Non-isobaric Thermal Instability

Mutliphase media have very complex structure and evolution. Accurate numerical simulations are necessary to make advances in our understanding of this rich physics. Because simulations can capture both the linear and nonlinear evolution of entropy and acoustic modes with a relatively wide range of sizes, it is important to thoroughly understand the stability of condensation and acoustic modes between the two extreme wavelength limits of isobaric and isochoric instability as identified by Field (1965). Partially motivated by a recent suggestion that large non-isobaric clouds can `shatter' into tiny cloudlets, we revisit the linear theory to survey all possible regimes of thermal instability (TI). We uncover seven regimes in total, one of which allows three unstable condensation modes. Using the code Athena++, we determine the numerical requirements to properly evolve small amplitude perturbations of the entropy mode into the nonlinear regime. Our 1D numerical simulations demonstrate that a single perturbation in an isobarically unstable plasma always settles into a single cloud, and the nonlinear evolution to the final state involves increasingly larger amplitude oscillations in cloud size, temperature and density as the wavelength increases. This is the hallmark behavior of non-isobaric multiphase gas dynamics and may be observable as correlations between changes in brightness and the associated periodic redshifts and blueshifts in systems that can be spatially resolved. Intriguingly, for an isochorically unstable case, we derive a characteristic cloud size for which the saturation process giving rise to these oscillations can be so energetic that the cloud may indeed break apart. However, we dub this process `splattering' instead of `shattering', as it is a different fragmentation mechanism triggered when the cloud suddenly `lands' on the stable cold branch of the equilibrium curve.