A Thermodynamically Induced Finite-Amplitude Convective Instability in Stellar Envelopes

Abstract

Stellar envelopes are subject to a finite-amplitude convective instability that originates with the reduction in the adiabatic exponent Γ1 = ad accompanying partial ionization of the principle plasma constituents, notably hydrogen. The instability is one-sided; low-Γ1 perturbations are unstable, while high-Γ1 perturbations are stable. Since a partially ionized fluid has a lower adiabatic exponent than either a fully recombined or fully ionized one, convective downflows are stabilized in the upper regions of a convective envelope where the nearly fully recombined fluid is embedded in a partially ionized background. They are significantly destabilized at a depth, however, where the partially ionized downflowing fluid has a lower Γ1 than does the highly ionized mean state. Convective upflows, by contrast, are stabilized at a depth where their fully ionized state contrasts with the partially ionized background and are destabilized only in the very upper layers where the mean state of the fluid is nearly fully recombined and the upflows are partially ionized. This Letter illustrates the instability mechanism, its finite-amplitude character, and its possible significance to both idealized compressible convection simulations and the solar convective envelope.