Lithium abundance versus effective temperature. The “lithium isochrones” by Baraffe et al. (2015) in the 2–100 Myr range are overlaid with dotted lines. Arrows refer to upper and lower limits. Open squares, circles, and pentagons represent the position of the class III, transitional disks, and sub-luminous or flat energy distribution targets, respectively. Spectral types as in Fig. 1 are also shown.