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

• Published in: Monthly notices of the Royal Astronomical Society Vol. 434; no. 2; pp. 966 - 977
• Main Authors: Sergison, Darryl J., Mayne, N. J., Naylor, Tim, Jeffries, R. D., Bell, Cameron P. M.
• Format: Journal Article
• Language: English
• Published: London Oxford University Press 2013
• Subjects: Astronomy; Astrophysics; Lithium; Star & galaxy formation; Stars & galaxies; open clusters and associations: individual: NGC 2264; open clusters and associations: individual: NGC 1976; stars: pre-main-sequence
• ISSN: 0035-8711; 1365-2966
• DOI: 10.1093/mnras/stt973

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 Å lithium spectral line strengths. This suggests that 'cold' accretion at high rates (Ṁ ≥ 5 × 10−4 M yr−1) occurs in the assembly phase of fewer than 0.5 per cent of 0.3 ≤ M ≤ 1.9 M stars. We also find that the dispersion in the strength of the 6708 Å 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.