The Gaia-ESO survey: 3D NLTE abundances in the open cluster NGC 2420 suggest atomic diffusion and turbulent mixing are at the origin of chemical abundance variations

Abstract

Context. Atomic diffusion and mixing processes in stellar interiors influence the structure and the surface composition of stars. Some of these processes cannot yet be modelled from the first principles, and they require calibrations. This limits their applicability in stellar models used for studies of stellar populations and Galactic evolution. Aims. Our main goal is to put constraints on the stellar structure and evolution models using new refined measurements of the chemical composition in stars of a Galactic open cluster. Methods. We used medium-resolution, 19 200 ≤ R ≤ 21 500, optical spectra of stars in the open cluster NGC 2420 obtained within the Gaia-ESO survey. The sample covers all evolutionary stages from the main sequence to the red giant branch. Stellar parameters were derived using a combined Bayesian analysis of spectra, 2MASS photometry, and astrometric data from Gaia DR2. The abundances of Mg, Ca, Fe, and Li were determined from non-local thermodynamic equilibrium (NLTE) synthetic spectra, which were computed using one-dimensional (1D) and averaged three-dimensional (3D) model atmospheres. We compare our results with a grid of Code d’Evolution Stellaire Adaptatif et Modulaire (CESTAM) stellar evolution models, which include atomic diffusion, turbulent, and rotational mixing. Results. We find prominent evolutionary trends in the abundances of Fe, Ca, Mg, and Li with the mass of the stars in the cluster. Furthermore, Fe, Mg, and Ca show a depletion at the cluster turn-off, but the abundances gradually increase and flatten near the base of the red giant branch. The abundance trend for Li displays a signature of rotational mixing on the main sequence and abrupt depletion on the sub-giant branch, which is caused by advection of Li-poor material to the surface. The analysis of abundances combined with the CESTAM model predictions allows us to place limits on the parameter space of the models and to constrain the zone in the stellar interior, where turbulent mixing takes place.

Lithium depletion and angular momentum transport in F-type and G-type stars in Galactic open clusters

“…On the other hand, the use of the Zahn (1992) prescription for the horizontal shear diffusivity (D h ) and that of Talon & Zahn (1997), which includes the effect of thermal and molecular diffusion for the vertical shear diffusivity (D v ) in model M ν R2 T 6.42 A , requires higher values for the parametric viscosity for the internal transport of angular momentum, with a mass-dependent efficiency as expected from IGW. The choice of these prescriptions thus impacts our ability to constrain the other transport processes (see Meynet et al 2013;Amard et al 2016) as additionally illustrated by the predictions obtained in this work and for instance by Semenova et al (2020) for the specific case of NGC 2420. Insight and validation of the different available prescriptions for the vertical turbulent shear transport from hydrodynamicists and multi-dimensional numerical simulations, similar to the ongoing work on the horizontal turbulent shear (Park et al 2020(Park et al , 2021, are now mandatory in order to overcome this impasse.…”

Mapping the Galactic Metallicity Gradient with Open Clusters: The State-of-the-Art and Future Challenges

“…It provides stellar parameters, metallicity, and elemental abundances, radial velocities, and additional products, such as gravity index, chromospheric activity tracers, mass accretion rate diagnostics, and veiling. Several works in recent years have been devoted to the study of open clusters observed by the GES, among many we recall the latest ones: Bertelli Motta et al [76], Magrini et al [77,78,79], Prisinzano et al [80], Hatzidimitriou et al [81], Casali et al [82,83], Baratella et al [84], Randich et al [85], Jackson et al [86], Bonito et al [87], Gutiérrez Albarrán et al [88], Semenova et al [89], Binks et al [90]. In the following sections, we adopt the average abundances from IDR6 for the member stars of 57 clusters with ages ≥ 120 Myr published in Magrini et al [79].…”

The SAPP pipeline for the determination of stellar abundances and atmospheric parameters of stars in the core program of the PLATO mission

“…For small convective cores, f is decreased linearly from 0.02 down to 0 for stellar masses from 1.4 down to 1.1 M . This prescription has been found to accurately describe results from binary stars (Higl et al 2018) and mid-age open clusters (Semenova et al 2020). No convective core overshooting in the main sequence is used below 1.1 M .…”