On the possibility of chemically driven convection in red giants. Implications for the He-core flash and mixing above the Red Giant Branch Bump

TL;DR

This paper challenges standard criteria for mixing in stellar interiors, proposing an alternative that suggests chemically driven convection could occur under smaller mean molecular weight inversions, impacting stellar evolution models.

Contribution

It derives a new criterion for instability, demonstrating chemically driven convection may occur more readily than previously thought in red giant stars.

Findings

Standard criteria do not accurately distinguish instability regimes.

Chemically driven convection can occur with smaller mean molecular weight inversions.

Carbon production during the He-flash can sustain steady convection.

Abstract

Turbulent mixing remains one of the primary uncertainties in the modeling of stellar interiors. In stellar evolution simulations, regions where mixing occurs are typically identified using instability criteria. A particularly interesting situation arises when nuclear reactions produce inversions in the mean molecular weight within stellar interiors. Under these conditions, the material can become unstable to either thermohaline or a Rayleigh-Taylor instabilities. We demonstrate that the standard criterion adopted in stellar evolution calculations does not accurately distinguish between these two regimes. We derive an alternative criterion and show that chemically driven convection in stellar interiors might be viable under much smaller mean molecular weight inversions than it is normally assumed. We investigate whether inversions in the mean molecular weight can trigger chemically driven convection above the red giant branch bump (RGBB) or during the helium core flash. We find that the inversion at the base of the convective envelope above the RGBB is too weak and short-lived to sustain steady-state convection. In contrast, rapid carbon production at the base of the He-flash-driven convective zone can maintain a steady chemically driven convective region. This process could significantly alter our understanding of the He-core flash and warrants further study.