Pre-MS depletion, accretion and primordial 7Li

TL;DR

This paper explores how pre-main sequence lithium depletion, combined with accretion processes and overshoot mixing, can explain observed lithium abundances in stars and stellar populations, challenging traditional views.

Contribution

It introduces a model where pre-MS lithium destruction is followed by accretion, accounting for observed lithium levels and explaining discrepancies in stellar lithium abundances.

Findings

Pre-MS lithium depletion can be offset by accretion restoring lithium levels.

Overshoot mixing significantly enhances lithium destruction during pre-MS.

The model explains the lithium abundance variations in different stellar populations.

Abstract

We reconsider the role of pre-main sequence (pre-MS) Li depletion on the basis of new observational and theoretical evidence: i) new observations of Halpha emissions in young clusters show that mass accretion could be continuing till the first stages of the MS, ii) theoretical implications from helioseismology suggest large overshooting values below the bottom of the convective envelopes. We argue here that a significant pre-MS 7Li destruction, caused by efficient overshoot mixing, could be followed by a matter accretion after 7Li depletion has ceased on MS thus restoring Li almost to the pristine value. As a test case we show that a halo dwarf of 0.85 Msun with an extended overshooting envelope starting with an initial abundance of A(Li) = 2.74 would burn Li completely, but an accretion rate of the type 1e-8xe^{-t/3e6} Msun yr$^{-1}$ would restore Li to end with an A(Li) = 2.31. A self-regulating process is required to produce similar final values in a range of different stellar masses to explain the PopII Spite plateau. However, this framework could explain why open cluster stars have lower Li abundances than the pre-solar nebula, the absence of Li in the most metal poor dwarfs and a number of other features which lack of a satisfactory explanation.