Convective shells in the interior of Cepheid variable stars: overshooting models based on hydrodynamic simulations

TL;DR

This study uses 2D hydrodynamic simulations to analyze convective overshooting in Cepheid stars, revealing layered overshoot structures and proposing a new diffusion model for stellar interior mixing.

Contribution

It introduces a novel statistical approach to characterize overshooting layers and proposes a new diffusion coefficient model for better stellar interior mixing representation.

Findings

Overshooting layers are decomposed into weak and strong layers.

The ratio of overshooting lengths depends on the convective shell's size and depth.

A new diffusion coefficient model addresses overlapping overshooting layers.

Abstract

Because Cepheid variable stars have long been used as a cosmic benchmark, the accuracy of stellar evolution models for Cepheids have wide-reaching effects. Our goal is to provide a detailed multi-dimensional picture of hydrodynamic convection and convective boundary mixing in the interior of Cepheids. We perform 2D hydrodynamic simulations of six stars with the fully compressible Multidimensional Stellar Implicit Code (MUSIC). Our simulations do not model the radial pulsations but focus on the interior structure of Cepheids, which involves an interior convective shell and a convective envelope. We develop a new statistical analysis to examine overshooting in this inner convection zone. Using the extreme value theory, we find that overshooting above the convective shell fills the space between these convectively unstable layers. We develop a new statistical analysis that provides a clearer picture of how overshooting fills this layer, and also allows us to formulate a detailed comparison between overshooting above and below the convective shell. Our analysis effectively decomposes the overshooting layer into two layers: a weak and a strong overshooting layer. Statistically, this is accomplished by decomposing the strongly non-Gaussian probability density function into a mixture of Gamma distributions. Using our mixture model, we show that the ratio of overshooting lengths above and below the convective shell depends directly on the radial extent of the convective shell as well as its depth in the star. We propose a new form for the diffusion coefficient, which addresses the need for overlapping overshooting layers between convective shells. We introduce the idea of super-mixing layer where overshooting from both the convective shell and the convective envelope results in efficient mixing and could be viewed as merging the two adjacent convective zones.