Modeling Li I and K I sensitivity to Pleiades activity

We compare schematic modeling of spots and plage on the surface of cool dwarfs with Pleiades data to assess effects of magnetic activity on the strengths of the Li i and K i resonance lines in Pleiades spectra. Comprehensive Li i and K i NLTE line formation computation is combined with comparatively well-established empirical solar spot and plage stratifications for solar-like stars. For other stars, we use theoretical constructs to model spots and plage that portray recipes commonly applied in stellar activity analyses. We find that — up to B−V ≈ 1.1 — neither the Li i 670.8 nm nor the K i 769.9 nm line is sensitive to the presence of a chromosphere, in contrast to what is often supposed. Instead, both lines respond to the effects of activity on the stratification in the deep photosphere. They do so in similar fashion, making the K i line a valid proxy to study Li i line formation without spread from abundance variations. The computed effects of activity on line strength are opposite between plage and spots, differ noticeably between the empirical and theoretical solar-like stratifications, and considerably affect stellar broad-band colors. Our results indicate that one can neither easily establish, nor easily exclude, magnetic activity asmajor provider ofK i line strength variation in thePleiades. Since Li i line formation follows K i line formation closely, the same holds for Li i and the apparent lithium abundance.