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

TL;DR

This study used spectral analysis of T-Tauri stars to investigate whether high-rate cold accretion occurs during early star formation, finding no evidence for such intense accretion in most stars.

Contribution

The paper provides observational constraints showing that high-rate cold accretion is rare in young stars, challenging some theoretical models of early stellar assembly.

Findings

No strong lithium depletion detected in 168 stars.

High-rate cold accretion occurs in fewer than 0.5% of stars.

Lithium line strength dispersion suggests possible age spread.

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 6708 angstrom lithium spectral line strengths. This suggests that 'cold' accretion at high rates (M_dot > 5 x 10-4 M_sol yr-1) occurs in the assembly phase of fewer than 0.5 per cent of 0.3 < M < 1.9 M_sol stars. We also find that the dispersion in the strength of the 6708 angstrom 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_sol yr-1 are responsible.