The evolution of lithium in FGK dwarf stars: The lithium-rotation connection and the Li desert

Abstract

We investigate two topics regarding solar mass FGK-type stars, the lithium-rotation connection (LRC), and the existence of the 'lithium desert'. We determine the minimum critical rotation velocity (v sin i), related with the LRC separating slow from rapid stellar rotators, to be 5 km s-1. This value also splits different stellar properties. For the first time we explore the behaviour of the LRC for some stellar associations with ages between 45 Myr and 120 Myr. This allows us to study the LRC age dependence at the beginning of the general spin-down stage for low-mass stars, which starts at ∼30-40 Myr. We find that each stellar group presents a characteristic minimum lithium depletion connected to a specific high rotation velocity and that this minimum changes with age. For instance, the minimum changes from ∼50 km s-1 to less than 20 km s-1 in 200 Myr. This desert was described as a limited region in the A(Li)-Teff map containing no stars. Using Teff from Gaia DR2 we detect 30 stars inside and/or near the same box originally defined as the lithium desert. Due to their intrinsic Teff errors some of these stars may be inside or outside the box, implying to a high probability that the box contains several stars. This leads us to consider that the lithium desert appears to be more a statistical distribution fluctuation than a real problem. We conclude that the lithium desert is rather due to a statistical distribution fluctuation than a real physical problem.