Fingering convection induced by atomic diffusion in stars: 3D numerical computations and applications to stellar models

摘要

Iron-rich layers are known to form in the stellar subsurface through acombination of gravitational settling and radiative levitation. Their presence,nature and detailed structure can affect the excitation process of variousstellar pulsation modes, and must therefore be modeled carefully in order tobetter interpret Kepler asteroseismic data. In this paper, we study theinterplay between atomic diffusion and fingering convection in A-type stars,and its role in the establishment and evolution of iron accumulation layers. Todo so, we use a combination of three-dimensional idealized numericalsimulations of fingering convection, and one-dimensional realistic stellarmodels. Using the three-dimensional simulations, we first validate the mixingprescription for fingering convection recently proposed by Brown et al. (2013),and identify what system parameters (total mass of iron, iron diffusivity,thermal diffusivity, etc.) play a role in the overall evolution of the layer.We then implement the Brown et al. (2013) prescription in the Toulouse-GenevaEvolution code to study the evolution of the iron abundance profile beneath thestellar surface. We find, as first discussed by Th\'eado et al. (2009), thatwhen the concurrent settling of helium is ignored, this accumulation rapidlycauses an inversion in the mean molecular weight profile, which then drivesfingering convection. The latter mixes iron with the surrounding material veryefficiently, and the resulting iron layer is very weak. However, taking heliumsettling into account partially stabilizes the iron profile against fingeringconvection, and a large iron overabundance can accumulate. The opacity alsoincreases significantly as a result, and in some cases ultimately triggersdynamical convection.