Lithium isotopes in population II dwarfs

Abstract

We investigate the history of $^6$Li and $^7$Li in population II dwarfs during the pre main sequence and main sequence. The evolution is followed using the CESAM code and taking into account the most recent physics. The effective temperature ranges from $\approx$ 4700 K to $\approx$ 6400 K and therefore concerns objects on the so-called Spite plateau and cooler. We find the $^7$Li pre main sequence depletion is unable to account for the observations in the halo whatever the effective temperature. This supports microscopic diffusion and an additional non standard mixing process both acting during the main sequence. On the contrary the models $^6$Li pre main sequence depletion appears too strong and is marginally compatible with recent detections. During the main sequence we introduce the effects of tachocline diffusion. This process is a rotationnally induced mixing acting at the top of the radiative core. We show that the differences in the early rotation history cannot result in scattered lithium abundances on the Spite plateau. Moreover the tachocline mixing process predicts $^7$Li abundances in good agreement with the observations. We briefly address the question of turn off $^7$Li poor stars. A modest accretion of lithium free matter would be enough to explain their low abundance in this element. We expect the scatter in $^7$Li abundances is correlated to variations in [Fe/O] ratio for dwarfs cooler than 5500 K. Finally the tachocline mixing is robust with respect to the recent $^6$Li observations around the turn-off. We similarly suggest the [Fe/O] should be higher in objects with $^6$Li and effective temperature below 6000 K.