The non-LTE formation of Li I lines in cool stars

Abstract

We study the non-LTE (non local thermodynamic equilibrium) formation of Li I lines in the spectra of cool stars for a grid of radiative-equilibrium model atmospheres with variation in effective temperature, gravity, metallicity and lithium abundance. We analyze the mechanisms by which departures from LTE (local thermodynamic equilibrium) arise for Li I lines, first for the young sun (prior to its lithium depletion) and then across the cool-star grid. There are various mechanisms which compete in their effects on emergent Li I line strengths. Their neglect produces errors in lithium abundance determinations that vary in sign as well as size, both across the stellar grid and between different Li I lines (Figs). The errors are appreciable for all cooler stars and largest for cool lithium-rich metal-poor giants. They reverse sign between lithium-rich stars and lithium-poor stars for the λ=670.8nm resonance line, but not for the λ=610.4nm subordinate line. The non-LTE corrections are large enough that they should be taken into account in ongoing debates on lithium synthesis and depletion. We provide convenient numerical approximations of our results (Table 1) to this purpose. We end the paper with some examples in which non-LTE corrections change the slope of published relationships.