Main Sequence Evolution with Layered Semiconvection

TL;DR

This paper investigates layered semiconvection in main sequence stars, showing that realistic layer heights lead to mixing behavior similar to models using the Schwarzschild criterion, with implications for stellar evolution and observable properties.

Contribution

First stellar evolution models incorporating a semiconvection prescription based on numerical simulations, revealing the critical layer height influencing mixing regimes and stellar evolution.

Findings

Layer height determines weak or strong semiconvective mixing.

Realistic layer heights result in evolution similar to Schwarzschild models.

Layered semiconvection affects convective core sizes and stellar observables.

Abstract

Semiconvection - mixing that occurs in regions that are stable when considering compositional gradients, but unstable when ignoring them - is shown to have the greatest potential impact on main sequence stars with masses in the range 1.2 - 1.7 solar masses. We present the first stellar evolution calculations using a prescription for semiconvection derived from extrapolation of direct numerical simulations of double-diffusive mixing down to stellar parameters. The dominant mode of semiconvection in stars is layered semiconvection, where the layer height is an adjustable parameter analogous to the mixing length in convection. The rate of mixing across the semiconvective region is sensitively dependent on the layer height. We find that there is a critical layer height that separates weak semiconvective mixing (where evolution is well-approximated by using the Ledoux criterion) from strong semiconvective mixing (where evolution is well-approximated by using the Schwarzschild criterion). This critical layer height is much smaller than the minimum layer height expected from simulations so we predict that for realistic layer heights, the evolution is nearly the same as a model ran with the Schwarzschild criterion. We also investigate the effects of compositional gradient smoothing, finding that it causes convective cores to artificially shrink in the absence of additional mixing beyond the convective boundary. Layered semiconvection with realistic layer heights provides enough such mixing that stars will still evolve as if the Schwarzschild criterion is employed. Finally, we discuss the potential of detecting such semiconvection and its implication on convective core sizes in solar-like oscillators.