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

Abstract

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 6708Å lithium spectral line strengths. This suggests that 'cold' accretion at high rates (M = 5 × 10-4 M⊙ yr-1) occurs in the assembly phase of fewer than 0.5 per cent of 0.3 = M⊙ = 1.9M⊙ stars. We also find that the dispersion in the strength of the 6708Å 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 × 10-4 M⊙ yr-1 are responsible. ©2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.