A Test of Pre-Main-Sequence Lithium Depletion Models

TL;DR

This study tests pre-main-sequence lithium depletion models using high-resolution spectra of Beta Pictoris Moving Group stars, revealing discrepancies between lithium depletion ages and HR diagram ages, especially for mid-M stars.

Contribution

It provides empirical evidence that current models under-predict lithium depletion, highlighting the need to refine stellar evolutionary models for young stars.

Findings

Models under-predict lithium depletion for BPMG stars.

Lithium depletion ages are systematically older than HR diagram ages.

Discrepancies may relate to M-dwarf radius predictions.

Abstract

Despite the extensive study of lithium depletion during pre-main-sequence contraction, studies of individual stars show discrepancies between ages determined from the HR diagram and ages determined from lithium depletion (Song et al. 2002, White & Hillenbrand 2005) indicating open questions in the pre-main-sequence evolutionary models. To further test these models, we present high resolution spectra for members of the Beta Pictoris Moving Group (BPMG), which is young and nearby. We measure equivalent widths of the 6707.8 Angstrom Li I line in these stars and use them to determine lithium abundances. We combine the lithium abundance with the predictions of pre-main-sequence evolutionary models in order to calculate a lithium depletion age for each star. We compare this age to the age predicted by the HR diagram of the same model. We find that the evolutionary models under-predict the amount of lithium depletion for the BPMG given its nominal HR diagram age of ~12 Myr (Zuckerman et al. 2001), particularly for the mid-M stars, which have no observable Li I line. This results in systematically older ages calculated from lithium depletion isochrones than from the HR diagram. We suggest that this discrepancy may be related to the discrepancy between measured M-dwarf radii and the smaller radii predicted by evolutionary models.