved if a lower convection efficiency is adopted during the pre-MS phase with respect to the MS one.

Single stars in the Hyades open cluster

“…We adopt the recent Asplund et al (2009) solar metal distribution, and the corresponding mixing length parameter calibrated on the Sun, namely α ML = 1.74. We also include a mild convective core overshooting (β ov = 0.2) for M ≥ 1.2M (Tognelli et al 2012). The models have been computed in the mass range [0.1, 2.8] M from the early pre-MS evolution up to the exhaustion of the central hydrogen.…”

“…In comparison to the Pleiades (d ∼ 120-140 pc; Percival et al 2005;van Leeuwen 2009), the Hyades open cluster is closer to the Sun (d ∼ 45 pc; Perryman et al 1998;van Leeuwen 2009). This allows us to resolve companions with smaller physical separations, and also to analyze less luminous, hence less massive, objects.…”

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

“…The prediction of surface abundances for lithium, beryllium, and boron in stars still represents an unsolved and challenging task for astrophysics since they strongly depend on the adopted input physics in theoretical models (e.g., nuclear reaction rates, opacity of the stellar matter, equation of state, efficiency of microscopic diffusion, etc., see, e.g., Piau Tognelli et al 2012) as well as on the assumed external convection efficiency. The difficulty in calculating these stellar abundances is proven, for example, by the still present discrepancy between theoretical predictions and observational 7 Li data (the so-called "lithium-problem," see, e.g., Charbonnel et al 2000;Deliyannis et al 2000;Pinsonneault et al 2000;Baraffe & Chabrier 2010;Talon & Charbonnel 2010).…”

AN UPDATED6Li(p, α)3He REACTION RATE AT ASTROPHYSICAL ENERGIES WITH THE TROJAN HORSE METHOD

“…In general, the effect of adopting different 6 Li burning reaction rates, although not negligible, is less important than the effects due to errors in the input parameters (e.g., original metallicity, helium abundance, external convection efficiency) and on the uncertainties in some other physical inputs adopted in the calculations (e.g., opacity evaluation), (see, e.g., the discussion in Pizzone et al 2005 andTognelli et al 2012).…”

“…However, uncertainties on the efficiency of some physical processes such as superadiabatic convection, microscopic diffusion, and radiative acceleration (see, e.g., Richard et al 2002Richard et al , 2005Michaud et al 2000;Turcotte et al 1998;Gratton et al 2001) affect the predicted abundances of light elements in stellar atmospheres. In addition, the predicted light element depletion is strongly sensitive to the adopted physical inputs, such as nuclear reaction rates, equation of state, and opacity of the stellar matter (see, e.g., Tognelli et al 2012). An idea of the uncertainties on the current theories is provided by the discrepancies among recent evolutionary models computed by different authors (Siess et al 2000;Yi et al 2001;Dotter et al 2008;Di Criscienzo et al 2009;Tognelli et al 2011).…”