The Gaia-ESO Survey: lithium depletion in the Gamma Velorum cluster and inflated radii in low-mass pre-main-sequence stars

We show that non-magnetic models for the evolution of pre-main-sequence(PMS) stars cannot simultaneously describe the colour-magnitude diagram(CMD) and the pattern of lithium depletion seen in the cluster of young,low-mass stars surrounding gamma(2) Velorum. The age of 7.5 +/- 1 Myrinferred from the CMD is much younger than that implied by the strong Lidepletion seen in the cluster M-dwarfs, and the Li depletion occurs atmuch redder colours than predicted. The epoch at which a star of a givenmass depletes its Li and the surface temperature of that star are bothdependent on its radius. We demonstrate that if the low-mass stars haveradii similar to 10 per cent larger at a given mass and age, then boththe CMD and the Li-depletion pattern of the Gamma Velorum cluster areexplained at a common age of similar or equal to 18-21 Myr. This radiusinflation could be produced by some combination of magnetic suppressionof convection and extensive cool starspots. Models that incorporateradius inflation suggest that PMS stars, similar to those in the GammaVelorum cluster, in the range 0.2 < M/M-circle dot < 0.7, are at least afactor of 2 older and similar to 7 per cent cooler than previouslythought and that their masses are much larger (by > 30 per cent) thaninferred from conventional, non-magnetic models in theHertzsprung-Russell diagram. Systematic changes of this size may be ofgreat importance in understanding the evolution of young stars, disclifetimes and the formation of planetary systems.