Lithium abundances and extra mixing processes in evolved stars of M67

TL;DR

This study analyzes lithium abundances and rotation in evolved stars of M67, providing observational constraints and testing non-standard stellar evolution models that include atomic diffusion and rotation-induced mixing.

Contribution

It offers the first consistent interpretation of lithium and rotation patterns in M67's evolved stars, integrating observations with advanced stellar evolution models.

Findings

Lithium abundance decreases along stellar evolution from turn-off to giant phase.

Rotation-induced mixing explains the lithium depletion observed.

Surface rotation velocities correlate with lithium abundance patterns.

Abstract

Aims. We present a spectroscopic analysis of a sample of evolved stars in M67 (turn-off, subgiant and giant stars) in order to bring observational constraints to evolutionary models taking into account non-standard transport processes. Methods. We determined the stellar parameters (Teff, log g, [Fe/H]), microturbulent and rotational velocities and, Lithium abundances (ALi) for 27 evolved stars of M67 with the spectral synthesis method based on MARCS model atmospheres. We also computed non-standard stellar evolution models, taking into account atomic diffusion and rotation-induced transport of angular momentum and chemicals that were compared with this set of homogeneous data. Results. The lithium abundances that we derive for the 27 stars in our sample follow a clear evolutionary pattern ranging from the turn-off to the Red Giant Branch. Our abundance determination confirms the well known decrease of lithium content for evolved stars. For the first time, we provide a consistent interpretation of both the surface rotation velocity and of the lithium abundance patterns observed in an homogeneous sample of TO and evolved stars of M67. We show that the lithium evolution is determined by the evolution of the angular momentum through rotation-induced mixing in low-mass stars, in particular for those with initial masses larger than 1.30 M_\odot at solar metallicity.