2025MNRAS.539.3364J

Lithium is predicted, and observed, to be depleted in contracting low-mass pre-main-sequence (PMS) stars. Yet, these stars reach the zero-age main-sequence (ZAMS) with a spread in lithium abundance at a given effective temperature that is not predicted by standard stellar evolutionary models and which appears to be correlated with rotation. Using a homogeneous data set provided by the Gaia-ESO spectroscopic survey, we have followed the evolving photospheric lithium content of cohorts of stars destined to be ZAMS late G-, K-, and M-dwarfs, in clusters at ages of 2-300 Myr. We show that a dispersion in the Li I 6708 Å line strength develops in the lower mass stars after 10-20 Myr on the PMS, as soon as Li depletion begins, even in fully convective stars. A model based on a surface star-spot coverage varying from star-to-star, leading to a differential Li-burning rate, can explain this temporal behaviour and its mass dependence. However, to fully explain the magnitude of the Li dispersion and its correlation with rotation, the spot coverage during Li-burning would need to be a factor of 2 larger on average than measured in ZAMS clusters like the Pleiades and continue increasing with rotation in PMS stars beyond the usual 'saturation limit' observed for other magnetic activity indicators.