Chemical mixing in low mass stars

Abstract

Context. When modelling stars with masses higher than 1.2 M⊙ with no observed chemical peculiarity, atomic diffusion is often neglected because, on its own, it causes unrealistic surface abundances compared with those observed. The reality is that atomic diffusion is in competition with other transport processes. Rotation is one of the processes able to prevent excessively strong surface abundance variations. Aims. The purpose of this study is to quantify the opposite or conjugated effects of atomic diffusion (including radiative acceleration) and rotationally induced mixing in stellar models of low mass stars, and to assess whether rotational mixing is able to prevent the strong abundance variations induced by atomic diffusion in F-type stars. Our second goal is to estimate the impact of neglecting both rotational mixing and atomic diffusion in stellar parameter inferences for stars with masses higher than 1.3 M⊙. Methods. Using the Asteroseismic Inference on a Massive Scale (AIMS) stellar parameter inference code, we infer the masses and ages of a set of representative artificial stars for which models were computed with the Code d’Evolution Stellaire Adaptatif et Modulaire (CESTAM; the T stands for Transport) evolution code, taking into account rotationally induced mixing and atomic diffusion, including radiative acceleration. The observed constraints are asteroseismic and classical properties. The grid of stellar models used for the optimization search include neither atomic diffusion nor rotationally induced mixing. The differences between real and retrieved parameters then provide an estimate of the errors made when neglecting transport processes in stellar parameter inference. Results. We show that for masses lower than 1.3 M⊙, rotation dominates the transport of chemical elements and strongly reduces the effect of atomic diffusion, with net surface abundance modifications similar to solar values. At higher mass, atomic diffusion and rotation are competing equally. Above 1.44 M⊙, atomic diffusion dominates in stellar models with initial rotation lower than 80 km s−1 producing a chemical peculiarity which is not observed in Kepler Legacy stars. This indicates that a transport process of chemical elements is missing, probably linked to the missing transport process of angular momentum needed to explain rotation profiles in solar-like stars. Importantly, neglecting rotation and atomic diffusion (including radiative acceleration) in the models, when inferring the parameters of F-type stars, may lead to respective errors of ≈5%, ≈2.5%, and ≈25% for stellar masses, radii, and ages. Conclusions. Atomic diffusion (including radiative acceleration) and rotational mixing should be taken into account in stellar models in order to determine accurate stellar parameters. When atomic diffusion and shellular rotation are both included, they enable stellar evolution codes to reproduce the observed metal and helium surface abundances for stars with masses up to 1.4 M⊙ at solar metallicity. However, if rotation is actually uniform for these stars (as observations seem to indicate), then an additional chemical mixing process is needed together with a revised formulation of rotational mixing. For higher masses, an additional mixing process is needed in any case.

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

“…Turbulent mixing helps to avoid strong surface abundance variations of all chemical elements, but it is not efficient enough to bring Li down to its nuclear destruction region. On the contrary, rotation can induce a Li destruction but cannot reduce strong surface abundance variations of chemical elements (Deal et al 2020). Indeed, the 1.0 M model, which includes rotational mixing, reduces the surface Li abundance by more than one order of magnitude.…”

“…We considered the magnetised wind braking of Matt et al (2015Matt et al ( , 2019 and an additional vertical viscosity of ν v = 10 8 cm 2 s −1 as calibrated by Ouazzani et al (2019) to take into account the fact that the current rotation theory underestimates the transport of angular momentum. The other aspects of these models are the same as the ones presented in Deal et al (2020). These models are only used to show that rotation may explain Li depletion on the main sequence.…”

“…Earlier theoretical studies of atomic diffusion and mixing in stellar structure calculations (Richard et al 2002;Deliyannis & Pinsonneault 1990;Proffitt & Michaud 1991;Richer et al 1992;Vauclair 1999;Chaboyer et al 2001;Richard 2005) are now being superseded by the new generation of stellar evolution models (e.g. Théado et al 2009;Vick et al 2013;Zhang et al 2019;Deal et al 2020) that include both atomic diffusion and transport processes as thermohaline convection, mass loss, rotation, or accretion. A comprehensive review of the subject can be found in Salaris & Cassisi (2017).…”

Robust asteroseismic properties of the bright planet host HD 38529

“…Deal et al (2018) showed that this is an important process when inferring the properties of main-sequence stars. Deal et al (2020) further showed that modest rotation (about 30 km s −1 ) is insufficient to prevent a discernible effect on the stellar properties. Given that HD 38529 probably rotated more slowly, it may have experienced significant heavy element enhancement at its surface on the main sequence, even if much of the effect has since been erased by the growing convective envelope.…”

“…To roughly quantify the effect of these neglected processes, we first computed evolutionary tracks up to the observed eff = 5578 K with = 1.48 M , [Fe/H] = 0.34 and a rotation rate of 5 d at age 10 Myr as described in Deal et al (2020). Each track used one of the following combinations of the extra chemical transport processes above: rotation, gravitational settling and radiative levitation; gravitational settling and radiative levitation; only gravitational settling; and no extra chemical transport.…”