No evidence for intense, cold accretion on to YSOs from measurements of Li in T-Tauri stars

We have used medium-resolution spectra to search for evidence that proto-stellar objects accrete at high rates during their early `assembly phase'. Models predict that depleted lithium and reduced luminosity in T-Tauri stars are key signatures of 'cold' high-rate accretion occurring early in a star's evolution. We found no evidence in 168 stars in NGC 2264 and the Orion nebula cluster for strong lithium depletion through analysis of veiling-corrected 6708 A lithium spectral line strengths. This suggests that 'cold' accretion at high rates (M >= 5 x 10(-4) M-circle dot yr(-1)) occurs in the assembly phase of fewer than 0.5 per cent of 0.3 < M-star < 1.9 M-circle dot stars. We also find that the dispersion in the strength of the 6708 A lithium line might imply an age spread that is similar in magnitude to the apparent age spread implied by the luminosity dispersion seen in colour-magnitude diagrams. Evidence for weak lithium depletion (< 10 per cent in equivalent width) that is correlated with luminosity is also apparent, but we are unable to determine whether age spreads or accretion at rates less than 5 x 10(-4) M-circle dot yr(-1) are responsible.