A thermodynamically induced finite-amplitude convective instability in stellar envelopes

Stellar envelopes are subject to a finite-amplitude convective instability that originates with the reduction in the adiabatic exponent Gamma (1) = (d ln P/d ln rho)(ad) accompanying partial ionization of the principle plasma constituents, notably hydrogen. The instability is one-sided; low-Gamma (1 ) perturbations are unstable, while high-Gamma (1) perturbations are stable . Since a partially ionized fluid has a lower adiabatic exponent than eithe r a fully recombined or fully ionized one, convective downflows are stabili zed in the upper regions of a convective envelope where the nearly fully re combined fluid is embedded in a partially ionized background. They are sign ificantly destabilized at a depth, however, where the partially ionized dow nflowing fluid has a lower than Gamma (1) does the highly ionized mean stat e. Convective upflows, by contrast, are stabilized at a depth where their f ully ionized state contrasts with the partially ionized background and are destabilized only in the very upper layers where the mean state of the flui d is nearly fully recombined and the upflows are partially ionized. This Le tter illustrates the instability mechanism, its finite-amplitude character, and its possible significance to both idealized compressible convection si mulations and the solar convective envelope.