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

Abstract

Iron-rich layers are known to form in the stellar subsurface through a combination of gravitational settling and radiative levitation. Their presence, nature and detailed structure can affect the excitation process of various stellar pulsation modes, and must therefore be modeled carefully in order to better interpret Kepler asteroseismic data. In this paper, we study the interplay between atomic diffusion and fingering convection in A-type stars, and its role in the establishment and evolution of iron accumulation layers. To do so, we use a combination of three-dimensional idealized numerical simulations of fingering convection, and one-dimensional realistic stellar models. Using the three-dimensional simulations, we first validate the mixing prescription 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-Geneva Evolution code to study the evolution of the iron abundance profile beneath the stellar surface. We find, as first discussed by Théado et al. (2009), that when the concurrent settling of helium is ignored, this accumulation rapidly causes an inversion in the mean molecular weight profile, which then drives fingering convection. The latter mixes iron with the surrounding material very efficiently, and the resulting iron layer is very weak. However, taking helium settling into account partially stabilizes the iron profile against fingering convection, and a large iron overabundance can accumulate. The opacity also increases significantly as a result, and in some cases ultimately triggers dynamical convection.

arXiv comment38 pages, 16 figures, submitted to ApJ

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• On Carbon Burning in Super Asymptotic Giant Branch Stars
• Are exoplanetesimals differentiated?
• Excitation of gravity waves by fingering convection, and the formation of compositional staircases in stellar interiors
• Accretion of planetary matter and the lithium problem in the 16 Cygni stellar system
• Hydrodynamical instabilities induced by atomic diffusion in A stars and their consequences
• 2D or not 2D: the effect of dimensionality on the dynamics of fingering convection at low Prandtl number
• Long-Term Lithium Abundance Signatures following Planetary Engulfment
• Importance of fingering convection for accreting white dwarfs in the framework of full evolutionary calculations: the case of the hydrogen-rich white dwarfs GD133 and G29-38
• The long-term evolution and appearance of Type Iax postgenitor stars
• The interaction between shear and fingering (thermohaline) convection
• Asteroseismology of the exoplanet-host F-type star 94 Ceti : impact of atomic diffusion on the stellar parameters
• Matter accretion in metal-poor stars down to extremely metal-poor stars and the lithium problem
• New simulations of accreting DA white dwarfs: inferring accretion rates from the surface contamination
• Modelling of the scandium abundance evolution in AmFm stars
• Magnetized oscillatory double-diffusive convection