Stars with M≤2:3M⊙ are considered Li depletion sites during their early and late evolutionary stages. Indeed 7Li, synthesized through electron-captures on 7Be, is burnt into two alpha particles, when the H-shell burns below convective envelopes of Red Giant Branch (RGB) and Asymptotic Giant Branch (AGB) stars. Furthermore, Li abundances observed in the spectra of these stars cover a wide range of values, which is difficult to be explained by stellar models, both Li-rich and Li-poor objects being observed. Difficulties arise in measuring very low Li abundances in O-rich AGBs and the lack of knowledge about the physical causes of extra-mixing, but the main source of uncertainty in investigating Li nucleosynthesis concerns the 7 Be life-time, as the avail- able estimations actually are valid only for the Sun [1]. Since in RGB and AGB H-shell burning temperatures and densities might be respectively up to five times higher and five orders of mag- nitudes lower than in the solar core, using for giants a 7Be life-time, extrapolated from the one valid in solar condition is at least hazardous. We present the consequence on Li nucleosynthesis of a new 7Be life-time estimation computed by a theoretical approach inherited from quantum chemistry. Extra-mixing models integrated with this new datum well reproduce the trends of Li destruction observed in RGBs and are suitable to account for Li observations in AGBs, explaining also the amount of nuclei from C to Al
Modeling the Li abundances of RGB and AGB stars with a new estimate for the 7Be half-life time
SIMONUCCI, Stefano;
2012-01-01
Abstract
Stars with M≤2:3M⊙ are considered Li depletion sites during their early and late evolutionary stages. Indeed 7Li, synthesized through electron-captures on 7Be, is burnt into two alpha particles, when the H-shell burns below convective envelopes of Red Giant Branch (RGB) and Asymptotic Giant Branch (AGB) stars. Furthermore, Li abundances observed in the spectra of these stars cover a wide range of values, which is difficult to be explained by stellar models, both Li-rich and Li-poor objects being observed. Difficulties arise in measuring very low Li abundances in O-rich AGBs and the lack of knowledge about the physical causes of extra-mixing, but the main source of uncertainty in investigating Li nucleosynthesis concerns the 7 Be life-time, as the avail- able estimations actually are valid only for the Sun [1]. Since in RGB and AGB H-shell burning temperatures and densities might be respectively up to five times higher and five orders of mag- nitudes lower than in the solar core, using for giants a 7Be life-time, extrapolated from the one valid in solar condition is at least hazardous. We present the consequence on Li nucleosynthesis of a new 7Be life-time estimation computed by a theoretical approach inherited from quantum chemistry. Extra-mixing models integrated with this new datum well reproduce the trends of Li destruction observed in RGBs and are suitable to account for Li observations in AGBs, explaining also the amount of nuclei from C to AlI documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
