Off-centre convective zones in mass accreting stellar models

TL;DR

This paper explains the physical origin of transient off-centre convective zones in mass accreting stellar models, showing they result from local density increases that enhance opacity and trigger convection when certain stability thresholds are exceeded.

Contribution

It identifies the physical mechanism behind off-centre convective zones in accreting stars using detailed MESA simulations, linking density, opacity, and stability criteria.

Findings

Off-centre convective zones are caused by local density increases near the convective core edge.

These zones are transient and detached, with mixing erasing local compositional gradients.

The mechanism explains observations in massive interacting stars.

Abstract

We report the physical origin of transient off-centre convective zones (oCZs) that arise in mass accreting stellar models. Using detailed MESA simulations of binary evolution, we find that these oCZs are not numerical artefacts but emerge due to a local increase in density near the retreating edge of the convective core. The density enhancement raises the local opacity, which amplifies the radiative temperature gradient $\nabla_{\rm rad}$. If this gradient surpasses the Ledoux threshold $\nabla_{\rm L}$, defined by both thermal and compositional stratification, the region becomes convectively unstable. The resulting oCZs are detached from the convective core and transient: mixing within the oCZ erases the local gradient in mean molecular weight, leaving a sharp $\nabla_{\mu}$ discontinuity at the boundary, stabilizing the adjacent layers. This mechanism naturally explains the presence and evolution of oCZs, as previously reported in massive interacting stars.